CHAPTER VI.

SOLANACEAE, PRIMULACEAE, POLYGONEAE, ETC.

Petunia violacea, crossed and self-fertilised plants compared for four generations. Effects of a cross with a fresh stock. Uniform colour of the flowers on the self-fertilised plants of the fourth generation. Nicotiana tabacum, crossed and self-fertilised plants of equal height. Great effects of a cross with a distinct sub-variety on the height, but not on the fertility, of the offspring. Cyclamen persicum, crossed seedlings greatly superior to the self-fertilised. Anagallis collina. Primula veris. Equal-styled variety of Primula veris, fertility of, greatly increased by a cross with a fresh stock. Fagopyrum esculentum. Beta vulgaris. Canna warscewiczi, crossed and self-fertilised plants of equal height. Zea mays. Phalaris canariensis.

25. SOLANACEAE. Petunia violacea.

DINGY PURPLE VARIETY.

The flowers of this plant are so seldom visited during the day by insects in this country, that I have never seen an instance; but my gardener, on whom I can rely, once saw some humble-bees at work. Mr. Meehan says, that in the United States bees bore through the corolla for the nectar, and adds that their “fertilisation is carried on by night-moths.” (6/1. ‘Proceedings of the Academy of Natural Science of Philadelphia’ August 2, 1870 page 90.)

In France M. Naudin, after castrating a large number of flowers whilst in bud, left them exposed to the visits of insects, and about a quarter produced capsules (6/2. ‘Annales des Sc. Nat.’ 4th series Bot. Tome 9 cah. 5); but I am convinced that a much larger proportion of flowers in my garden are cross-fertilised by insects, for protected flowers with their own pollen placed on the stigma never yielded nearly a full complement of seed; whilst those left uncovered produced fine capsules, showing that pollen from other plants must have been brought to them, probably by moths. Plants growing vigorously and flowering in pots in the greenhouse, never yielded a single capsule; and this may be attributed, at least in chief part, to the exclusion of moths.

Six flowers on a plant covered by a net were crossed with pollen from a distinct plant and produced six capsules, containing by weight 4.44 grains of seed. Six other flowers were fertilised with their own pollen and produced only three capsules, containing only 1.49 grains weight of seed. From this it follows that an equal number of crossed and self-fertilised capsules would have contained seeds by weight as 100 to 67. I should not have thought the proportional contents of so few capsules worth giving, had not nearly the same result been confirmed by several subsequent trials.

Seeds of the two lots were placed on sand, and many of the self-fertilised seeds germinated before the crossed, and were rejected. Several pairs in an equal state of germination were planted on the opposite sides of Pots 1 and 2; but only the tallest plant on each side was measured. Seeds were also sown thickly on the two sides of a large pot (3), the seedlings being afterwards thinned, so that an equal number was left on each side; the three tallest on each side being measured. The pots were kept in the greenhouse, and the plants were trained up sticks. For some time the young crossed plants had no advantage in height over the self-fertilised; but their leaves were larger. When fully grown and in flower the plants were measured, as follows:--

TABLE 6/76. Petunia violacea (first generation).

Heights of plants measured in inches.

Column 1: Number (Name) of Pot.

Column 2: Crossed Plants.

Column 3: Self-fertilised Plants.

Pot 1 : 30 : 20 4/8.

Pot 2 : 34 4/8 : 27 4/8.

Pot 3 : 34 : 28 4/8. Pot 3 : 30 4/8 : 27 4/8. Pot 3 : 25 : 26.

Total : 154 : 130.

The five tallest crossed plants here average 30.8, and the five tallest self-fertilised 26 inches in height, or as 100 to 84.

Three capsules were obtained by crossing flowers on the above crossed plants, and three other capsules by again self-fertilising flowers on the self-fertilised plants. One of the latter capsules appeared as fine as any one of the crossed capsules; but the other two contained many imperfect seeds. From these two lots of seeds the plants of the following generation were raised.

CROSSED AND SELF-FERTILISED PLANTS OF THE SECOND GENERATION.

As in the last generation, many of the self-fertilised seeds germinated before the crossed.

Seeds in an equal state of germination were planted on the opposite sides of three pots. The crossed seedlings soon greatly exceeded in height the self-fertilised. In Pot 1, when the tallest crossed plant was 10 1/2 inches high, the tallest self-fertilised was only 3 1/2 inches; in Pot 2 the excess in height of the crossed was not quite so great. The plants were treated as in the last generation, and when fully grown measured as before. In Pot 3 both the crossed plants were killed at an early age by some animal, so that the self-fertilised had no competitors. Nevertheless these two self-fertilised plants were measured, and are included in Table 6/77. The crossed plants flowered long before their self-fertilised opponents in Pots 1 and 2, and before those growing separately in Pot 3.

TABLE 6/77. Petunia violacea (Second generation).

Heights of plants measured in inches.

Column 1: Number (Name) of Pot.

Column 2: Crossed Plants.

Column 3: Self-fertilised Plants.

Pot 1 : 57 2/8 : 13 4/8. Pot 1 : 36 2/8 : 8.

Pot 2 : 44 4/8 : 33 2/8. Pot 2 : 24 : 28.

Pot 3 : 0 : 46 2/8. Pot 3 : 0 : 28 4/8.

