Species and Varieties, Their Origin by Mutation Part 7

Let us now take an instance of a character that is apt to vary in both ways, for this is obviously the best way of making clear what is meant by a negative and a positive change.

In the family of the composites we find a group of genera with two forms of florets on each flower-head. The hermaphrodite ones are tubular with 5, or rarely 4, equal teeth, and occupy the center of the head. These are often called the flosculous florets or disk-florets. Those of the circ.u.mference are ligulate and ordinarily unis.e.xual, without stamens. In many cases they are sterile, having only an imperfect ovary. They are large and brightly colored and are generally designated as ray-florets.

As instances we may cite the camomile (_Anthemis n.o.bilis_), the wild camomile (_Matricaria Chamomilla_), [131] the yarrow (_Achillea Millefolium_), the daisies, the Dahlia and many others. Species occur in this group of plants from time to time that lack the ray-florets, as in the tansy (_Tanacetum vulgare_) and some _artemisias_. And the genus of the marigolds or _Bidens_ is noted for containing both of these types.

The smaller and the three-toothed marigold (_B. cernua_ and _B.

tripart.i.ta_) are very common plants of wet soil and swamps, ordinarily lacking the ray-florets, and in some countries they are very abundant and wholly constant in this respect, never forming radiate flower-heads.

On the other hand the white-flowered and the purple marigold (_B.

leucantha_ and _B. atropurpurea_) are cultivated species of our gardens, prized for their showy flower-heads with large white or deeply colored, nearly black-purple florets.

Here we have opportunity to observe positive and negative varieties of the same character. The smaller, and the three-toothed marigold occur from time to time, provided with ray florets, showing a positive variation. And the white marigold has produced in our gardens a variety without rays. Such varieties are quite constant, never returning to the old species. Positive and negative varieties of this kind are by no means rare among the compositae.

[132] In systematic works the positive ones are as a rule called "radiate," and the negative ones "discoid." Discoid forms of the ordinary camomile, of the daisy, of some asters (_Aster Tripolium_), and of some centauries have been described. Radiate forms have been observed in the tansy (_Tanacetum vulgare_), the common horse-weed or Canada fleabane (_Erigeron canadensis_) and the common groundsel (_Senecio vulgaris_). Taken broadly the negative varieties seem to be somewhat more numerous than the positive ones, but it is very difficult to come to a definite conclusion on this point.

Quite the contrary is the case with regard to the color-varieties of red and blue flowers. Here the loss of color is so common that every one could give long lists of examples of it. Linnaeus himself supposed that no blue or red-colored wild species would be without a white variety. It is well known that he founded his often criticized prescript never to trust to color in recognizing or describing a species, on this belief.

On the other hand there are some red varieties of white-flowered species. But they are very rare, and little is known about their characters or constancy. Blue varieties of white species are not found.

The yarrow (_Achillea Millefolium_) has a red-flowered form, which occurs [133] from time to time in sunny and sandy localities. I have isolated it and cultivated it during a series of years and during many generations. It is quite true to its character, but the degree of its coloring fluctuates between pink and white and is extremely variable.

Perhaps it can be considered as an inconstant variety. A redflowered form of the common _Begonia semperflorens_ is cultivated under the name of "Vernon," the white hawthorn (_Crataegus Oxyacantha_) is often seen with red flowers, and a pink-flowered variety of the "Silverchain" or "b.a.s.t.a.r.d acacia" (_Robinia Pseud-Acacia_) is not rarely cultivated. The "Crown" variety of the yellow wall-flower and the black varieties, are also to be considered as positive color variations, the black being due in the latter cases to a very great amount of the red pigment.

Among fruits there are also some positive red varieties of greenish or yellowish species, as for instance the red gooseberry (_Ribes Grossularia_) and the red oranges. The red hue is far more common in leaves, as seen among herbs, in cultivated varieties of _Coleus_ and in the brown leaved form of the ordinary white clover, among trees and shrubs in the hazelnut (_Corylus_), the beech (_f.a.gus_), the birch (_Betula_), the barberry (_Berberis_) and many others. But though most of these forms are very ornamental and abundant [134] in parks and gardens, little is as yet known concerning the origin of their varietal attributes and their constancy, when propagated by seeds. Besides the ray-florets and the colors, there are of course a great many other characters in which varieties may differ from their species. In most of the cases it is easy to discern whether the new character is a positive or a negative one. And it is not at all necessary to scrutinize very narrowly the list of forms to become convinced that the negative form is the one which prevails nearly everywhere, and that positive aberrations are in a general sense so rare that they might even be taken for exceptions to the rule.

