Chapter 24

the ovules in the flowers of Wistaria sinensis, and carefully estimated the number of pollen-grains, and he found that for each ovule there were 7000 grains. (10/10. Quoted in ‘Gardeners’ Chronicle’ 1846 page 771.) With Mirabilis, three or four of the very large pollen-grains are sufficient to fertilise an ovule; but I do not know how many grains a flower produces. With Hibiscus, Kolreuter found that sixty grains were necessary to fertilise all the ovules of a flower, and he calculated that 4863 grains were produced by a single flower, or eighty-one times too many. With Geum urbanum, however, according to Gartner, the pollen is only ten times too much. (10/11. Kolreuter ‘Vorlaufige Nachricht’ 1761 page 9. Gartner ‘Beitrage zur Kenntniss’ etc. page 346.) As we thus see that the open state of all ordinary flowers, and the consequent loss of much pollen, necessitate the development of so prodigious an excess of this precious substance, why, it may be asked, are flowers always left open? As many plants exist throughout the vegetable kingdom which bear cleistogene flowers, there can hardly be a doubt that all open flowers might easily have been converted into closed ones. The graduated steps by which this process could have been effected may be seen at the present time in Lathyrus nissolia, Biophytum sensitivum, and several other plants. The answer to the above question obviously is, that with permanently closed flowers there could be no cross-fertilisation.

The frequency, almost regularity, with which pollen is transported by insects from flower to flower, often from a considerable distance, well deserves attention. (10/12. An experiment made by Kolreuter ‘Forsetsung’ etc. 1763 page 69, affords good evidence on this head. Hibiscus vesicarius is strongly dichogamous, its pollen being shed before the stigmas are mature. Kolreuter marked 310 flowers, and put pollen from other flowers on their stigmas every day, so that they were thoroughly fertilised; and he left the same number of other flowers to the agency of insects. Afterwards he counted the seeds of both lots: the flowers which he had fertilised with such astonishing care produced 11,237 seeds, whilst those left to the insects produced 10,886; that is, a less number by only 351; and this small inferiority is fully accounted for by the insects not having worked during some days, when the weather was cold with continued rain.) This is best shown by the impossibility in many cases of raising two varieties of the same species pure, if they grow at all near together; but to this subject I shall presently return; also by the many cases of hybrids which have appeared spontaneously both in gardens and a state of nature. With respect to the distance from which pollen is often brought, no one who has had any experience would expect to obtain pure cabbage-seed, for instance, if a plant of another variety grew within two or three hundred yards. An accurate observer, the late Mr. Masters of Canterbury, assured me that he once had his whole stock of seeds “seriously affected with purple bastards,” by some plants of purple kale which flowered in a cottager’s garden at the distance of half a mile; no other plant of this variety growing any nearer. (10/13. Mr. W.C. Marshall caught no less than seven specimens of a moth (Cucullia umbratica) with the pollinia of the butterfly-orchis (Habenaria chlorantha) sticking to their eyes, and, therefore, in the proper position for fertilising the flowers of this species, on an island in Derwentwater, at the distance of half a mile from any place where this plant grew: ‘Nature’ 1872 page 393.) But the most striking case which has been recorded is that by M. Godron, who shows by the nature of the hybrids produced that Primula grandiflora must have been crossed with pollen brought by bees from P. officinalis, growing at the distance of above two kilometres, or of about one English mile and a quarter. (10/14. ‘Revue des Sc. Nat.’ 1875 page 331.)