Total : 162.0 : 157.5.

The four crossed plants average 40.5, and the six self-fertilised 26.25 inches in height; or as 100 to 65. But this great inequality is in part accidental, owing to some of the self-fertilised plants being very short, and to one of the crossed being very tall.

Twelve flowers on these crossed plants were again crossed, and eleven capsules were produced; of these, five were poor and six good; the latter contained by weight 3.75 grains of seeds. Twelve flowers on the self-fertilised plants were again fertilised with their own pollen and produced no less than twelve capsules, and the six finest of these contained by weight 2.57 grains of seeds. It should however be observed that these latter capsules were produced by the plants in Pot 3, which were not exposed to any competition. The seeds in the six fine crossed capsules to those in the six finest self-fertilised capsules were in weight as 100 to 68. From these seeds the plants of the next generation were raised.

CROSSED AND SELF-FERTILISED PLANTS OF THE THIRD GENERATION.

TABLE 6/78. Petunia violacea (third generation; plants very young).

Heights of plants measured in inches.

Column 1: Number (Name) of Pot.

Column 2: Crossed Plants.

Column 3: Self-fertilised Plants.

Pot 1 : 1 4/8 : 5 6/8. Pot 1 : 1 : 4 4/8.

Pot 2 : 5 7/8 : 8 3/8. Pot 2 : 5 6/8 : 6 7/8.

Pot 3 : 4 : 5 5/8.

Pot 4 : 1 4/8 : 5 3/8.

Total : 19.63 : 36.50.

The above seeds were placed on sand, and after germinating were planted in pairs on the opposite sides of four pots; and all the remaining seeds were thickly sown on the two sides of a fifth large pot. The result was surprising, for the self-fertilised seedlings very early in life beat the crossed, and at one time were nearly double their height. At first the case appeared like that of Mimulus, in which after the third generation a tall and highly self-fertile variety appeared. But as in the two succeeding generations the crossed plants resumed their former superiority over the self-fertilised, the case must be looked at as an anomaly. The sole conjecture which I can form is that the crossed seeds had not been sufficiently ripened, and thus produced weakly plants, as occurred with Iberis. When the crossed plants were between 3 and 4 inches in height, the six finest in four of the pots were measured to the summits of their stems, and at the same time the six finest of the self-fertilised plants. The measurements are given in Table 6/78, and it may be here seen that all the self-fertilised plants exceed their opponents in height, whereas when subsequently measured the excess of the self-fertilised depended chiefly on the unusual tallness of two of the plants in Pot 2. The crossed plants here average 3.27, and the self-fertilised 6.08 inches in height; or as 100 to 186.

When fully grown they were again measured, as follows:--

TABLE 6/79. Petunia violacea (third generation; plants fully grown).

Heights of plants measured in inches.

Column 1: Number (Name) of Pot.

Column 2: Crossed Plants.

Column 3: Self-fertilised Plants.

Pot 1 : 41 4/8 : 40 6/8. Pot 1 : 48 : 39. Pot 1 : 36 : 48.

Pot 2 : 36 : 47. Pot 2 : 21 : 80 2/8. Pot 2 : 36 2/8 : 86 2/8.

Pot 3 : 52 : 46.

Pot 4 : 57 : 43 6/8.

Total : 327.75 : 431.00.

The eight crossed plants now averaged 40.96, and the eight self-fertilised plants 53.87 inches in height, or as 100 to 131; and this excess chiefly depended, as already stated, on the unusual tallness of two of the self-fertilised plants in Pot 2. The self-fertilised had therefore lost some of their former great superiority over the crossed plants. In three of the pots the self-fertilised plants flowered first; but in Pot 3 at the same time with the crossed.

The case is rendered the more strange, because the crossed plants in the fifth pot (not included in the two last tables), in which all the remaining seeds had been thickly sown, were from the first finer plants than the self-fertilised, and had larger leaves. At the period when the two tallest crossed plants in this pot were 6 4/8 and 4 5/8 inches high, the two tallest self-fertilised were only 4 inches. When the two crossed plants were 12 and 10 inches high, the two self-fertilised were only 8 inches. These latter plants, as well as many others on the same side of this pot never grew any higher, whereas several of the crossed plants grew to the height of two feet! On account of this great superiority of the crossed plants, the plants on neither side of this pot have been included in the two last tables.

Thirty flowers on the crossed plants in Pots 1 and 4 (Table 6/79) were again crossed, and produced seventeen capsules. Thirty flowers on the self-fertilised plants in the same two pots were again self-fertilised, but produced only seven capsules. The contents of each capsule of both lots were placed in separate watch-glasses, and the seeds from the crossed appeared to the eye to be at least double the number of those from the self-fertilised capsules.

In order to ascertain whether the fertility of the self-fertilised plants had been lessened by the plants having been self-fertilised for the three previous generations, thirty flowers on the crossed plants were fertilised with their own pollen. These yielded only five capsules, and their seeds being placed in separate watch-glasses did not seem more numerous than those from the capsules on the self-fertilised plants self-fertilised for the fourth time. So that as far as can be judged from so few capsules, the self-fertility of the self-fertilised plants had not decreased in comparison with that of the plants which had been intercrossed during the three previous generations. It should, however, be remembered that both lots of plants had been subjected in each generation to almost exactly similar conditions.