Many organs and many qualities may be lost in the origination of a variety. In some instances the petals may disappear, as in _Nigella_, or the stamens, as in the Guelder-rose (_Viburnum Opulus_) and the _Hortensia_ and in some bulbs even the whole flowers may be wanting, as in the beautiful "Plumosa" form of the cultivated grape-hyacinth or _Muscari comosum_. Fruits of the pineapples and bananas without seeds are on record as well as some varieties of apples and pears, of raisins and oranges. And some years ago Mr. Riviere of Algeria described a date growing in his garden that forms fruit without pits. The stoneless plum of Mr. [135] Burbank of Santa Rosa, California, is also a very curious variety, the kernel of which is fully developed but naked, no hard substance intervening between it and the pulp.

More curious still are the unbranched varieties consisting of a single stem, as may be seen sometimes in the corn or maize and in the fir.

Fir-trees of some three or four meters in height without a single branch, wholly naked and bearing leaves only on the shoots of the last year's growth at the apex of the tree, may be seen. Of course they cannot bear seed, and so it is with the sterile maize, which never produces any seed-spikes or staminate flowers. Other seedless varieties can be propagated by buds; their origin is in most cases unknown, and we are not sure as to whether they should be cla.s.sified with the constant or with the inconstant varieties.

A very curious loss is that of starch in the grains of the sugar-corn and the sugar-peas. It is replaced by sugar or some allied substance (dextrine). Equally remarkable is the loss of the runners in the so-called "Gaillon" strawberries.

Among trees the pendulous or weeping, and the broomlike or fastigiate forms are very marked varieties, which occur in species belonging to quite different orders. The ash, the beach, some willows, many other trees and some [136] finer species of garden-plants, as _Sophora j.a.ponica_, have given rise to weeping varieties, and the yew-tree or _Taxus_ has a fastigiate form which is much valued because of its ascending branches and pyramidal habit. So it is with the pyramidal varieties of oaks, elms, the b.a.s.t.a.r.d-acacia and some others.

It is generally acknowledged that these forms are to be considered as varieties on the ground of their occurrence in so wide a range of species, and because they always bear the same attributes. The pendulous forms owe their peculiarity to a lengthening of the branches and a loss of their habit of growing upwards; they are too weak to retain a vertical position and the response to gravity, which is ordinarily the cause of the upright growth, is lacking in them. As far as we know, the cause of this weeping habit is the same in all instances. The fastigiate trees and shrubs are a counterpart of the weeping forms. Here the tendency to grow in a horizontal direction is lacking, and with it the bilateral and symmetric structure of the branches has disappeared. In the ordinary yew-tree the upright stem bears its needles equally distributed around its circ.u.mference, but on the branches the needles are inserted in two rows, one to the left and one to the right. All the needles turn their upper surfaces upwards, [137] and their lower surfaces downwards, and all of them are by this means placed in a single horizontal plane, and branching takes place in the same plane. Evidently this general arrangement is another response to gravity, and it is the failure of this reaction which induces the branches to grow upwards and to behave like stems.

Both weeping and fastigiate characters are therefore to be regarded as steps in a negative direction, and it is highly important that even such marked departures occur without transitions or intermediate forms. If these should occur, though ever so rarely, they would probably have been brought to notice, on account of the great prospect the numerous instances would offer. The fact that they are lacking, proves that the steps, though apparently great, are in reality to be considered as covering single units, that cannot be divided into smaller parts.

Unfortunately we are still in the dark as to the question of the inheritance of these forms, since in most cases it is difficult to obtain pure seed.

We now consider the cases of the loss of superficial organs, of which the nectarines are example. These are smooth peaches, lacking the soft hairy down, that is a marked peculiarity of the true peaches. They occur in different [138] races of the peach. As early as the beginning of the past century, Gallesio described no less than eight subvarieties of nectarines, each related to a definite race of peach. Most of them reproduce themselves truly from seed, as is well known in this country concerning the clingstones, freestones and some other types. Nectarines have often varied, giving rise to new sorts, as in the case of the white nectarine and many others differing greatly in appearance and flavor. On the other hand it is to be remarked, that the trees do not differ in other respects and cannot be distinguished while young, the varietal mark being limited to the loss of the down on the fruit. Peaches have been known to produce nectarines, and nectarines to yield true peaches.