All those who have long attended to hybridisation, insist in the strongest terms on the liability of castrated flowers to be fertilised by pollen brought from distant plants of the same species. (10/15. See, for instance, the remarks by Herbert ‘Amaryllidaceae’ 1837 page 349. Also Gartner’s strong expressions on this subject in his ‘Bastarderzeugung’ 1849 page 670 and ‘Kenntniss der Befruchtung’ 1844 pages 510, 573. Also Lecoq ‘De la Fecondation’ etc. 1845 page 27. Some statements have been published during late years of the extraordinary tendency of hybrid plants to revert to their parent forms; but as it is not said how the flowers were protected from insects, it may be suspected that they were often fertilised with pollen brought from a distance from the parent-species.) The following case shows this in the clearest manner: Gartner, before he had gained much experience, castrated and fertilised 520 flowers on various species with pollen of other genera or other species, but left them unprotected; for, as he says, he thought it a laughable idea that pollen should be brought from flowers of the same species, none of which grew nearer than between 500 and 600 yards. (10/16. ‘Kenntniss der Befruchtung’ pages 539, 550, 575, 576.) The result was that 289 of these 520 flowers yielded no seed, or none that germinated; the seed of 29 flowers produced hybrids, such as might have been expected from the nature of the pollen employed; and lastly, the seed of the remaining 202 flowers produced perfectly pure plants, so that these flowers must have been fertilised by pollen brought by insects from a distance of between 500 and 600 yards. (10/17. Henschel’s experiments quoted by Gartner ‘Kenntniss’ etc. page 574, which are worthless in all other respects, likewise show how largely flowers are intercrossed by insects. He castrated many flowers on thirty-seven species, belonging to twenty-two genera, and put on their stigmas either no pollen, or pollen from distinct genera, yet they all seeded, and all the seedlings raised from them were of course pure.) It is of course possible that some of these 202 flowers might have been fertilised by pollen left accidentally in them when they were castrated; but to show how improbable this is, I may add that Gartner, during the next eighteen years, castrated no less than 8042 flowers and hybridised them in a closed room; and the seeds from only seventy of these, that is considerably less than 1 per cent, produced pure or unhybridised offspring. (10/18. ‘Kenntniss’ etc. pages 555, 576.)

From the various facts now given, it is evident that most flowers are adapted in an admirable manner for cross-fertilisation. Nevertheless, the greater number likewise present structures which are manifestly adapted, though not in so striking a manner, for self-fertilisation. The chief of these is their hermaphrodite condition; that is, their including within the same corolla both the male and female reproductive organs. These often stand close together and are mature at the same time; so that pollen from the same flower cannot fail to be deposited at the proper period on the stigma. There are also various details of structure adapted for self-fertilisation. (10/19. Hermann Muller ‘Die Befruchtung’ etc. page 448.) Such structures are best shown in those curious cases discovered by Hermann Muller, in which a species exists under two forms,--one bearing conspicuous flowers fitted for cross-fertilisation, and the other smaller flowers fitted for self-fertilisation, with many parts in the latter slightly modified for this special purpose. (10/20. ‘Nature’ 1873 pages 44, 433.)

As two objects in most respects opposed, namely, cross-fertilisation and self-fertilisation, have in many cases to be gained, we can understand the co-existence in so many flowers of structures which appear at first sight unnecessarily complex and of an opposed nature. We can thus understand the great contrast in structure between cleistogene flowers, which are adapted exclusively for self-fertilisation, and ordinary flowers on the same plant, which are adapted so as to allow of at least occasional cross-fertilisation. (10/21. Fritz Muller has discovered in the animal kingdom ‘Jenaische Zeitschr.’ B. 4 page 451, a case curiously analogous to that of the plants which bear cleistogene and perfect flowers. He finds in the nests of termites in Brazil, males and females with imperfect wings, which do not leave the nests and propagate the species in a cleistogene manner, but only if a fully-developed queen after swarming does not enter the old nest. The fully-developed males and females are winged, and individuals from distinct nests can hardly fail often to intercross. In the act of swarming they are destroyed in almost infinite numbers by a host of enemies, so that a queen may often fail to enter an old nest; and then the imperfectly developed males and females propagate and keep up the stock.) The former are always minute, completely closed, with their petals more or less rudimentary and never brightly coloured; they never secrete nectar, never are odoriferous, have very small anthers which produce only a few grains of pollen, and their stigmas are but little developed. Bearing in mind that some flowers are cross-fertilised by the wind (called anemophilous by Delpino), and others by insects (called entomophilous), we can further understand, as was pointed out by me several years ago, the great contrast in appearance between these two classes of flowers. (10/22. ‘Journal of the Linnean Society’ volume 7 Botany 1863 page 77.) Anemophilous flowers resemble in many respects cleistogene flowers, but differ widely in not being closed, in producing an extraordinary amount of pollen which is always incoherent, and in the stigma often being largely developed or plumose. We certainly owe the beauty and odour of our flowers and the storage of a large supply of honey to the existence of insects.

