Chapter 18

Unfortunately the practical value of these calculations is not very great. They indicate the size of the cultures required to get all the possible combinations, and show that in ordinary cases many thousands of individuals have to be cultivated, in order to exhaust the whole range of possibilities. They further show that among all these thousands, only very few are constant in all their characters; in fact, it may easily be seen that with seven differentiating points among the 16,384 named above, only one individual will have all the seven qualities in pure active, and only one will have them all in a purely dormant condition. Then there will be some with some attributes active and others latent, but their numbers will also be very small. All others will split up in the succeeding generation in regard to one or more of their apparently active marks. And since only in very rare cases the stable hybrids can be distinguished by external characters from the unstable ones, the stability of each individual bearing a desired combination of characters would have to be established by experiment [306] after pure fertilization. Mendel's law teaches us to predict the difficulties, but hardly shows any way to avoid them. It lays great stress on the old prescript of isolation and pure fertilization, but it will have to be worked out and applied to a large number of practical cases before it will gain a preeminent influence in horticultural practice.

Or, as Bailey states it, we are only beginning to find a pathway through the bewildering maze of hybridization.

This pathway is to be laid out with regard to the following considerations. We are not to cross species or varieties, or even accidental plants. We must cross unit-characters, and consider the plants only as the bearers of these units. We may assume that these units are represented in the hereditary substance of the cell-nucleus by definite bodies of too small a size to be seen, but constituting together the chromosomes. We may call these innermost representatives of the unit-characters pangenes, in accordance with Darwin's hypothesis of pangenesis, or give them any other name, or we may even wholly abstain from such theoretical discussion, and limit ourselves to the conception of the visible character-units. These units then may be present, or lacking and in the first case active, or latent.

[307] True elementary species differ from each other in a number of unit-characters, which do not contrast. They have arisen by progressive mutation. One species has one kind of unit, another species has another kind. On combining these, there can be no interchange. Mendelism assumes such an interchange between units of the same character, but in a different condition. Activity and latency are such conditions, and therefore Mendel's law obviously applies to them. They require pairs of antagonistic qualities, and have no connection whatever with those qualities, which do not find an opponent in the other parent. Now, only pure varieties afford such pure conditions. When undergoing further modifications, some of them may be in the progressive line and others in the retrogressive. Progressive modifications give new units, which are not in contrast with any other, retrograde changes turn active units into the latent condition and so give rise to pairs. Ordinary species generally originate in this way, and hence differ from each other partly in specific, partly in varietal characters. As to the first, they give in their hybrids stable peculiarities, while as to the latter, they split up according to Mendel's law.

Unpaired or unbalanced characters lie side by side with paired or balanced qualities, and they [308] do so in nearly all the crosses made for practical purposes, and in very many scientific experiments. Even Mendel's peas were not pure in this respect, much less do the campions noted above differ only in Mendelian characters.

Comparative and systematic studies must be made to ascertain the true nature of every unit in every single plant, and crossing experiments must be based on these distinctions in order to decide what laws are applicable in any case.

[309] D. EVER-SPORTING VARIETIES

LECTURE XI

STRIPED FLOWERS

Terminology is an awkward thing. It is as disagreeable to be compelled to make new names, as to be constrained to use the old faulty ones. Different readers may associate different ideas with the same terms, and unfortunately this is the case with much of the terminology of the science of heredity and variability. What are species and what are varieties? How many different conceptions are conveyed by the terms constancy and variability? We are compelled to use them, but we are not at all sure that we are rightly understood when we do so.

Gradually new terms arise and make their way. They have a more limited applicability than the old ones, and are more narrowly circumscribed. They are not to supplant the older terms, but permit their use in a more general way.

[310] One of these doubtful terms is the word _sport_. It often means bud-variation, while in other cases it conveys the same idea as the old botanical term of mutation. But then all sorts of seemingly sudden variations are occasionally designated by the same term by one writer or another, and even accidental anomalies, such as teratological ascidia, are often said to arise by sports.

If we compare all these different conceptions, we will find that their most general feature is the suddenness and the rarity of the phenomenon. They convey the idea of something unexpected, something not always or not regularly occurring. But even this demarcation is not universal, and there are processes that are regularly repeated and nevertheless are called sports. These at least should be designated by another name.

