Species and Varieties, Their Origin by Mutation Part 23

Returning now to the seeds of this pair of plants, I had, of course, not the least occasion to ascribe to it any higher value than the harvest of former years. The consequence was that I had no reason to make large sowings, and grew only enough young plants to have about 50 in bloom in the summer of 1894. Among [473] these, stray peloric flowers were observed in somewhat larger number than in the previous generations, 11 plants bearing one or two, or even three such abnormalities. This however, could not be considered as a real advance, since such plants may occur in varying, though ordinarily small numbers in every generation.

Besides them a single plant was seen to bear only peloric flowers; it produced racemes on several stems and their branches. All were peloric without exception. I kept it through the winter, taking care to preserve a complete isolation of its roots. The other plants were wholly destroyed. Such annihilation must include both the stems and roots and the latter of course requires considerable labor. The following year, however, gave proof of the success of the operation, since my plant bloomed luxuriously for the second time and remained true to the type of the first year, producing peloric flowers exclusively.

Here we have the first experimental mutation of a normal into a peloric race. Two facts were clear and simple. The ancestry was known for over a period of four generations, living under the ordinary care and conditions of an experimental garden, isolated from other toad-flaxes, but freely fertilized by bees or at times by myself. This ancestry was quite constant as to [474] the peloric peculiarity, remaining true to the wild type as it occurs everywhere in my country, and showing in no respect any tendency to the production of a new variety.

The mutation took place at once. It was a sudden leap from the normal plants with very rare peloric flowers to a type exclusively peloric. No intermediate steps were observed. The parents themselves had borne thousands of flowers during two summers, and these were inspected nearly every day, in the hope of finding some pelories and of saving their seed separately. Only one such flower was seen. If there had been more, say a few in every hundred flowers, it might be allowable to consider them as previous stages, showing a preparation of the impending change. But nothing of this kind was observed. There was simply no visible preparation for the sudden leap.

This leap, on the other hand, was full and complete. No reminiscence of the former condition remained. Not a single flower on the mutated plant reverted to the previous type. All were thoroughly affected by the new attribute, and showed the abnormally augmented number of spurs, the tubular structure of the corolla and the round and narrow entrance of its throat. The whole plant departed absolutely from the old type of its progenitors.

[475] Three ways were open to continue my experiment. The first was indicated by the abundant harvest from the parent-plants of the mutation. It seemed possible to compare the numerical proportion of the mutated seeds with those of normal plants. In order to ascertain this proportion I sowed the greatest part of my 10 cu. cm. of seed and planted some 2,000 young plants in little pots with well-manured soil. I got some 1,750 flowering plants and observed among them 16 wholly peloric individuals. The numerical proportion of the mutation was therefore in this instance to be calculated equal to about 1% of the whole crop.

This figure is of some importance. For it shows that the chance of finding mutations requires the cultivation of large groups of individuals. One plant in each hundred may mutate, and cultures of less than a hundred specimens must therefore be entirely dependent on chance for the appearance of new forms, even if such should accidentally have been produced and lay dormant in the seed. In other cases mutations may be more numerous, or on the contrary, more rare. But the chance of mutative changes in larger numbers is manifestly much reduced by this experiment, and they may be expected to form a very small proportion of the culture.

[476] The second question which arose from the above result was this.

Could the mutation be repeated? Was it to be ascribed to some latent cause which might be operative more than once? Was there some hidden tendency to mutation, which, ordinarily weak, was strengthened in my cultures by some unknown influence? Was the observed mutation to be explained by a common cause with the other cases recorded by field-observations? To answer this question I had only to continue my experiment, excluding the mutated individuals from any intercrossing with their brethren. To this end I saved the seeds from duly isolated groups in different years and sowed them at different times. For various causes I was not prepared to have large cultures from these seeds, but notwithstanding this, the mutation repeated itself. In one instance I obtained two, in another, one peloric plant with exclusively many-spurred flowers. As is easily understood, these were related as "nieces" to the first observed mutants. They originated in quite the same way, by a sudden leap, without any preparation and without any intermediate steps.