Here we have another instance of positive and negative steps with reference to the same character, but I cannot withhold an expression of some doubt as to the possibility of crossing and subsequently splitting up of the hybrids as a more probable explanation of at least some of the cases quoted by various writers.

Smooth or glabrous varieties often occur, and some of them have already been cited as instances of the multiplication of varietal names.

Positive aberrations are rather rare, and are mostly restricted to a greater density of the [139] p.u.b.escence in some hairy species, as in _Galeopsis Ladanum canescens_, _Lotus corniculatus hirsutus_ and so on.

But _Veronica scutellata_ is smooth and has a p.u.b.escent variety, and Cytisus prostratus and _C. spinescens_ are each recorded to have a ciliate form.

Comparable with the occurrence and the lack of hairs, is the existence or deficiency of the glaucous effect in leaves, as is well known in the common _Ricinus_. Here the glaucous appearance is due to wax distributed in fine particles over the surface of the leaves, and in the green variety this wax is lacking. Other instances could be given as in the green varieties of _Papaver alpinum_ and _Rumex scutatus_. No positive instances are recorded in this case.

Spines and p.r.i.c.kles may often disappear and give rise to unarmed and defenceless types. Of the thorn-apples both species, the whiteflowered _Datura Stramonium_ and the purple _D. Tatula_ have such varieties.

Spinach has a variety called the "Dutch," which lacks the p.r.i.c.kles of the fruit; it is a very old form and absolutely constant, as are also the thornless thorn-apples. Last year a very curious instance of a partial loss of p.r.i.c.kles was discovered by Mr. c.o.c.kerell of East Las Vegas in New Mexico. It is a variety of the American c.o.c.klebur, often called sea-burdock, or the [140] hedgehog-burweed, a stout and common weed of the western states. Its Latin name is _Xanthium canadense_ or _X. commune_ and the form referred to is named by Mr. c.o.c.kerell, _X.

Wootoni_, in honor of Professor E.o. Wooton who described the first collected specimens.

The burs of the common species are densely covered with long p.r.i.c.kles, which are slightly hooked at the apex. In the new form, which is similar in all other respects to the common c.o.c.klebur, the burs are more slender and the p.r.i.c.kles much less numerous, about 25 to the bur and mostly stouter at the base. It occurs abundantly in New Mexico, always growing with the common species, and seems to be quite constant from seed. Mr.

c.o.c.kerell kindly sent me some burs of both forms, and from these I raised in my garden last year a nice lot of the common, as well as of the _Wootoni_ plants.

Spineless varieties are recorded for the b.a.s.t.a.r.d-acacia, the holly and the garden gooseberry (_Ribes Grossularia_, or _R. Uva-crispa_). A spineless sport of the p.r.i.c.kly Broom (_Ulex europaeus_) has been seen from time to time, but it has not been propagated.

Summarizing the foregoing facts, we have excellent evidence of varieties being produced either by the loss of some marked peculiarity or by the acquisition of others that are already [141] present in allied species.

There are a great many cases however, in which the morphologic cause of the dissimilarity is not so easily discerned. But there is no reason to doubt that most of them will be found to conform to the rule on closer investigation. Therefore we can consider the following as the princ.i.p.al difference between elementary species and varieties; that the first arise by the acquisition of entirely new characters, and the latter by the loss of existing qualities or by the gain of such peculiarities as may already be seen in other allied species.

If we suppose elementary species and varieties originated by sudden leaps or mutations, then the elementary species have mutated in the line of progression, some varieties have mutated in the line of retrogression, while others have diverged from their parental types in a line of depression, or in the way of repet.i.tion. This conception agrees quite well with the current idea that in the building up of the vegetable kingdom according to the theory of descent, it is species that form the links of the chain from the lower forms to the more highly organized later derivatives. Otherwise expressed, the system is built up of species, and varieties are only local and lateral, but never of real importance for the whole structure.

[142] Heretofore we have generally a.s.sumed, that varieties differ from the parent-species in a single character only, or at least that only one need be considered. We now come to the study of those varieties, which differ in more than one character. Of these there are two types. In the first the points of dissimilarity are intimately connected with one another, in the second they are more or less independent.

The mutually related peculiarities may be termed correlative, and we therefore speak, in such cases, of correlative variability. This phenomenon is of the highest importance and is of general occurrence.

But before describing some examples, it is as well to note that in the lecture on fluctuating variability, cases of a totally different nature will be dealt with, which unfortunately are designated by the same term.

Such merely fluctuating variations are therefore to be left out of the present discussion.