In order to avoid confusion as far as possible, with the least change in existing terminology, I shall use the term "ever-sporting varieties" for such forms as are regularly propagated by seed, and of pure and not hybrid origin, but which sport in nearly every generation. The term is a new one, but the facts are for the most part new, and require to be considered in a new light. Its meaning will become clearer at once when the illustrations afforded by [311] striped flowers are introduced. In the following discussion it will be found most convenient to give a summary of what is known concerning them, and follow this by a consideration of the detailed evidence obtained experimentally, which supports the usage cited.

The striped variety of the larkspur of our gardens is known to produce monochromatic flowers, in addition to striped ones. They may be borne by the same racemes, or on different branches, or some seedlings from the same parent-plant may bear monochromatic flowers while others may be striped. Such deviations are usually called sports. But they occur yearly and regularly and may be observed invariably when the cultures are large enough. Such a variety I shall call "ever-sporting."

The striped larkspur is one of the oldest garden varieties. It has kept its capacity of sporting through centuries, and therefore may in some sense be said to be quite stable. Its changes are limited to a rather narrow circle, and this circle is as constant as the peculiarities of any other constant species or variety. But within this circle it is always changing from small stripes to broad streaks, and from them to pure colors. Here the variability is a thing of absolute constancy, while the constancy consists in eternal changes. Such apparent [312] contradictions are unavoidable, when we apply the old term to such unusual though not at all new cases. Combining the stability and the qualities of sports in one word, we may evidently best express it by the new term of eversporting variety.

We will now discuss the exact nature of such varieties, and of the laws of heredity which govern them. But before doing so, I might point out, that this new type is a very common one. It embraces most of the so-called variable types in horticulture, and besides these a wide range of anomalies.

Every ever-sporting variety has at least two different types, around and between which it varies in numerous grades, but to which it is absolutely limited. Variegated leaves fluctuate between green and white, or green and yellow, and display these colors in nearly all possible patterns. But there variability ends, and even the patterns are ordinarily narrowly prescribed in the single varieties. Double flowers afford a similar instance. On one side the single type, on the other the nearly wholly double model are the extreme limits, between which the variability is confined. So it is also with monstrosities. The race consists of anomalous and normal individuals, and displays between them all possible combinations of normal and monstrous [313] parts. But its variability is restricted to this group. And large as the group may seem on first inspection, it is in reality very narrow. Many monstrosities, such as fasciated branches, pitchers, split leaves, peloric flowers, and others constitute such ever-sporting varieties, repeating their anomalies year by year and generation after generation, changing as much as possible, but remaining absolutely true within their limits as long as the variety exists.

It must be a very curious combination of the unit-characters which causes such a state of continuous variability. The pure quality of the species must be combined with the peculiarity of the variety in such a way, that the one excludes the other, or modifies it to some extent, although both never fully display themselves in the same part of the same plant. A corolla cannot be at once monochromatic and striped, nor can the same part of a stem be twisted and straight. But neighboring organs may show the opposite attributes side by side.

In order to look closer into the real mechanism of this form of variability, and of this constant tendency to occasional reversions, it will be best to limit ourselves first to a single case, and to try to gather all the evidence, which can be obtained by an examination of the hereditary relations of its sundry constituents.

[314] This may best be done by determining the degree of inheritance for the various constituents of the race during a series of years. It is only necessary to apply the two precautions of excluding all cross-fertilization, and of gathering the seeds of each individual separately. We do not need to ascertain whether the variety as such is permanent; this is already clear from the simple fact of its antiquity in so many cases. We wish to learn what part each individual, or each group of individuals with similar characters, play in the common line of inheritance. In other words, we must build up a genealogical tree, embracing several generations and a complete set of the single cases occurring within the variety, in order to allow of its being considered as a part of the genealogy of the whole. It should convey to us an idea of the hereditary relations during the life-time of the variety.

It is manifest that the construction of such a genealogical tree requires a number of separate experiments. These should be extended over a series of years. Each should include a number of individuals large enough to allow the determination of the proportion of the different types among the offspring of a single plant. A species which is easily fertilized by its own pollen, and which bears capsules with [315] large quantities of seeds, obviously affords the best opportunities. As such, I have chosen the common snapdragon of the gardens, _Antirrhinum majus_. It has many striped varieties, some tall, others of middle height, or of dwarfed stature. In some the ground-color of the flowers is yellow, in others it is white, the yellow disappearing, with the exception of a large mark in the throat. On these ground-colors the red pigment is seen lying in streaks of pure carmine, with white intervals where the yellow fails, but combined with yellow to make a fiery red, and with yellow intervals when that color is present. This yellow color is quite constant and does not vary in any marked degree, notwithstanding the fact that it seems to make narrower and broader stripes, according to the parts of the corolla left free by the red pigment. But it is easily seen that this appearance is only a fallacious one.