Seeds from the crossed plants again crossed, and from the self-fertilised again self-fertilised, produced by the plants in Pot 1 (Table 6/79), in which the three self-fertilised plants were on an average only a little taller than the crossed, were used in the following experiment. They were kept separate from two similar lots of seeds produced by the two plants in Pot 4 in the same table, in which the crossed plant was much taller than its self-fertilised opponent.

CROSSED AND SELF-FERTILISED PLANTS OF THE FOURTH GENERATION (RAISED FROM THE PLANTS IN POT 1, TABLE 6/79).

Crossed and self-fertilised seeds from plants of the last generation in Pot 1 in Table 6/79, were placed on sand, and after germinating, were planted in pairs on the opposite sides of four pots. The seedlings when in full flower were measured to the base of the calyx. The remaining seeds were sown crowded on the two sides of Pot 5; and the four tallest plants on each side of this pot were measured in the same manner.

TABLE 6/80. Petunia violacea (fourth generation; raised from plants of the third generation in Pot 1, table 6/79).

Heights of plants measured in inches.

Column 1: Number (Name) of Pot.

Column 2: Crossed Plants.

Column 3: Self-fertilised Plants.

Pot 1 : 29 2/8 : 30 2/8. Pot 1 : 36 2/8 : 34 6/8. Pot 1 : 49 : 31 3/8.

Pot 2 : 33 3/8 : 31 5/8. Pot 2 : 37 3/8 : 38 2/8. Pot 2 : 56 4/8 : 38 4/8.

Pot 3 : 46 : 45 1/8. Pot 3 : 67 2/8 : 45. Pot 3 : 54 3/8 : 23 2/8.

Pot 4 : 51 6/8 : 34. Pot 4 : 51 7/8 : 0.

Pot 5 : 49 4/8 : 22 3/8. Pot 5 : 46 3/8 : 24 2/8. Pot 5 : 40 : 24 6/8. Pot 5 : 53 : 30. Crowded plants.

Total : 701.88 : 453.50.

The fifteen crossed plants average 46.79, and the fourteen (one having died) self-fertilised plants 32.39 inches in height; or as 100 to 69. So that the crossed plants in this generation had recovered their wonted superiority over the self-fertilised plants; though the parents of the latter in Pot 1, Table 6/79, were a little taller than their crossed opponents.

CROSSED AND SELF-FERTILISED PLANTS OF THE FOURTH GENERATION (RAISED FROM THE PLANTS IN POT 4, TABLE 6/79).

Two similar lots of seeds, obtained from the plants in Pot 4 in Table 6/79, in which the single crossed plant was at first shorter, but ultimately much taller than its self-fertilised opponent, were treated in every way like their brethren of the same generation in the last experiment. We have in Table 6/81 the measurements of the present plants. Although the crossed plants greatly exceeded in height the self-fertilised; yet in three out of the five pots a self-fertilised plant flowered before any one of the crossed; in a fourth pot simultaneously; and in a fifth (namely Pot 2) a crossed plant flowered first.

TABLE 6/81. Petunia violacea (fourth generation; raised from plants of the third generation in Pot 4, Table 6/79).

Heights of plants measured in inches.

Column 1: Number (Name) of Pot.

Column 2: Crossed Plants.

Column 3: Self-fertilised Plants.

Pot 1 : 46 : 30 2/8. Pot 1 : 46 : 28.

Pot 2 : 50 6/8 : 25. Pot 2 : 40 2/8 : 31 3/8. Pot 2 : 37 3/8 : 22 4/8.

Pot 3 : 54 2/8 : 22 5/8. Pot 3 : 61 1/8 : 26 6/8. Pot 3 : 45 : 32.

Pot 4 : 30 : 24 4/8. Pot 4 : 29 1/8 : 26.

Pot 5 : 37 4/8 : 40 2/8. Pot 5 : 63 : 18 5/8. Pot 5 : 41 2/8 : 17 4/8. Crowded plants.

Total : 581.63 : 349.36.

The thirteen crossed plants here average 44.74, and the thirteen self-fertilised plants 26.87 inches in height; or as 100 to 60. The crossed parents of these were much taller, relatively to the self-fertilised parents, than in the last case; and apparently they transmitted some of this superiority to their crossed offspring. It is unfortunate that I did not turn these plants out of doors, so as to observe their relative fertility, for I compared the pollen from some of the crossed and self-fertilised plants in Pot 1, Table 6/81, and there was a marked difference in its state; that of the crossed plants contained hardly any bad and empty grains, whilst such abounded in the pollen of the self-fertilised plants.

THE EFFECTS OF A CROSS WITH A FRESH STOCK.

I procured from a garden in Westerham, whence my plants originally came, a fresh plant differing in no respect from mine except in the colour of the flowers, which was a fine purple. But this plant must have been exposed during at least four generations to very different conditions from those to which my plants had been subjected, as these had been grown in pots in the greenhouse. Eight flowers on the self-fertilised plants in Table 6/81, of the last or fourth self-fertilised generation, were fertilised with pollen from this fresh stock; all eight produced capsules containing together by weight 5.01 grains of seeds. The plants raised from these seeds may be called the Westerham-crossed.