ON THE RELATION BETWEEN THE STRUCTURE AND CONSPICUOUSNESS OF FLOWERS, THE VISITS OF INSECTS, AND THE ADVANTAGES OF CROSS-FERTILISATION.

It has already been shown that there is no close relation between the number of seeds produced by flowers when crossed and self-fertilised, and the degree to which their offspring are aaffected by the two processes. I have also given reasons for believing that the inefficiency of a plant’s own pollen is in most cases an incidental result, or has not been specially acquired for the sake of preventing self-fertilisation. On the other hand, there can hardly be a doubt that dichogamy, which prevails according to Hildebrand in the greater number of species (10/23. ‘Die Geschlecter Vertheiling’ etc. page 32.),--that the heterostyled condition of certain plants,--and that many mechanical structures--have all been acquired so as both to check self-fertilisation and to favour cross-fertilisation. The means for favouring cross-fertilisation must have been acquired before those which prevent self-fertilisation; as it would manifestly be injurious to a plant that its stigma should fail to receive its own pollen, unless it had already become well adapted for receiving pollen from another individual. It should be observed that many plants still possess a high power of self-fertilisation, although their flowers are excellently constructed for cross-fertilisation--for instance, those of many papilionaceous species.

It may be admitted as almost certain that some structures, such as a narrow elongated nectary, or a long tubular corolla, have been developed in order that certain kinds of insects alone should obtain the nectar. These insects would thus find a store of nectar preserved from the attacks of other insects; and they would thus be led to visit frequently such flowers and to carry pollen from one to the other. (10/24. See the interesting discussion on this subject by Hermann Muller, ‘Die Befruchtung’ etc. page 431.) It might perhaps have been expected that plants having their flowers thus peculiarly constructed would profit in a greater degree by being crossed, than ordinary or simple flowers; but this does not seem to hold good. Thus Tropaeolum minus has a long nectary and an irregular corolla, whilst Limnanthes douglasii has a regular flower and no proper nectary, yet the crossed seedlings of both species are to the self-fertilised in height as 100 to 79. Salvia coccinea has an irregular corolla, with a curious apparatus by which insects depress the stamens, while the flowers of Ipomoea are regular; and the crossed seedlings of the former are in height to the self-fertilised as 100 to 76, whilst those of the Ipomoea are as 100 to 77. Fagopyrum is dimorphic, and Anagallis collina is non-dimorphic, and the crossed seedlings of both are in height to the self-fertilised as 100 to 69.

With all European plants, excepting the comparatively rare anemophilous kinds, the possibility of distinct individuals intercrossing depends on the visits of insects; and Hermann Muller has proved by his valuable observations, that large conspicuous flowers are visited much more frequently and by many more kinds of insects, than are small inconspicuous flowers. He further remarks that the flowers which are rarely visited must be capable of self-fertilisation, otherwise they would quickly become extinct. (10/25. ‘Die Befruchtung’ etc. page 426. ‘Nature’ 1873 page 433.) There is, however, some liability to error in forming a judgment on this head, from the extreme difficulty of ascertaining whether flowers which are rarely or never visited during the day (as in the above given case of Fumaria capreolata) are not visited by small nocturnal Lepidoptera, which are known to be strongly attracted by sugar. (10/26. In answer to a question by me, the editor of an entomological journal writes--“The Depressariae, as is notorious to every collector of Noctuae, come very freely to sugar, and no doubt naturally visit flowers:” the ‘Entomologists’ Weekly Intelligencer’ 1860 page 103.) The two lists given in the early part of this chapter support Muller’s conclusion that small and inconspicuous flowers are completely self-fertile: for only eight or nine out of the 125 species in the two lists come under this head, and all of these were proved to be highly fertile when insects were excluded. The singularly inconspicuous flowers of the Fly Ophrys (O. muscifera), as I have elsewhere shown, are rarely visited by insects; and it is a strange instance of imperfection, in contradiction to the above rule, that these flowers are not self-fertile, so that a large proportion of them do not produce seeds. The converse of the rule that plants bearing small and inconspicuous flowers are self-fertile, namely, that plants with large and conspicuous flowers are self-sterile, is far from true, as may be seen in our second list of spontaneously self-fertile species; for this list includes such species as Ipomoea purpurea, Adonis aestivalis, Verbascum thapsus, Pisum sativum, Lathyrus odoratus, some species of Papaver and of Nymphaea, and others.