The purple thorn-apple, which is considered by some writers as a variety of the white-flowered species or _Datura Stramonium_, and by others as a separate species, _D. Tatula_, will serve as an ill.u.s.tration. But as its distinguis.h.i.+ng attributes, as far as we are concerned with them here, are of the nature described above as characteristic of varietal peculiarities no objection [143] can be made to our using them as a case of correlative variability.

The essential character of the purple thornapple lies in the color of the flowers, which are of a very beautiful pale blue. But this color is not limited to the corolla. It is also to be seen in the stems and in the stalks and veins of the leaves, which are stained with a deep purple, the blue color being added to the original green. Even on the surface of the leaves it may spread into a purplish hue. On the stems it is to be met with everywhere, and even the young seedlings show it. This is of some importance, as the young plants when unfolding their cotyledons and primary leaves, may be distinguished by this means from the seedlings of the white flowered species.

In crossing experiments it is therefore possible to distinguish the whites and the blues, even in young seedlings, and experience shows that the correlation is quite constant. The color can always be relied upon; if lacking in the seedlings, it will be lacking in the stems and flowers also; but if the axis of the young plant is ever so slightly tinged, the color will show itself in its beauty in the later stages of the life of the plant.

This is what we term correlation. The colors of the different organs are always in agreement. It is true that they require the concurrence of [144] light for development, and that in the dark or in a faint light the seedlings are apt to remain green when they should become purple, but aside from such consideration all organs always come true to their color, whether pure green and white, or whether these are combined with the blue tinge. This constancy is so absolute that the colors of the different organs convey the suggestion, that they are only separate marks of a single character.

It is on this suggestion that we must work, as it indicates the cause of the correlation. Once present, the faculty of producing the anthocyan, the color in question, will come into activity wherever and whenever opportunity presents itself. It is the cell-sap of the ordinary cell tissue or parenchyma, which is colored by the anthocyan, and for this reason all organs possessing this tissue, may exhibit the color in question.

Thus the color is not a character belonging to any single organ or cell, nor is it bound to a morphologic unit; it is a free, physiologic quality. It is not localized, but belongs to the entire plant. If we wish to a.s.sume for its basis material representative particles, these particles must be supposed to be diffused throughout the whole body of the plant.

This conception of a physiologic unit as the [145] cause of colors and other qualities is evidently opposed to the current idea of the cells and tissues as the morphologic units of the plants. But I do not doubt, that in the long run it will recommend itself as much to the scientist as to the breeder. For the breeder, when desiring to keep his varieties up to their standard, or when breeding to a definite idea, obviously keeps his standard and his ideal for the whole plant, even if he breeds only for flowers or for fruit.

I have chosen the color of the purple thornapple as a first example, but the colors of other plants show so many diverging aspects, all pointing so clearly to the same conclusion, that it would be well to take a more extensive view of this interesting subject.

First we must consider the correlation in the colors of flowers and fruits. If both are colored in the species, whether red or brown or purple or nearly black, and a variety lacking this hue is known, it will be lacking in both organs. If the color is pure, the flowers and berries will become white, but such cases are rare. Ordinarily a yellowish or greenish tinge underlies the ornamental color, and if this latter disappears, the yellowish ground will become manifest. So for instance in the Belladonna, a beautiful perennial herb with great s.h.i.+ny black, but very poisonous, fruits. Its flowers are brown, but in [146] some woods a variety with greenish flowers and bright yellow berries occurs, which is also frequently seen in botanic gardens. The anthocyan dye is lacking in both organs, and the same is the case with the stems and the leaves. The lady's laurel or _Daphne Mezereum_ has red corollas, purple leaves and red fruits; its white flowered variety may be distinguished by lack of the red hue in the stems and leaves, and by their beautiful yellow berries. Many other instances could be given, since the loss of color in berries is a very common occurrence, so common that for instance, in the heath-family or Ericaceae, with only a few exceptions, all berry-bearing species have white-fruited varieties.

The same correlation is observed in the seeds. The white-flowered flax may be seen to yield yellow and not brown seeds as in the blue species.

Many varieties of flowers may be recognized by the color of their seeds, as in the poppies, stocks and others. Other white-flowered varieties may be distinguished when germinating, their young axes being of a pure instead of a purplish green. It is a test ordinarily used by gardeners, to purify their flower beds long before the blooming time, when thinning or weeding them. Even in wild plants, as in _Erodium_, _Calluna_, _Brunella_ and others, a botanist may recognize the rare white-flowered [147] variety by the pure green color of the leaves, at times when it is not in flower. Some sorts of peas bear colored flowers and a red mark on the stipules of their leaves. Among bulbous plants many varieties may be recognized even in the dry bulbs by the different tinges of the outer scales.