Eight flowers on the crossed plants of the last or fourth generation in Table 6/81 were again crossed with pollen from one of the other crossed plants, and produced five capsules, containing by weight 2.07 grains of seeds. The plants raised from these seeds may be called the INTERCROSSED; and these form the fifth intercrossed generation.

Eight flowers on the self-fertilised plants of the same generation in Table 6/81 were again self-fertilised, and produced seven capsules, containing by weight 2.1 grains of seeds. The SELF-FERTILISED plants raised from these seeds form the fifth self-fertilised generation. These latter plants and the intercrossed are comparable in all respects with the crossed and self-fertilised plants of the four previous generations.

From the foregoing data it is easy to calculate that:

Ten Westerham-crossed capsules would have contained 6.26 grains weight of seed.

Ten intercrossed capsules would have contained 4.14 grains weight of seed.

Ten self-fertilised capsules would have contained 3.00 grains weight of seed.

We thus get the following ratios:--

Seeds from the Westerham-crossed capsules to those from the capsules of the fifth self-fertilised generation, in weight as 100 to 48.

Seeds from the Westerham-crossed capsules to those from the capsules of the fifth intercrossed generation, in weight as 100 to 66.

Seeds from the intercrossed capsules to those from the self-fertilised capsules, in weight as 100 to 72.

So that a cross with pollen from a fresh stock greatly increased the productiveness of the flowers on plants which had been self-fertilised for the four previous generations, in comparison not only with the flowers on the same plants self-fertilised for the fifth time, but with the flowers on the crossed plants crossed with pollen from another plant of the same old stock for the fifth time.

These three lots of seeds were placed on sand, and were planted in an equal state of germination in seven pots, each made tripartite by three superficial partitions. Some of the remaining seeds, whether or not in a state of germination, were thickly sown in an eighth pot. The pots were kept in the greenhouse, and the plants trained up sticks. They were first measured to the tops of their stems when coming into flower; and the twenty-two Westerham-crossed plants then averaged 25.51 inches; the twenty-three intercrossed plants 30.38; and the twenty-three self-fertilised plants 23.40 inches in height. We thus get the following ratios:--

The Westerham-crossed plants in height to the self-fertilised as 100 to 91.

The Westerham-crossed plants in height to the intercrossed as 100 to 119.

The intercrossed plants in height to the self-fertilised as 100 to 77.

These plants were again measured when their growth appeared on a casual inspection to be complete. But in this I was mistaken, for after cutting them down, I found that the summits of the stems of the Westerham-crossed plants were still growing vigorously; whilst the intercrossed had almost, and the self-fertilised had quite completed their growth. Therefore I do not doubt, if the three lots had been left to grow for another month, that the ratios would have been somewhat different from those deduced from the measurements in Table 6/82.

TABLE 6/82. Petunia violacea.

Heights of plants measured in inches.

Column 1: Number (Name) of Pot.

Column 2: Westerham-Crossed Plants (from self-fertilised Plants of fourth generation crossed by a fresh stock).

Column 3: Intercrossed Plants (Plants of one and the same stock intercrossed for five generations).

Column 4: Self-fertilised Plants (self-fertilised for five generations).

Pot 1 : 64 5/8 : 57 2/8 : 43 6/8. Pot 1 : 24 : 64 : 56 3/8. Pot 1 : 51 4/8 : 58 6/8 : 31 5/8.

Pot 2 : 48 7/8 : 59 7/8 : 41 5/8. Pot 2 : 54 4/8 : 58 2/8 : 41 2/8. Pot 2 : 58 1/8 : 53 : 18 2/8.

Pot 3 : 62 : 52 2/8 : 46 6/8. Pot 3 : 53 2/8 : 54 6/8 : 45. Pot 3 : 62 7/8 : 61 6/8 : 19 4/8.

Pot 4 : 44 4/8 : 58 7/8 : 37 5/8. Pot 4 : 49 2/8 : 65 2/8 : 33 2/8. Pot 4 : .. : 59 6/8 : 32 2/8.

Pot 5 : 43 1/8 : 35 6/8 : 41 6/8. Pot 5 : 53 7/8 : 34 6/8 : 26 4/8. Pot 5 : 53 2/8 : 54 6/8 : 0.

Pot 6 : 37 4/8 : 56 : 46 4/8. Pot 6 : 61 : 63 5/8 : 29 6/8. Pot 6 : 0 : 57 7/8 : 14 4/8.

Pot 7 : 59 6/8 : 51 : 43. Pot 7 : 43 4/8 : 49 6/8 : 12 2/8. Pot 7 : 50 5/8 : 0 : 0.

Pot 8 : 37 7/8 : 38 5/8 : 21 6/8. Pot 8 : 37 2/8 : 44 5/8 : 14 5/8.

Total : 1051.25 : 1190.50 : 697.88.

The twenty-one Westerham-crossed plants now averaged 50.05 inches; the twenty-two intercrossed plants, 54.11 inches; and the twenty-one self-fertilised plants, 33.23 inches in height. We thus get the following ratios:--

The Westerham-crossed plants in height to the self-fertilised as 100 to 66.

The Westerham-crossed plants in height to the intercrossed as 100 to 108.

The intercrossed plants in height to the self-fertilised as 100 to 61.