The rarity of the visits of insects to small flowers, does not depend altogether on their inconspicuousness, but likewise on the absence of some sufficient attraction; for the flowers of Trifolium arvense are extremely small, yet are incessantly visited by hive and humble-bees, as are the small and dingy flowers of the asparagus. The flowers of Linaria cymbalaria are small and not very conspicuous, yet at the proper time they are freely visited by hive-bees. I may add that, according to Mr. Bennett, there is another and quite distinct class of plants which cannot be much frequented by insects, as they flower either exclusively or often during the winter, and these seem adapted for self-fertilisation, as they shed their pollen before the flowers expand. (10/27. ‘Nature’ 1869 page 11.)

That many flowers have been rendered conspicuous for the sake of guiding insects to them is highly probable or almost certain; but it may be asked, have other flowers been rendered inconspicuous so that they may not be frequently visited, or have they merely retained a former and primitive condition? If a plant were much reduced in size, so probably would be the flowers through correlated growth, and this may possibly account for some cases; but the size and colour of the corolla are both extremely variable characters, and it can hardly be doubted that if large and brightly-coloured flowers were advantageous to any species, these could be acquired through natural selection within a moderate lapse of time, as indeed we see with most alpine plants. Papilionaceous flowers are manifestly constructed in relation to the visits of insects, and it seems improbable, from the usual character of the group, that the progenitors of the genera Vicia and Trifolium produced such minute and unattractive flowers as those of V. hirsuta and T. procumbens. We are thus led to infer that some plants either have not had their flowers increased in size, or have actually had them reduced and purposely rendered inconspicuous, so that they are now but little visited by insects. In either case they must also have acquired or retained a high degree of self-fertility.

If it became from any cause advantageous to a species to have its capacity for self-fertilisation increased, there is little difficulty in believing that this could readily be effected; for three cases of plants varying in such a manner as to be more fertile with their own pollen than they originally were, occurred in the course of my few experiments, namely, with Mimulus, Ipomoea, and Nicotiana. Nor is there any reason to doubt that many kinds of plants are capable under favourable circumstances of propagating themselves for very many generations by self-fertilisation. This is the case with the varieties of Pisum sativum and of Lathyrus odoratus which are cultivated in England, and with Ophrys apifera and some other plants in a state of nature. Nevertheless, most or all of these plants retain structures in an efficient state which cannot be of the least use excepting for cross-fertilisation. We have also seen reason to suspect that self-fertilisation is in some peculiar manner beneficial to certain plants; but if this be really the case, the benefit thus derived is far more than counter-balanced by a cross with a fresh stock or with a slightly different variety.