Mutation is proved by this experience to be of an iterative nature. It is the expression of some concealed condition, or as it is generally [477] called, of some hidden tendency. The real nature of this state of the hereditary qualities is as yet wholly unknown. It would not be safe to formulate further conclusions before the evidence offered by the evening-primroses is considered.

Thirdly, the question arises, whether the mutation is complete, not only as to the morphologic character, but also as to the hereditary const.i.tution of the mutated individuals. But here unfortunately the high degree of sterility of the peloric plants, as previously noted, makes the experimental evidence a thing of great difficulty. During the course of several years I isolated and planted together the peloric individuals already mentioned, all in all some twenty plants. Each individual was nearly absolutely sterile when treated with its own pollen, and the aid of insects was of no avail. I intercrossed my plants artificially, and pollinated more than a thousand flowers. Not a single one gave a normal fruit, but some small and nearly rudimentary capsules were produced, bearing a few seeds. From these I had 119 flowering plants, out of which 106 were peloric and 13 one-spurred. The great majority, some 90%, were thus shown to be true to their new type. Whether the 10% reverting ones were truly atavists, or whether they were [478] only vicinists, caused by stray pollen grains from another culture, cannot of course be decided with sufficient cert.i.tude.

Here I might refer to the observations concerning the invisible dimorphous state of the flowers of the normal toad-flax. Individuals of the same type, when fertilized with each other, are nearly, but not absolutely, sterile. The yield of seeds of my peloric plants agrees fairly well with the harvest which I have obtained from some of the nearly sterile pairs of individuals in my former trial. Hence the suggestion is forced upon us that perhaps, owing to some unknown cause, all the peloric individuals of my experiment belonged to one and the same type, and were sterile for this reason only. If this is true, then it is to be presumed that all previous investigators have met the same condition, each having at hand only one of the two required types. And this discussion has the further advantage of showing the way, in which perhaps a full and constant race of peloric toad-flaxes may be obtained.

Two individuals of different type are required to start from. They seem as yet never to have arisen from one group of mutations. But if it were possible to combine the products of two mutations obtained in different countries and under different conditions, there would be a chance [479]

that they might belong to the supposed opposite types, and thus be fertile with one another. My peloric plants are still available, and the occurrence of this form elsewhere would give material for a successful experiment. The probability thereof is enhanced by the experience that my peloric plants bear large capsules and a rich harvest of seeds when fertilized from plants of the normal one-spurred race, while they remain nearly wholly barren by artificial fertilization with others. I suppose that they are infertile with the normal toad-flaxes of their own s.e.xual disposition, but fertile with those of the opposite const.i.tution. At all events the fact that they may bear abundant seed when properly pollinated is an indication of successful experiments on the possibility of gaining a hereditary race with exclusively peloric flowers. And such a race would be a distinct gain for sundry physiologic inquiries, and perhaps not wholly dest.i.tute of value from an horticultural point of view.

Returning now to the often recorded occurrence of peloric toad-flaxes in the wild state and recalling our discussion about the improbability of a dispersion from one locality to another by seed, and the probability of independent origin for most of these cases, we are confronted with the conception that a latent [480] tendency to mutation must be universally present in the whole species. Another observation, although it is of a negative character, gains in importance from this point of view. I refer to the total lack of intermediate steps between normal and peloric individuals. If such links had ordinarily been produced previous to the purely peloric state they would no doubt have been observed from time to time. This is so much the more probable as _Linaria_ is a perennial herb, and the ancestors of a mutation might still be in a flowering condition together with their divergent offspring. But no such intermediates are on record. The peloric toad-flaxes are, as a rule, found surrounded by the normal type, but without intergrading forms.

This discontinuity has already been insisted upon by Hofmeister and others, even at the time when the theory of descent was most under discussion, and any link would surely have been produced as a proof of a slow and continuous change. But no such proof has been found, and the conclusion seems admissible that the mutation of toad-flaxes ordinarily, if not universally, takes place by a sudden step. Our experiment may simply be considered as a thoroughly controlled instance of an often recurring phenomenon. It teaches us how, in the [481] main, the peloric mutations must be a.s.sumed to proceed.