Leaving the colors, we come now to another instance of correlation, which is still more astonis.h.i.+ng. For it is as rare, as color-varieties are common. It is afforded by some plants the leaves of which, instead of being entire or only divided into large parts, are cleft to a greater extent by repeated fissures of the marginal lobes. Such foliar variations are often seen in gardens, where they are cultivated for their beauty or singularity, as the laciniated alders, fern-leaved, beeches and limes, oakleaved laburnums, etc. Many of them are described under the varietal name of _laciniata_. In some cases this fissure extends to the petals of the flowers, and changes them in a way quite a.n.a.logous to the aberrancy of the leaves.

This is known to occur with a variety of brambles, and is often seen in botanic gardens in one of the oldest and most interesting of all anomalies, the laciniated variety of the greater celandine or _Chelidonium majus_. Many other instances could be given. Most of them belong to the [148] group of negative variations, as we have defined them. But the same thing occurs also with positive varieties, though of course, such cases are very rare. The best known instance is that of the ever-flowering begonia, _Begonia semperflorens_, which has green leaves and white flowers, but which has produced garden varieties with a brown foliage and pink flowers. Here also the new quality manifests itself in different organs.

Enough has now been said on correlative changes, to convince us that they are as a rule to be considered as the expression of some general internal or physiologic quality, which is not limited to a single organ, but affects all parts of the organism, provided they are capable of undergoing the change. Such characters are therefore to be considered as units, and should be referred to the group of single characters.

Opposed to these are the true compound characters, which consist of different units. These may be segregated by the production of varieties, and thereby betray the separate factors of the complex group.

The most beautiful instances of such complex characters are offered by the colors of some of the most prized garden-flowers. Rarely these are of a single hue, often two or three shades contribute to the effect, and in some cases special [149] spots or lines or tracings are to be seen on a white or on a colored background. That such spots and lines are separate units is obvious and is demonstrated by the fact that sometimes spotless varieties occur, which in all other respects have kept the colors of the species. The complexity of the color is equally evident, whenever it is built up of const.i.tuents of the anthocyan and of the yellow group. The anthocyan dye is limited to the sap-cavity of the cells, while the yellow and pure orange colors are fixed in special organs of the protoplasm. The observation under the microscope shows at once the different units, which though lying in the same cell and in almost immediate vicinity of each other are always wholly separated from one another by the wall of the vacuole or sapfilled cell-cavity.

The combination of red and yellow gives a brown tinge, as in the cultivated wall-flower, or those bright hues of a dark orange-red, which are so much sought in tulips. By putting such flowers for a short time in boiling water, the cells die and release the red pigment, which becomes diffused in the surrounding fluids and the petals are left behind with their yellow tinge. In this way it is easy to separate the const.i.tuents, and demonstrate the compound nature of the original colors.

[150] But the diversity of the color patterns is far from being exhausted with these simple instances. Apart from them, or joined to them, other complications are frequently seen, which it is impossible to a.n.a.lyze in such an artificial way. Here we have to return to our former principle, the comparison of different varieties. a.s.suming that single units may be lost, irrespective of the others, we may expect to find them segregated by variation, wherever a sufficiently wide range of color-varieties is in cultivation. In fact, in most cases a high degree of dissimilarity may be reached in the simplest way by such a separation of the components, and by their combination into most diverse smaller groups. A very nice instance of such an a.n.a.lysis of flower-colors is afforded by the ordinary snapdragon. The beautiful brown red color of this common garden-plant is composed on one side of yellow elements, on the other of red units. Of the yellow there are two, one staining the whole corolla with a light hue, as is to be seen in the pure yellow variety called _luteum. This form has been produced by the loss of the whole group of the red const.i.tuents. If the yellow tinge is also lost, there arises a white variety, but this is not absolutely colorless, but shows the other yellow const.i.tuent. This last stains only some small parts [151] of the lips of the flower around the throat, brightening, as it seems, the entrance for the visiting insects. In many of the red or reddish varieties this one yellow patch remains, while the general yellow hue fails. In the variety called "Brilliant" the yellow ground makes the red color more s.h.i.+ny, and if it is absent the pure carmine tinge predominates.

It is readily seen, that in the ordinary form the lips are of a darker red than the tube. This evident dissimilarity indicates some complexity.