Notwithstanding the several considerations just advanced, it seems to me highly improbable that plants bearing small and inconspicuous flowers have been or should continue to be subjected to self-fertilisation for a long series of generations. I think so, not from the evil which manifestly follows from self-fertilisation, in many cases even in the first generation, as with Viola tricolor, Sarothamnus, Nemophila, Cyclamen, etc.; nor from the probability of the evil increasing after several generations, for on this latter head I have not sufficient evidence, owing to the manner in which my experiments were conducted. But if plants bearing small and inconspicuous flowers were not occasionally intercrossed, and did not profit by the process, all their flowers would probably have been rendered cleistogene, as they would thus have largely benefited by having to produce only a small quantity of safely-protected pollen. In coming to this conclusion, I have been guided by the frequency with which plants belonging to distinct orders have been rendered cleistogene. But I can hear of no instance of a species with all its flowers rendered permanently cleistogene. Leersia makes the nearest approach to this state; but as already stated, it has been known to produce perfect flowers in one part of Germany. Some other plants of the cleistogene class, for instance Aspicarpa, have failed to produce perfect flowers during several years in a hothouse; but it does not follow that they would fail to do so in their native country, any more than with Vandellia, which with me produced only cleistogene flowers during certain years. Plants belonging to this class commonly bear both kinds of flowers every season, and the perfect flowers of Viola canina yield fine capsules, but only when visited by bees. We have also seen that the seedlings of Ononis minutissima, raised from the perfect flowers fertilised with pollen from another plant, were finer than those from self-fertilised flowers; and this was likewise the case to a certain extent with Vandellia. As therefore no species which at one time bore small and inconspicuous flowers has had all its flowers rendered cleistogene, I must believe that plants now bearing small and inconspicuous flowers profit by their still remaining open, so as to be occasionally intercrossed by insects. It has been one of the greatest oversights in my work that I did not experimentise on such flowers, owing to the difficulty of fertilising them, and to my not having seen the importance of the subject. (10/28. Some of the species of Solanum would be good ones for such experiments, for they are said by Hermann Muller ‘Befruchtung’ page 434, to be unattractive to insects from not secreting nectar, not producing much pollen, and not being very conspicuous. Hence probably it is that, according to Verlot ‘Production des Varieties’ 1865 page 72, the varieties of “les aubergines et les tomates” (species of Solanum) do not intercross when they are cultivated near together; but it should be remembered that these are not endemic species. On the other hand, the flowers of the common potato (S. tuberosum), though they do not secrete nectar Kurr ‘Bedeutung der Nektarien’ 1833 page 40, yet cannot be considered as inconspicuous, and they are sometimes visited by diptera (Muller), and, as I have seen, by humble-bees. Tinzmann (as quoted in ‘Gardeners’ Chronicle’ 1846 page 183, found that some of the varieties did not bear seed when fertilised with pollen from the same variety, but were fertile with that from another variety.)

It should be remembered that in two of the cases in which highly self-fertile varieties appeared amongst my experimental plants, namely, with Mimulus and Nicotiana, such varieties were greatly benefited by a cross with a fresh stock or with a slightly different variety; and this likewise was the case with the cultivated varieties of Pisum sativum and Lathyrus odoratus, which have been long propagated by self-fertilisation. Therefore until the contrary is distinctly proved, I must believe that as a general rule small and inconspicuous flowers are occasionally intercrossed by insects; and that after long-continued self-fertilisation, if they are crossed with pollen brought from a plant growing under somewhat different conditions, or descended from one thus growing, their offspring would profit greatly. It cannot be admitted, under our present state of knowledge, that self-fertilisation continued during many successive generations is ever the most beneficial method of reproduction.

THE MEANS WHICH FAVOUR OR ENSURE FLOWERS BEING FERTILISED WITH POLLEN FROM A DISTINCT PLANT.

We have seen in four cases that seedlings raised from a cross between flowers on the same plant, even on plants appearing distinct from having been propagated by stolons or cuttings, were not superior to seedlings from self-fertilised flowers; and in a fifth case (Digitalis) superior only in a slight degree. Therefore we might expect that with plants growing in a state of nature a cross between the flowers on distinct individuals, and not merely between the flowers on the same plant, would generally or often be effected by some means. The fact of bees and of some Diptera visiting the flowers of the same species as long as they can, instead of promiscuously visiting various species, favours the intercrossing of distinct plants. On the other hand, insects usually search a large number of flowers on the same plant before they fly to another, and this is opposed to cross-fertilisation. The extraordinary number of flowers which bees are able to search within a very short space of time, as will be shown in a future chapter, increases the chance of cross-fertilisation; as does the fact that they are not able to perceive without entering a flower whether other bees have exhausted the nectar. For instance, Hermann Muller found that four-fifths of the flowers of Lamium album which a humble-bee visited had been already exhausted of their nectar. (10/29. ‘Die Befruchtung’ etc. page 311.) In order that distinct plants should be intercrossed, it is of course indispensable that two or more individuals should grow near one another; and this is generally the case. Thus A. de Candolle remarks that in ascending a mountain the individuals of the same species do not commonly disappear near its upper limit quite gradually, but rather abruptly. This fact can hardly be explained by the nature of the conditions, as these graduate away in an insensible manner, and it probably depends in large part on vigorous seedlings being produced only as high up the mountain as many individuals can subsist together.