This conception may still be broadened. We may include in it all similar occurrences, in allied and other species. There is hardly a limit to the possibilities which are opened up by this experience. But it will be well to refrain from hazardous theorizing, and consider only those cases which may be regarded as exact repet.i.tions of the same phenomenon and of which our culture is one of the most recent instances on record. We will limit ourselves to the probable origin of peloric variations at large, of which little is known, but some evidence may be derived from the recorded facts. Only one case can be said to be directly a.n.a.logous to our observations.

This refers to the peloric race of the common snapdragon, or _Antirrhinum majus_ of our gardens. It is known to produce peloric races from time to time in the same way as does the toadflax. But the snapdragon is self-fertile and so is its peloric variety. Some cases are relatively old, and some of them have been recorded and in part observed by Darwin. Whence they have sprung and in what manner they were produced, seems never to have been noted. Others are of later origin, and among these one or two varieties have been accidentally produced [482] in the nursery of Mr. Chr. Lorenz in Erfurt, and are now for sale, the seeds being guaranteed to yield a large proportion of peloric individuals. The peloric form in this case appeared at once, but was not isolated, and was left free to visiting insects, which of course crossed it with the surrounding varieties. Without doubt the existence of two color-varieties of the peloric type, one of a very dark red, indicating the "Black prince" variety as the pollen-parent, and the other with a white tube of the corolla, recalling the form known as "Delila," is due to these crossings. I had last year (1903) a large lot of plants, partly normal and partly peloric, but evidently of hybrid origin, from seeds from this nursery, showing moreover all intermediate steps between nearly wholly peloric individuals and apparently normal ones. I have saved the seeds of the isolated types and before seeing the flowers of their offspring, nothing can be said about the purity and constancy of the type, when freed from hybrid admixtures. The peloric snapdragon has five small unequal spurs at the base of its long tube, and in this respect agrees with the peloric toad-flax.

Other pelories are terminal and quite regular, and occur in some species of _Linaria_, where I observed them in _Linaria dalmatica_. The [483]

terminal flowers of many branches were large and beautifully peloric, bearing five long and equal spurs. About their origin and inheritance nothing is known.

A most curious terminal pelory is that of the common foxglove or _Digitalis purpurea_. As we have seen in a previous lecture, it is an old variety. It was described and figured for the first time by Vrolik of Amsterdam, and the original specimens of his plates are still to be seen in the collections of the botanic garden of that university. Since his time it has been propagated by seed as a commercial variety, and may be easily obtained. The terminal flower of the central stem and those of the branches only are affected, all other flowers being wholly normal.

Almost always it is accompanied by other deviations, among which a marked increase of the number of the parts of the corolla and other whorls is the most striking. Likewise supernumerary petals on the outer side of the corolla, and a production of a bud in the center of the capsule may be often met with. This bud as a rule grows out after the fading away of the flower, bursting through the green carpels of the unripe fruit, and producing ordinarily a secondary raceme of flowers.

This raceme is a weak but exact repet.i.tion of the first, bearing symmetrical foxgloves all [484] along and terminating in a peloric structure. On the branches these anomalies are more or less reduced, according to the strength of the branch, and conforming to the rule of periodicity, given in our lecture on the "five-leaved" clover. Through all this diminution the peloric type remains unchanged and therefore becomes so much the purer, the weaker the branches on which it stands.

I am not sure whether such peloric flowers have ever been purely pollinated and their seed saved separately, but I have often observed that the race comes pure from the seed of the zygomorphic flowers. It is as yet doubtful whether it is a half race or a double race, and whether it might be purified and strengthened by artificial selection. Perhaps the determination of the hereditary percentage described when dealing with the tricotyls might give the clue to the acquisition of a higher specialized race. The variety is old and widely disseminated, but must be subjected to quite a number of additional experiments before it can be said to be sufficiently understood.