We here see that the Westerham-crossed (the offspring of plants self-fertilised for four generations and then crossed with a fresh stock) have gained greatly in height, since they were first measured, relatively to the plants self-fertilised for five generations. They were then as 100 to 91, and now as 100 to 66 in height. The intercrossed plants (i.e., those which had been intercrossed for the last five generations) likewise exceed in height the self-fertilised plants, as occurred in all the previous generations with the exception of the abnormal plants of the third generation. On the other hand, the Westerham-crossed plants are exceeded in height by the intercrossed; and this is a surprising fact, judging from most of the other strictly analogous cases. But as the Westerham-crossed plants were still growing vigorously, while the intercrossed had almost ceased to grow, there can hardly be a doubt that if left to grow for another month they would have beaten the intercrossed in height. That they were gaining on them is clear, as when measured before they were as 100 to 119, and now as only 100 to 108 in height. The Westerham-crossed plants had also leaves of a darker green, and looked altogether more vigorous than the intercrossed; and what is much more important, they produced, as we shall presently see, much heavier seed-capsules. So that in fact the offspring from the self-fertilised plants of the fourth generation crossed by a fresh stock were superior to the intercrossed, as well as to the self-fertilised plants of the fifth generation--of which latter fact there could not be the least doubt.

These three lots of plants were cut down close to the ground and weighed. The twenty-one Westerham-crossed plants weighed 32 ounces; the twenty-two intercrossed plants, 34 ounces, and the twenty-one self-fertilised plants 7 1/4 ounces. The following ratios are calculated for an equal number of plants of each kind. But as the self-fertilised plants were just beginning to wither, their relative weight is here slightly too small; and as the Westerham-crossed were still growing vigorously, their relative weight with time allowed would no doubt have greatly increased.

The Westerham-crossed plants in weight to the self-fertilised as 100 to 22.

The Westerham-crossed plants in weight to the intercrossed as 100 to 101.

The intercrossed plants in weight to the self-fertilised as 100 to 22.3.

We here see, judging by weight instead of as before by height, that the Westerham-crossed and the intercrossed have an immense advantage over the self-fertilised. The Westerham-crossed are inferior to the intercrossed by a mere trifle; but it is almost certain that if they had been allowed to go on growing for another month, the former would have completely beaten the latter.

As I had an abundance of seeds of the same three lots, from which the foregoing plants had been raised, these were sown in three long parallel and adjoining rows in the open ground, so as to ascertain whether under these circumstances the results would be nearly the same as before. Late in the autumn (November 13) the ten tallest plants were carefully selected out of each row, and their heights measured, with the following result:--

TABLE 6/83. Petunia violacea (plants growing in the open ground).

Heights of plants measured in inches.

Column 1: Westerham-Crossed Plants (from self-fertilised Plants of the fourth generation crossed by a fresh stock).

Column 2: intercrossed Plants (Plants of one and the same stock intercrossed for five generations).

Column 3: self-fertilised Plants (self-fertilised for five generations).

34 2/8 : 38 : 27 3/8. 36 2/8 : 36 2/8 : 23. 35 2/8 : 39 5/8 : 25. 32 4/8 : 37 : 24 1/8. 37 : 36 : 22 4/8. 36 4/8 : 41 3/8 : 23 3/8. 40 7/8 : 37 2/8 : 21 5/8. 37 2/8 : 40 : 23 4/8. 38 2/8 : 41 2/8 : 21 3/8. 38 5/8 : 36 : 21 2/8.

366.76 : 382.76 : 233.13.

The ten Westerham-crossed plants here average 36.67 inches in height; the ten intercrossed plants, 38.27 inches; and the ten self-fertilised, 23.31 inches. These three lots of plants were also weighed; the Westerham-crossed plants weighed 28 ounces; the intercrossed plants, 41 ounces; and the self-fertilised, 14.75 ounces. We thus get the following ratios:--

The Westerham-crossed plants in height to the self-fertilised as 100 to 63.

The Westerham-crossed plants in weight to the self-fertilised as 100 to 53.

The Westerham-crossed plants in height to the intercrossed as 100 to 104.

The Westerham-crossed plants in weight to the intercrossed as 100 to 146.

The intercrossed plants in height to the self-fertilised as 100 to 61.

The intercrossed plants in weight to the self-fertilised as 100 to 36.

Here the relative heights of the three lots are nearly the same (within three or four per cent) as with the plants in the pots. In weight there is a much greater difference: the Westerham-crossed exceed the self-fertilised by much less than they did before; but the self-fertilised plants in the pots had become slightly withered, as before stated, and were in consequence unfairly light. The Westerham-crossed plants are here inferior in weight to the intercrossed plants in a much higher degree than in the pots; and this appeared due to their being much less branched, owing to their having germinated in greater numbers and consequently being much crowded. Their leaves were of a brighter green than those of the intercrossed and self-fertilised plants.

RELATIVE FERTILITY OF THE THREE LOTS OF PLANTS.