With respect to dioecious plants, distinct individuals must always fertilise each other. With monoecious plants, as pollen has to be carried from flower to flower, there will always be a good chance of its being carried from plant to plant. Delpino has also observed the curious fact that certain individuals of the monoecious walnut (Juglans regia) are proterandrous, and others proterogynous, and these will reciprocally fertilise each other. (10/30. ‘Ult. Osservazioni’ etc. part 2 fasc 2 page 337.) So it is with the common nut (Corylus avellana) (10/31. ‘Nature’ 1875 page 26.), and, what is more surprising, with some few hermaphrodite plants, as observed by Hermann Muller. (10/32. ‘Die Befruchtung’ etc. pages 285, 339.) These latter plants cannot fail to act on each other like dimorphic or trimorphic species, in which the union of two individuals is necessary for full and normal fertility. With ordinary hermaphrodite species, the expansion of only a few flowers at the same time is one of the simplest means for favouring the intercrossing of distinct individuals; but this would render the plants less conspicuous to insects, unless the flowers were of large size, as in the case of several bulbous plants. Kerner thinks that it is for this object that the Australian Villarsia parnassifolia produces daily only a single flower. (10/33. ‘Die Schutzmittel’ etc page 23.) Mr. Cheeseman also remarks, that as certain Orchids in New Zealand which require insect-aid for their fertilisation bear only a single flower, distinct plants cannot fail to intercross. (10/34. ‘Transactions of the New Zealand Institute’ volume 5 1873 page 356.)

Dichogamy, which prevails so extensively throughout the vegetable kingdom, much increases the chance of distinct individuals intercrossing. With proterandrous species, which are far more ccommon than proterogynous, the young flowers are exclusively male in function, and the older ones exclusively female; and as bees habitually alight low down on the spikes of flowers in order to crawl upwards, they get dusted with pollen from the uppermost flowers, which they carry to the stigmas of the lower and older flowers on the next spike which they visit. The degree to which distinct plants will thus be intercrossed depends on the number of spikes in full flower at the same time on the same plant. With proterogynous flowers and with depending racemes, the manner in which insects visit the flowers ought to be reversed in order that distinct plants should be intercrossed. But this whole subject requires further investigation, as the great importance of crosses between distinct individuals, instead of merely between distinct flowers, has hitherto been hardly recognised.

In some few cases the special movements of certain organs almost ensure pollen being carried from plant to plant. Thus with many orchids, the pollen-masses after becoming attached to the head or proboscis of an insect do not move into the proper position for striking the stigma, until ample time has elapsed for the insect to fly to another plant. With Spiranthes autumnalis, the pollen-masses cannot be applied to the stigma until the labellum and rostellum have moved apart, and this movement is very slow. (10/35. ‘The Various Contrivances by which British and Foreign Orchids are fertilised’ first edition page 128.) With Posoqueria fragrans (one of the Rubiaceae) the same end is gained by the movement of a specially constructed stamen, as described by Fritz Muller.

We now come to a far more general and therefore more important means by which the mutual fertilisation of distinct plants is effected, namely, the fertilising power of pollen from another variety or individual being greater than that of a plant’s own pollen. The simplest and best known case of prepotent action in pollen, though it does not bear directly on our present subject, is that of a plant’s own pollen over that from a distinct species. If pollen from a distinct species be placed on the stigma of a castrated flower, and then after the interval of several hours, pollen from the same species be placed on the stigma, the effects of the former are wholly obliterated, excepting in some rare cases. If two varieties are treated in the same manner, the result is analogous, though of directly opposite nature; for pollen from any other variety is often or generally prepotent over that from the same flower. I will give some instances: the pollen of Mimulus luteus regularly falls on the stigma of its own flower, for the plant is highly fertile when insects are excluded. Now several flowers on a remarkably constant whitish variety were fertilised without being castrated with pollen from a yellowish variety; and of the twenty-eight seedlings thus raised, every one bore yellowish flowers, so that the pollen of the yellow variety completely overwhelmed that of the mother-plant. Again, Iberis umbellata is spontaneously self-fertile, and I saw an abundance of pollen from their own flowers on the stigmas; nevertheless, of thirty seedlings raised from non-castrated flowers of a crimson variety crossed with pollen from a pink variety, twenty-four bore pink flowers, like those of the male or pollen-bearing parent.