The most widespread peloric variety is that of _Gloxinia_. It has erect instead of drooping flowers; and with the changed position the structure is also changed. Like other pelories it has five equal stamens instead of four unequal [485] ones, and a corolla with five equal segments instead of an upper and a lower lip. It shows the peloric condition in all of its flowers and is often combined with a small increase of the number of the parts of the whorls. It is for sale under the name of _erecta_, and may be had in a wide range of color-types. It seems to be quite constant from seed.

Many other instances of peloric flowers are on record. Indian cress or _Tropaeolum majus_ loses the spur in some double varieties and with it most of its symmetrical structure; it seems to be considered justly as a peloric malformation. Other species produce such anomalies only from time to time and nothing is known about their hereditary tendency. One of the most curious instances is the terminal flower of the raceme of the common laburnum, which loses its whole papilionaceous character and becomes as regularly quinate as a common b.u.t.tercup.

Some families are more liable to pelorism than others. Obviously all the groups, the flowers of which are not symmetrical, are to be excluded.

But then we find that l.a.b.i.ates and their allies among the dicotyledonous plants, and orchids among the monocotyledonous ones are especially subjected to this alteration. In both groups many genera and a long list of species [486] could be quoted as proof. The family of the l.a.b.i.ates seems to be essentially rich in terminal pelories, as for instance in the wild sage or _Salvia_ and the dead-nettle or _Lamium_. Here the pelories have long and straight corolla-tubes, which are terminated by a whorl of four or five segments. Such forms often occur in the wild state and seem to have a geographic distribution as narrowly circ.u.mscribed as in the case of many small species. Those of the l.a.b.i.ates chiefly belong to southern Europe and are unknown at least in some parts of the other countries. On the contrary terminal pelories of _Scrophularia nodosa_ are met with from time to time in Holland. Such facts clearly point to a common origin, and as only the terminal flowers are affected by the malformation, the fertility of the whole plant is evidently not seriously infringed upon.

Before leaving the l.a.b.i.ates, we may cite a curious instance of pelorism in the toad-flax, which is quite different from the ordinary peloric variety. This latter may be considered from a morphologic standpoint to be owing to a five-fold repet.i.tion of the middle part of the underlip.

This conception would at once explain the occurrence of five spurs and of the orange border all around the corolla-tube. We might readily imagine that any other of the five [487] parts of the corolla could be repeated five-fold, in which case there would be no spur, and no orange hue on the upper corolla-ring. Such forms really occur, though they seem to be more rare than the five-spurred pelories. Very little is known about their frequency and hereditary qualities.

Orchids include a large number of peloric monstrosities and moreover a wild pelory which is systematically described not only as a separate species but even as a new genus. It bears the name of _Uropedium lindenii_, and is so closely related to _Cypripedium caudatum_ that many authors take it for the peloric variety of this plant. It occurs in the wild state in some parts of Mexico, where the _Cypripedium_ also grows.

Its claims to be a separate genus are lessened by the somewhat monstrous condition of the s.e.xual organs, which are described as quite abnormal.

But here also, intermediates are lacking, and this fact points to a sudden origin.

Many cases of pelorism afford promising material for further studies of experimental mutations. The peloric toad-flax is only the prototype of what may be expected in other cases. No opportunity should be lost to increase the as yet too scanty, evidence on this point.

[488]

LECTURE XVII

THE PRODUCTION OF DOUBLE FLOWERS

Mutations occur as often among cultivated plants as among those in the wild state. Garden flowers are known to vary markedly. Much of their variability, however, is due to hybridism, and the combination of characters previously separate has a value for the breeder nearly equal the production of really new qualities. Nevertheless there is no doubt that some new characters appear from time to time.

In a previous lecture we have seen that varietal characters have many features in common. One of them is their frequent recurrence both in the same and in other, often very distantly related, species. This recurrence is an important factor in the choice of the material for an experimental investigation of the nature of mutations.