None of the plants in pots in the greenhouse ever produced a capsule; and this may be attributed in chief part to the exclusion of moths. Therefore the fertility of the three lots could be judged of only by that of the plants growing out of doors, which from being left uncovered were probably cross-fertilised. The plants in the three rows were exactly of the same age and had been subjected to closely similar conditions, so that any difference in their fertility must be attributed to their different origin; namely, to the one lot being derived from plants self-fertilised for four generations and then crossed with a fresh stock; to the second lot being derived from plants of the same old stock intercrossed for five generations; and to the third lot being derived from plants self-fertilised for five generations. All the capsules, some nearly mature and some only half-grown, were gathered, counted, and weighed from the ten finest plants in each of the three rows, of which the measurements and weights have already been given. The intercrossed plants, as we have seen, were taller and considerably heavier than the plants of the other two lots, and they produced a greater number of capsules than did even the Westerham-crossed plants; and this may be attributed to the latter having grown more crowded and being in consequence less branched. Therefore the average weight of an equal number of capsules from each lot of plants seems to be the fairest standard of comparison, as their weights will have been determined chiefly by the number of the included seeds. As the intercrossed plants were taller and heavier than the plants of the other two lots, it might have been expected that they would have produced the finest or heaviest capsules; but this was very far from being the case.

The ten tallest Westerham-crossed plants produced 111 ripe and unripe capsules, weighing 121.2 grains. Therefore 100 of such capsules would have weighed 109.18 grains.

The ten tallest intercrossed plants produced 129 capsules, weighing 76.45 grains. Therefore 100 of these capsules would have weighed 59.26 grains.

The ten tallest self-fertilised plants produced only 44 capsules, weighing 22.35 grains. Therefore 100 of these capsules would have weighed 50.79 grains.

From these data we get the following ratios for the fertility of the three lots, as deduced from the relative weights of an equal number of capsules from the finest plants in each lot:--

Westerham-crossed plants to self-fertilised plants as 100 to 46.

Westerham-crossed plants to intercrossed plants as 100 to 54.

Intercrossed plants to self-fertilised plants as 100 to 86.

We here see how potent the influence of a cross with pollen from a fresh stock has been on the fertility of plants self-fertilised for four generations, in comparison with plants of the old stock when either intercrossed or self-fertilised for five generations; the flowers on all these plants having been left to be freely crossed by insects or to fertilise themselves. The Westerham-crossed plants were also much taller and heavier plants than the self-fertilised, both in the pots and open ground; but they were less tall and heavy than the intercrossed plants. This latter result, however, would almost certainly have been reversed, if the plants had been allowed to grow for another month, as the Westerham-crossed were still growing vigorously, whilst the intercrossed had almost ceased to grow. This case reminds us of the somewhat analogous one of Eschscholtzia, in which plants raised from a cross with a fresh stock did not grow higher than the self-fertilised or intercrossed plants, but produced a greater number of seed-capsules, which contained a far larger average number of seeds.

COLOUR OF THE FLOWERS ON THE ABOVE THREE LOTS OF PLANTS.

The original mother-plant, from which the five successive self-fertilised generations were raised, bore dingy purple flowers. At no time was any selection practised, and the plants were subjected in each generation to extremely uniform conditions. The result was, as in some previous cases, that the flowers on all the self-fertilised plants, both in the pots and open ground, were absolutely uniform in tint; this being a dull, rather peculiar flesh colour. This uniformity was very striking in the long row of plants growing in the open ground, and these first attracted my attention. I did not notice in which generation the original colour began to change and to become uniform, but I have every reason to believe that the change was gradual. The flowers on the intercrossed plants were mostly of the same tint, but not nearly so uniform as those on the self-fertilised plants, and many of them were pale, approaching almost to white. The flowers on the plants from the cross with the purple-flowered Westerham stock were, as might have been expected, much more purple and not nearly so uniform in tint. The self-fertilised plants were also remarkably uniform in height, as judged by the eye; the intercrossed less so, whilst the Westerham-crossed plants varied much in height.

Nicotiana tabacum.

This plant offers a curious case. Out of six trials with crossed and self-fertilised plants, belonging to three successive generations, in one alone did the crossed show any marked superiority in height over the self-fertilised; in four of the trials they were approximately equal; and in one (i.e., in the first generation) the self-fertilised plants were greatly superior to the crossed. In no case did the capsules from flowers fertilised with pollen from a distinct plant yield many more, and sometimes they yielded much fewer seeds than the capsules from self-fertilised flowers. But when the flowers of one variety were crossed with pollen from a slightly different variety, which had grown under somewhat different conditions,--that is, by a fresh stock,--the seedlings derived from this cross exceeded in height and weight those from the self-fertilised flowers in an extraordinary degree.

Twelve flowers on some plants of the common tobacco, raised from purchased seeds, were crossed with pollen from a distinct plant of the same lot, and these produced ten capsules. Twelve flowers on the same plants were fertilised with their own pollen, and produced eleven capsules. The seeds in the ten crossed capsules weighed 31.7 grains, whilst those in ten of the self-fertilised capsules weighed 47.67 grains; or as 100 to 150. The much greater productiveness of the self-fertilised than of the crossed capsules can hardly be attributed to chance, as all the capsules of both lots were very fine and healthy ones.