The variety of snapdragon chosen was of medium height and with the yellow ground-color, and is known by horticulturists as _A. majus luteum rubro-striatum_. As the yellow tinge showed itself to be invariable; I may limit my description to the red stripes.

Some flowers of this race are striped, others are not. On a hasty survey there seem to be three types, pure yellow, pure red, and stripes [316] with all their intermediate links of narrower and broader, fewer and more numerous streaks. But on a close inspection one does not succeed in finding pure yellow racemes. Little lines of red may be found on nearly every flower. They are the extreme type on this side of the range of variability. From them an almost endless range of patterns passes over to the broadest stripes and even to whole sections of a pure red. But then, between these and the wholly red flowers we observe a gap, which may be narrower by the choice of numerous broad striped individuals, but which is never wholly filled up. Hence we see that the red flowers are a separate type within the striped variety.

This red type springs yearly from the striped form, and yearly reverts to it. This is what in the usual descriptions of this snapdragon, is called its sporting. The breadth of the streaks is considered to be an ordinary case of variability, but the red flowers appear suddenly, without the expected links. Therefore they are to be considered as sports. Similarly the red forms may suddenly produce striped ones, and this too is to be taken as a sport, according to the usual conception of the word.

Such sports may occur in different ways. Either by seeds, or by buds, or even within the single spikes. Both opposite reversions, [317] from striped to red and from red to stripes, occur by seed, even by the strictest exclusion of cross-fertilization. As far as my experiments go, they are the rule, and parent-plants that do not give such reversions, at least in some of their offspring, are very rare, if not wholly wanting. Bud-variations and variations within the spike I have as yet only observed on the striped individuals, and never on the red ones, though I am confident that they might appear in larger series of experiments. Both cases are more common on individuals with broad stripes than on plants bearing only the narrower red lines, as might be expected, but even on the almost purely yellow individuals they may be seen from time to time. Bud-variations produce branches with spikes of uniform red flowers. Every bud of the plant seems to have equal chances to be transformed in this way. Some striped racemes bear a few red flowers, which ordinarily are inserted on one side of the spike only. As they often cover a sharply defined section of the raceme, this circumstance has given rise to the term of sectional variability to cover such cases. Sometimes the section is demarcated on the axis of the flower-spike by a brownish or reddish color, sharply contrasting with the green hue of the remaining parts. Sectional variation may be looked at as a [318] special type of bud-variation, and from this point of view we may simplify our inquiry and limit ourselves to the inheritance of three types, the striped plants, the red plants and the red asexual variants of the striped individuals. In each case the heredity should be observed not only for one, but at least for two successive generations.

Leaving these introductory remarks I now come at once to the genealogical tree, as it may be deduced from my experiments:

Year 1896 95% Striped 84% Red | | 1895 Striped Individual Red Indiv. \ / 1895 98% Striped 71% Red | | 1894 Striped branches. Red branches. \ / 1894 98% Striped 76% Red | | 1893 90% Striped Indiv. 10% Red Indiv. \ / 1892 Striped Individual

This experiment was begun in the year 1892 with one individual out of a large lot of striped plants grown from seeds which I had purchased from a firm in Erfurt. The capsules were gathered separately from this individual and about 40 flowering plants were obtained from the seeds in the following year. Most of them had neatly striped flowers, some displayed broader stripes and spare flowers were seen with one [319] half wholly red. Four individuals were found with only uniform red flowers. These were isolated and artificially pollinated, and the same was done with some of the best striped individuals. The seeds from every parent were sown separately, so as to allow the determination of the proportion of uniform red individuals in the progeny.

Neither group was constant in its offspring. But as might be expected, the type of the parent plant prevailed in both groups, and more strongly so in the instances with the striped, than with the red ones. Or, in other words seed-reversions were more numerous among the already reverted reds than among the striped type itself. I counted 2% reversion in the latter case, but 24% from the red parents.

Among the striped plants from the striped parents, I found some that produced bud variations. I succeeded in isolating these red flowering branches in paper bags and in pollinating them with their own pollen, and subjected the striped spikes of the same individuals to a similar treatment. Three individuals gave a sufficient harvest from both types, and these six lots of seeds were sown separately. The striped flowers repeated their character in 98% of their offspring, the red twigs in only 71%, the [320] remaining individuals sporting into the opposite group.