Some varieties are reputed to occur more often and more readily than others. White-colored varieties, though so very common, seem for the most part to be of ancient date, but only few [489] have a known origin, however. Without any doubt many of them have been found in a wild state and were introduced into culture. On the other hand double flowers are exceedingly rare in the wild state, and even a slight indication of a tendency towards doubling, the stray petaloid stamens, are only rarely observed growing wild. In cultivation, however, double flowers are of frequent occurrence; hence the conclusion that they have been produced in gardens and nurseries more frequently than perhaps any other type of variety.

In the beginning of my experimental work I cherished the hope of being able to produce a white variety. My experiments, however, have not been successful, and so I have given them up temporarily. Much better chances for a new double variety seemed to exist, and my endeavors in this direction have finally been crowned with success.

For this reason I propose to deal now with the production of double flowers, to inquire what is on record about them in horticultural literature, and to give a full description of the origin thereof in an instance which it was my good fortune to observe in my garden.

Of course the historical part is only a hasty survey of the question and will only give such evidence as may enable us to get an idea of the [490] chances of success for the experimental worker. In the second half of the seventeenth century (1671), my countryman, Abraham Munting, published a large book on garden plants with many beautiful figures. It is called "Waare Oeffeninge der Planters," or "True Exercises With Plants." The descriptions pertain to ordinary typical species in greater part, but garden varieties receive special attention. Among these a long list of double flowers are to be seen. Double varieties of poppies, liverleaf (Hepatica), wallflowers (_Cheiranthus_), violets, _Caltha_, _Althaea_, _Colchic.u.m_, and periwinkles (_Vinca_), and a great many other common flowers were already in cultivation at that time.

Other double forms have been since added. Many have been introduced from j.a.pan, especially the j.a.panese marigold, _Chrysanthemum indic.u.m_. Others have been derived from Mexico, as for instance the double zinnias. The single dahlias only seem to have been originally known to the inhabitants of Mexico. They were introduced into Spain at about 1789, and the first double ones were produced in Louvain, Belgium, in 1814.

The method of their origin has not been described, and probably escaped the originators themselves. But in historical records we find the curious statement that it took place after three years' work. This indicates [491] a distinct plan, and the possibility of carrying it to a practical conclusion within a few years' time.

Something more is known about other cases. Garden anemones, _Anemone coronaria_, are said to have become double in the first half of the last century in an English nursery. The owner, Williamson, observing in his beds a flower with a single broadened stamen, saved its seeds separately, and in the next generations procured beautifully filled flowers. These he afterwards had crossed by bees with a number of colored varieties, and in this way succeeded in producing many new double types of anemone.

The first double petunia is known to have suddenly and accidentally arisen from ordinary seed in a private garden at Lyons about 1855. From this one plant all double races and-varieties have been derived by natural and partly by artificial crosses. Carriere, who reported this fact, added that likewise other species were known at that time to produce new double varieties rapidly. The double fuchsias originated about the same time (1854) and ten years later the range of double varieties of this plant had become so large that Carriere found it impossible to enumerate all of them.

Double carnations seem to be relatively old, double corn-flowers and double blue-bells being [492] of a later period. A long list could easily be made, to show that during the whole history of horticulture double varieties have arisen from time to time. As far as we can judge, such appearances have been isolated and sudden. Sometimes they sprang into existence in the full display of their beauty, but most commonly they showed themselves for the first time, exhibiting only spare supernumerary petals. Whenever such sports were worked up, a few years sufficed to reach the entire development of the new varietal attribute.

From this superficial survey of historical facts, the inference is forced upon us that the chance of producing a new double variety is good enough to justify the attempt. It has frequently succeeded for practical purposes, why should it not succeed as well for purely scientific investigation? At all events the type recommends itself to the student of nature, both on account of its frequency, and of the apparent insignificance of the first step, combined with the possibility of rapidly working up from this small beginning of one superfluous petal towards the highest degree of duplication.