The seeds were placed on sand, and several pairs in an equal state of germination were planted on the opposite sides of three pots. The remaining seeds were thickly sown on the two sides of Pot 4, so that the plants in this pot were much crowded. The tallest plant on each side of each pot was measured. Whilst the plants were quite young the four tallest crossed plants averaged 7.87 inches, and the four tallest self-fertilised 14.87 inches in height; or as 100 to 189. The heights at this age are given in the two left columns of Table 6/84.

When in full flower the tallest plants on each side were again measured, see the two right hand columns in Table 6/84. But I should state that the pots were not large enough, and the plants never grew to their proper height. The four tallest crossed plants now averaged 18.5, and the four tallest self-fertilised plants 32.75 inches in height; or as 100 to 178. In all four pots a self-fertilised plant flowered before any one of the crossed.

In Pot 4, in which the plants were extremely crowded, the two lots were at first equal; and ultimately the tallest crossed plant exceeded by a trifle the tallest self-fertilised plant. This recalled to my mind an analogous case in the one generation of Petunia, in which the self-fertilised plants were throughout their growth taller than the crossed in all the pots except in the crowded one. Accordingly another trial was made, and some of the same crossed and self-fertilised seeds of tobacco were sown thickly on opposite sides of two additional pots; the plants being left to grow up much crowded. When they were between 13 and 14 inches in height there was no difference between the two sides, nor was there any marked difference when the plants had grown as tall as they could; for in one pot the tallest crossed plant was 26 1/2 inches in height, and exceeded by 2 inches the tallest self-fertilised plant, whilst in the other pot, the tallest crossed plant was shorter by 3 1/2 inches than the tallest self-fertilised plant, which was 22 inches in height.

TABLE 6/84. Nicotiana tabacum (first generation).

Heights of plants measured in inches.

Column 1: Number (Name) of Pot.

Column 2: Crossed Plants, May 20, 1868.

Column 3: self-fertilised Plants, May 20, 1868.

Column 4: Crossed Plants, December 6, 1868.

Column 5: self-fertilised Plants, December 6, 1868.

Pot 1 : 15 4/8 : 26 : 40 : 44.

Pot 2 : 3 : 15 : 6 4/8 : 43.

Pot 3 : 8 : 13 4/8 : 16 : 33.

Pot 4 : 5 : 5 : 11 4/8 : 11.

Total : 31.5 : 59.5 : 74.0 : 131.0.

As the plants did not grow to their proper height in the above small pots in Table 6/84, four crossed and four self-fertilised plants were raised from the same seed, and were planted in pairs on the opposite sides of four very large pots containing rich soil; so that they were not exposed to at all severe mutual competition. When these plants were in flower I neglected to measure them, but record in my notes that all four self-fertilised plants exceeded in height the four crossed plants by 2 or 3 inches. We have seen that the flowers on the original or parent-plants which were crossed with pollen from a distinct plant yielded much fewer seeds than those fertilised with their own pollen; and the trial just given, as well as that in Table 6/84, show us clearly that the plants raised from the crossed seeds were inferior in height to those from the self-fertilised seeds; but only when not greatly crowded. When crowded and thus subjected to very severe competition, the crossed and self-fertilised plants were nearly equal in height.

CROSSED AND SELF-FERTILISED PLANTS OF THE SECOND GENERATION.

Twelve flowers on the crossed plants of the last generation growing in the four large pots just mentioned, were crossed with pollen from a crossed plant growing in one of the other pots; and twelve flowers on the self-fertilised plants were fertilised with their own pollen. All these flowers of both lots produced fine capsules. Ten of the crossed capsules contained by weight 38.92 grains of seeds, and ten of the self-fertilised capsules 37.74 grains; or as 100 to 97. Some of these seeds in an equal state of germination were planted in pairs on the opposite sides of five large pots. A good many of the crossed seeds germinated before the self-fertilised, and were of course rejected. The plants thus raised were measured when several of them were in full flower.

TABLE 6/85. Nicotiana tabacum (second generation).

Heights of plants measured in inches.

Column 1: Number (Name) of Pot.

Column 2: Crossed Plants.

Column 3: Self-fertilised Plants.

Pot 1 : 14 4/8 : 27 6/8. Pot 1 : 78 4/8 : 8 6/8. Pot 1 : 9 : 56.

Pot 2 : 60 4/8 : 16 6/8. Pot 2 : 44 6/8 : 7. Pot 2 : 10 : 50 4/8.

Pot 3 : 57 1/8 : 87 (A). Pot 3 : 1 2/8 : 81 2/8 (B).

Pot 4 : 6 6/8 : 19. Pot 4 : 31 : 43 2/8. Pot 4 : 69 4/8 : 4.

Pot 5 : 99 4/8 : 9 4/8. Pot 5 : 29 2/8 : 3.

Total : 511.63 : 413.75.

The thirteen crossed plants here average 39.35, and the thirteen self-fertilised plants 31.82 inches in height; or as 100 to 81. But it would be a very much fairer plan to exclude all the starved plants of only 10 inches and under in height; and in this case the nine remaining crossed plants average 53.84, and the seven remaining self-fertilised plants 51.78 inches in height, or as 100 to 96; and this difference is so small that the crossed and self-fertilised plants may be considered as of equal heights.

In addition to these plants, three crossed plants were planted separately in three large pots, and three self-fertilised plants in three other large pots, so that they were not exposed to any competition; and now the self-fertilised plants exceeded the crossed in height by a little, for the three crossed averaged 55.91, and the three self-fertilised 59.16 inches; or as 100 to 106.