In these two cases flowers were fertilised with pollen from a distinct variety, and this was shown to be prepotent by the character of the offspring. Nearly similar results often follow when two or more self-fertile varieties are allowed to grow near one another and are visited by insects. The common cabbage produces a large number of flowers on the same stalk, and when insects are excluded these set many capsules, moderately rich in seeds. I planted a white Kohl-rabi, a purple Kohl-rabi, a Portsmouth broccoli, a Brussels sprout, and a Sugar-loaf cabbage near together and left them uncovered. Seeds collected from each kind were sown in separate beds; and the majority of the seedlings in all five beds were mongrelised in the most complicated manner, some taking more after one variety, and some after another. The effects of the Kohl-rabi were particularly plain in the enlarged stems of many of the seedlings. Altogether 233 plants were raised, of which 155 were mongrelised in the plainest manner, and of the remaining 78 not half were absolutely pure. I repeated the experiment by planting near together two varieties of cabbage with purple-green and white-green lacinated leaves; and of the 325 seedlings raised from the purple-green variety, 165 had white-green and 160 purple-green leaves. Of the 466 seedlings raised from the white-green variety, 220 had purple-green and 246 white-green leaves. These cases show how largely pollen from a neighbouring variety of the cabbage effaces the action of the plant’s own pollen. We should bear in mind that pollen must be carried by the bees from flower to flower on the same large branching stem much more abundantly than from plant to plant; and in the case of plants the flowers of which are in some degree dichogamous, those on the same stem would be of different ages, and would thus be as ready for mutual fertilisation as the flowers on distinct plants, were it not for the prepotency of pollen from another variety. (10/36. A writer in the ‘Gardeners’ Chronicle’ 1855 page 730, says that he planted a bed of turnips (Brassica rapa) and of rape (B. napus) close together, and sowed the seeds of the former. The result was that scarcely one seedling was true to its kind, and several closely resembled rape.)

Several varieties of the radish (Raphanus sativus), which is moderately self-fertile when insects are excluded, were in flower at the same time in my garden. Seed was collected from one of them, and out of twenty-two seedlings thus raised only twelve were true to their kind. (10/37. Duhamel as quoted by Godron ‘De l’Espece’ tome 2 page 50, makes an analogous statement with respect to this plant.)

The onion produces a large number of flowers, all crowded together into a large globular head, each flower having six stamens; so that the stigmas receive plenty of pollen from their own and the adjoining anthers. Consequently the plant is fairly self-fertile when protected from insects. A blood-red, silver, globe and Spanish onion were planted near together; and seedlings were raised from each kind in four separate beds. In all the beds mongrels of various kinds were numerous, except amongst the ten seedlings from the blood-red onion, which included only two. Altogether forty-six seedlings were raised, of which thirty-one had been plainly crossed.

A similar result is known to follow with the varieties of many other plants, if allowed to flower near together: I refer here only to species which are capable of fertilising themselves, for if this be not the case, they would of course be liable to be crossed by any other variety growing near. Horticulturists do not commonly distinguish between the effects of variability and intercrossing; but I have collected evidence on the natural crossing of varieties of the tulip, hyacinth, anemone, ranunculus, strawberry, Leptosiphon androsaceus, orange, rhododendron and rhubarb, all of which plants I believe to be self-fertile. (10/38. With respect to tulips and some other flowers, see Godron ‘De l’Espece’ tome 1 page 252. For anemones ‘Gardeners’ Chronicle’ 1859 page 98. For strawberries see Herbert in ‘Transactions of the Horticultural Society’