Compared with the tedious experimental production of the peloric toad-flax, the attempt to produce a double flower has a distinct attraction. The peloric toad-flax is nothing new; the [493] experiment was only a repet.i.tion of what presumably takes place often within the same species. To attempt to produce a double variety we may choose any species, and of course should select one which as yet has not been known to produce double flowers. By doing so we will, if we succeed, produce something new. Of course, it does not matter whether the new variety has an horticultural interest or not, and it seems preferable to choose a wild or little cultivated species, to be quite sure that the variety in question is not already in existence. Finally the prospect of success seems to be enhanced if a species is chosen, the nearest allies of which are known to have produced double flowers.

For these reasons and others I chose for my experiment the corn-marigold, or _Chrysanthemum segetum_. It is also called the golden cornflower. In the wheat and rye fields of central Europe it a.s.sociates with the blue-bottle or blue corn-flower. It is sometimes cultivated and the seeds are offered for sale by many nurserymen. It has a cultivated variety, called _grandiflorum_, which is esteemed for its brilliancy and long succession of golden bloom. This variety has larger flower-heads, surrounded with a fuller border of ray-florets. The species belongs to a genus many species of which have produced [494] double varieties. One of them is the j.a.panese marigold, others are the _carinatum_ and the _imbricatum_ species. Nearly allied are quite a number of garden-plants with double flower-heads, among which are the double camomiles.

My attention was first drawn to the structure of the heads and especially to the number of the ray-florets of the corn-marigold. The species appertains to that group of composites which have a head of small tubular florets surrounded by a broad border of rays. These rays, when counted, are observed to occur in definite numbers, which are connected with each other by a formula, known as "the series" of Braun and Schimper. In this formula, which commences with 1 and 2, each number is equal to the sum of the two foregoing figures. Thus 5, 8 and 13 are very frequent occurrences, and the following number, 21, is a most general one for apparently full rays, such as in daisies, camomiles, _Arnica_ and many other wild and cultivated species.

These numbers are not at all constant. They are only the averages, around which the real numbers fluctuate. There may even be an overlapping of the extremes, since the fluctuation around 13 may even go beyond 8 and 21, and so on. But such extremes are only found in stray flowers, occurring on the same [495] individuals with the lesser degrees of deviation.

Now the marigold averages 13, and the _grandiflorum_ 21 rays. The wild species is pure in this respect, but the garden-variety is not. The seeds which are offered for sale usually contain a mixture of both forms and their hybrids. So I had to isolate the pure types from this mixture and to ascertain their constancy and mutual independency. To this end I isolated from the mixture first the 13-rayed, and afterwards the 21-rayed types. As the marigolds are not sufficiently self-fertile, and are not easily pollinated artificially, it seemed impossible to carry on these two experiments at the same time and in the same garden. I devoted the first three years to the lower form, isolated some individuals with 12-13 rays out of the mixture of 1892 and counted the ray-florets on the terminal head of every plant of the ensuing generation next year. I cultivated and counted in this way above 150 individuals and found an average of exactly 13 with comparatively few individuals displaying 14 or only 12 rays, and with the remainder of the plants grouped symmetrically around this average. I continued the experiment for still another year and found the same group of figures. I was then satisfied as to the purity of the isolated strain. Next year I sowed a new mixture in [496] order to isolate the reputed pure _grandiflorum_ type. During the beginning of the flowering period I ruthlessly threw away all plants displaying less than 21 rays in the first or terminal head. But this selection was not to be considered as complete, because the 13-rayed race may eventually transgress its boundary and come over to the 21 and more. This made a second selection necessary. On the selected plants all the secondary heads were inspected and their ray-florets counted. Some individuals showed an average of about 13 and were destroyed. Others gave doubtful figures and were likewise eliminated, and only 6 out of a lot of nearly 300 flowering plants reached an average of 21 for all of the flowers.

Our summer is a short one, compared with the long and beautiful summer of California, and it was too late to cut off the faded and the open flowers, and await new ones, which might be purely fertilized after the destruction of all minor plants. So I had to gather the seed from flowers, which might have been partially fertilized by the wrong pollen.

This however, is not so great a drawback in selection experiments as might be supposed at first sight. The selection of the following year is sure to eliminate the offspring of such impure parentage.