CROSSED AND SELF-FERTILISED PLANTS OF THE THIRD GENERATION.

TABLE 6/86. Nicotiana tabacum (third generation). Seedlings from the self-fertilised plant A in pot 3, Table 6/85, of the last or second generation.

Heights of plants measured in inches.

Column 1: Number (Name) of Pot.

Column 2: From Self-fertilised Plant, crossed by a Crossed Plant.

Column 3: From Self-fertilised Plant again self-fertilised, forming the third Self-fertilised generation.

Pot 1 : 100 2/8 : 98. Pot 1 : 91 : 79.

Pot 2 : 110 2/8 : 59 1/8. Pot 2 : 100 4/8 : 66 6/8.

Pot 3 : 104 : 79 6/8.

Pot 4 : 84 2/8 : 110 4/8. Pot 4 : 76 4/8 : 64 1/8.

Total : 666.75 : 557.25.

As I wished to ascertain, firstly, whether those self-fertilised plants of the last generation, which greatly exceeded in height their crossed opponents, would transmit the same tendency to their offspring, and secondly, whether they possessed the same sexual constitution, I selected for experiment the two self-fertilised plants marked A and B in Pot 3 in Table 6/85, as these two were of nearly equal height, and were greatly superior to their crossed opponents. Four flowers on each plant were fertilised with their own pollen, and four others on the same plants were crossed with pollen from one of the crossed plants growing in another pot. This plan differs from that before followed, in which seedlings from crossed plants again crossed, have been compared with seedlings from self-fertilised plants again self-fertilised. The seeds from the crossed and self-fertilised capsules of the above two plants were placed in separate watch-glasses and compared, but were not weighed; and in both cases those from the crossed capsules seemed to be rather less numerous than those from the self-fertilised capsules. These seeds were planted in the usual manner, and the heights of the crossed and self-fertilised seedlings, when fully grown, are given in Tables 6/86 and 6/87.

The seven crossed plants in the first of these two tables average 95.25, and the seven self-fertilised 79.6 inches in height; or as 100 to 83. In half the pots a crossed plant, and in the other half a self-fertilised plant flowered first.

We now come to the seedlings raised from the other parent-plant B.

TABLE 6/87. Nicotiana tabacum (third generation). Seedlings from the self-fertilised plant B in pot 3, Table 6/85, of the last or second generation.

Heights of plants measured in inches.

Column 1: Number (Name) of Pot.

Column 2: From Self-fertilised Plant, crossed by a Crossed Plant.

Column 3: From Self-fertilised Plant again self-fertilised, forming the third Self-fertilised generation.

Pot 1 : 87 2/8 : 72 4/8. Pot 1 : 49 : 14 2/8.

Pot 2 : 98 4/8 : 73. Pot 2 : 0 : 110 4/8.

Pot 3 : 99 : 106 4/8. Pot 3 : 15 2/8 : 73 6/8.

Pot 4 : 97 6/8 : 48 6/8.

Pot 5 : 48 6/8 : 81 2/8. Pot 5 : 0 : 61 2/8.

Total : 495.50 : 641.75.

The seven crossed plants (for two of them died) here average 70.78 inches, and the nine self-fertilised plants 71.3 inches in height; or as 100 to barely 101. In four out of these five pots, a self-fertilised plant flowered before any one of the crossed plants. So that, differently from the last case, the self-fertilised plants are in some respects slightly superior to the crossed.

If we now consider the crossed and self-fertilised plants of the three generations, we find an extraordinary diversity in their relative heights. In the first generation, the crossed plants were inferior to the self-fertilised as 100 to 178; and the flowers on the original parent-plants which were crossed with pollen from a distinct plant yielded much fewer seeds than the self-fertilised flowers, in the proportion of 100 to 150. But it is a strange fact that the self-fertilised plants, which were subjected to very severe competition with the crossed, had on two occasions no advantage over them. The inferiority of the crossed plants of this first generation cannot be attributed to the immaturity of the seeds, for I carefully examined them; nor to the seeds being diseased or in any way injured in some one capsule, for the contents of the ten crossed capsules were mingled together and a few taken by chance for sowing. In the second generation the crossed and self-fertilised plants were nearly equal in height. In the third generation, crossed and self-fertilised seeds were obtained from two plants of the previous generation, and the seedlings raised from them differed remarkably in constitution; the crossed in the one case exceeded the self-fertilised in height in the ratio of 100 to 83, and in the other case were almost equal. This difference between the two lots, raised at the same time from two plants growing in the same pot, and treated in every respect alike, as well as the extraordinary superiority of the self-fertilised over the crossed plants in the first generation, considered together, make me believe that some individuals of the present species differ to a certain extent from others in their sexual affinities (to use the term employed by Gartner), like closely allied species of the same genus. Consequently if two plants which thus differ are crossed, the seedlings suffer and are beaten by those from the self-fertilised flowers, in which the sexual elements are of the same nature. It is known that with our domestic animals certain individuals are sexually incompatible, and will not produce offspring, although fertile with other individuals. (6/3. I have given evidence on this head in my ‘Variation of Animals and Plants under Domestication’