In the following year I continued the experiment with the seeds of the offspring of the red bud-variations. The striped individuals gave 95%, but in the red ones only 84% of the progeny remained true to the parent type.

From these figures it is manifest that the red and striped types differ from one another not only in their visible attributes, but also in the degree of their heredity. The striped individuals repeat their peculiarity in 90-98% of their progeny, 2-10% sporting into the uniform red color. On the other hand the red individuals are constant in 71-84% of their offspring, while 16-29% go over to the striped type. Or, briefly, both types are inherited to a high degree, but the striped type is more strictly inherited than the red one.

Moreover the figures show that the degree of inheritance is not contingent upon the question as to how the sport may have arisen. Bud-sports show the same degree of inheritance as seed-sports. Sexual and asexual variability therefore seem to be one and the same process in this instance. But the deeper meaning of this and other special features of our genealogical tree are still awaiting further investigation. It seems that much important evidence might [321] come from an extension of this line of work. Perhaps it might even throw some light on the intimate nature of the bud-variations of ever-sporting varieties in general. Sectional variations remain to be tested as to the degree of inheritance exhibited, and the different occurrences as to the breadth of the streaks require similar treatment.

In ordinary horticultural practice it is desirable to give some guarantee as to what may be expected to come from the seeds of brightly striped flowers. Neither the pure red type, nor the nearly yellow racemes are the object of the culture, as both of them may be had pure from their, own separate varieties. In order to insure proper striping, both extremes are usually rejected and should be rooted out as soon as the flowering period begins. Similarly the broad-striped ones should be rejected, as they give a too large amount of uniform red flowers. Clearly, but not broadly striped individuals always yield the most reliable seed.

Summing up once more the results of our pedigree-experiment, we may assert that the striped variety of the snapdragon is wholly permanent, including the two opposite types of uniform color and of stripes. It must have been so since it first originated from the invariable uniform [322] varieties, about the middle of the last century, in the nursery of Messrs. Vilmorin, and probably it will remain so as long as popular taste supports its cultivation. It has never been observed to transgress its limits or to sport into varieties without reversions or sports. It fluctuates from one extreme to the other yearly, always recurring in the following year, or even in the same summer by single buds. Highly variable within its limits, it is absolutely constant or permanent, when considered as a definite group.

Similar cases occur not rarely among cultivated plants. In the wild state they seem to be wholly wanting. Neither are they met with as occasional anomalies nor as distinct varieties. On the contrary, many garden-flowers that are colored in the species, and besides this have a white or yellow variety, have also striped sorts. The oldest instance is probably the marvel of Peru, _Mirabilis Jalappa_, which already had more than one striped variety at the time of its introduction from Peru into the European gardens, about the beginning of the seventeenth century. Stocks, liver-leaf (_Hepatica_), dame's violet (_Hesperis_), Sweet William (_Dianthus barbatus_), and periwinkles (_Vinca minor_) seem to be in the same condition, as their striped varieties were already quoted [323] by the writers of the same century. Tulips, hyacinths, _Cyclamen_, _Azalea_, _Camellia_, and even such types of garden-plants as the meadow crane's-bill (_Geranium pratensev) have striped varieties. It is always the red or blue color which occurs in stripes, the underlying ground being white or yellow, according to the presence or absence of the yellow in the original color mixture.

All these varieties are known to be permanent, coming true during long series of successive generations. But very little is known concerning the more minute details of their hereditary qualities. They come from seed, when this is taken from striped individuals, and thence revert from time to time to the corresponding monochromatic type. But whether they would do so when self-fertilized, and whether the reversionary individuals are always bound to return towards the center of the group or towards the opposite limit, remains to be investigated. Presumably there is nowhere a real transgression of the limits, and never or only very rarely and at long intervals of time a true production of another race with other hereditary qualities.

In order to satisfy myself on these points, I made some pedigree-cultures with the striped forms of dame's violet (_Hesperis matronalis_) [324] and of _Clarkia pulchella_. Both of them are ever-sporting varieties. The experiments were conducted during five generations with the violet, and during four with the striped Clarkia, including the progeny of the striped and of the monochromatic red offspring of a primitive striped plant. I need not give the figures here for the numerical relations between the different types of each group, and shall limit myself to the statement that they behaved in exactly the same manner as the snapdragon.