Species and Varieties, Their Origin by Mutation Part 25

In such groups of nearly allied forms the single members must evidently be of common origin. It is not necessary for them to have originated all in the same place or at the same time. In some cases, as with _Draba verna_, the present geographic distribution points to a common birthplace, from whence the various forms may about the same period have radiated in all directions. The violets on the other hand seem to include widely diffused original forms, from which branches have started at different times and in different localities.

The origin of such groups of allied forms must therefore be the object of our research. Perhaps we might find a whole group, perhaps only part of it. In my opinion we have the right to a.s.sume that if _Draba_ and violets and [519] others have formerly mutated in this way, other species must at present be in the same changeable condition. And if mutations in groups, or such periodic mutations should be the rule, it is to be premised that these periods recur from time to time, and that many species must even now be in mutating condition, while others are not.

It is readily granted that the constant condition of species is the normal one, and that mutating periods must be the exception. This fact does not tend to increase our prospect of discovering a species in a state of mutability. Many species will have to be tested before finding an instance. On the other hand, a direct trial seems to be the only way to reach the goal. No such special guides as those that led us to the choice of pelories and double flowers are available. The only indication of value is the presumption that a condition of mutability might be combined with a general state of variability at large, and that groups of plants of very uniform features might be supposed to be constant in this respect too. On the contrary, anomalies and deviations if existent in the members of one strain, or found together in one native locality of a species, might be considered as an indication in the desired direction.

Few plants vary in the wild state in such a [520] measure as to give distinct indications. All have to be given a trial in the garden under conditions as similar as possible to their natural environments.

Cultivated plants are of course to be excluded. Practically they have already undergone the experience in question and can not be expected to change their habits soon enough. Moreover they are often of hybrid origin. The best way is to experiment with the native plants of one's own country.

I have made such experiments with some hundred species that grow wild in Holland. Some were very variable, as for instance, the jointed charlock (_Rapha.n.u.s Raphanistrum_) and the narrow-leaved plantain (_Plantago lanceolata_). Others seemed more uniform, but many species, collected without showing any malformation, subsequently produced them in my garden, either on the introduced plants themselves or among their offspring. From this initial material I have procured a long series of hereditary races, each with some peculiar anomaly for its special character. But this result was only a secondary gain, a meager consolation for the negative fact that no real mutability could be discovered.

My plants were mostly annuals or biennials, or such perennials as under adequate treatment might produce flowers and seeds during their [521]

first summer. It would be of no special use to enumerate them. The negative result does not apply to the species as such, but only to the individual strain, which I collected and cultivated. Many species, which are quite constant with us, may be expected to be mutable in other parts of their range.

Only one of all my tests met my expectations. This species proved to be in a state of mutation, producing new elementary forms continually, and it soon became the chief member of my experimental garden. It was one of the evening primroses.

Several evening-primroses have at different times been introduced into European gardens from America. From thence they have spread into the vicinity, becoming common and exhibiting the behavior of indigenous types. _Oenothera biennis_ was introduced about 1614 from Virginia, or nearly three centuries ago. _O. muricata_, with small corollas and narrow leaves, was introduced in the year 1789 by John Hunneman, and _O.

suaveolens_, or sweet-scented primrose, a form very similar to the _biennis_, about the same time, in 1778, by John Fothergill. This form is met with in different parts of France, while the _biennis_ and _muricata_ are very common in the sandy regions of Holland, where I have observed them for [522] more than 40 years. They are very constant and have proven so in my experiments. Besides these three species, the large-flowered evening-primrose, or _Oenothera lamarckiana_, is found in some localities in Holland and elsewhere. We know little concerning its origin. It is supposed to have come from America in the same way as its congeners, but as yet I have not been able to ascertain on what grounds this supposition rests. As far as I know, it has not been seen growing wild in this country, though it may have been overlooked. The fact that the species of this group are subject to many systematic controversies and are combined by different writers into systematic species in different ways, being often considered as varieties of one or two types, easily accounts for it having been overlooked. However, it would be of great interest to ascertain whether _O. lamarckiana_ yet grows in America, and whether it is in the same state of mutability here as it is in Holland.

The large-flowered evening-primrose was also cultivated about the beginning of the last century in the gardens of the Museum d'Histoire Naturelle, at Paris, where it was noticed by Lamarck, who at once distinguished it as an undescribed species. He wrote a complete description [523] of it and his type specimens are still preserved in the herbarium of the Museum, where I have compared them with the plants of my own culture. Shortly afterwards it was renamed by Seringe, in honor of its eminent discoverer, whose name it now bears. So Lamarck unconsciously discovered and described himself the plant, which after a century, was to become the means of an empirical demonstration of his far-reaching views on the common origin of all living beings.

_Oenothera lamarckiana_ is considered in Europe as a garden-plant, much prized for parks and ornamental planting. It is cultivated by seed-merchants and offered for sale. It has escaped from gardens, and having abundant means for rapid multiplication, has become wild in many places. As far as I know its known localities are small, and it is to be presumed that in each of them the plant has escaped separately from culture. It was in this state that I first met with this beautiful species.

Lamarck's evening-primrose is a stately plant, with a stout stem, attaining often a height of 1.6 meters and more. When not crowded the main stem is surrounded by a large circle of smaller branches, growing upwards from its base so as often to form a dense bush. These branches in their turn have numerous lateral [524] branches. Most of them are crowned with flowers in summer, which regularly succeed each other, leaving behind them long spikes of young fruits. The flowers are large and of a bright yellow color, attracting immediate attention, even from a distance. They open towards evening, as the name indicates, and are pollinated by humble-bees and moths. On bright days their duration is confined to one evening, but during cloudy weather they may still be found open on the following morning. Contrary to their congeners they are dependent on visiting insects for pollination. _O. biennis_ and _O.

muricata_ have their stigmas in immediate contact with the anthers within the flower-buds, and as the anthers open in the morning preceding the evening of the display of the petals, fecundation is usually accomplished before the insects are let in. But in _O. lamarckiana_ no such self-fertilization takes place. The stigmas are above the anthers in the bud, and as the style increases in length at the time of the opening of the corolla, they are elevated above the anthers and do not receive the pollen. Ordinarily the flowers remained sterile if not visited by insects or pollinated by myself, although rare instances of self-fertilization were seen.

In falling off, the flowers leave behind them a stout ovary with four cells and a large number [525] of young seeds. The capsule when ripe, opens at its summit with four valves, and contains often from two to three hundred seeds. A hundred capsules on the main stem is an average estimate, and the lateral branches may ripen even still more fruits, by which a very rapid dissemination is ensured.

This striking species was found in a locality near Hilvers, in the vicinity of Amsterdam, where it grew in some thousands of individuals.

Ordinarily biennial, it produces rosettes in the first, and stems in the second year. Both the stems and the rosettes were at once seen to be highly variable, and soon distinct varieties could be distinguished among them.

The first discovery of this locality was made in 1886. Afterwards I visited it many times, often weekly or even daily during the first few years, and always at least once a year up to the present time. This stately plant showed the long-sought peculiarity of producing a number of new species every year. Some of them were observed directly on the field, either as stems or as rosettes. The latter could be transplanted into my garden for further observation, and the stems yielded seeds to be sown under like control. Others were too weak to live a sufficiently long time in the field. They were discovered by sowing seed from indifferent plants [526] of the wild locality in the garden. A third and last method of getting still more new species from the original strain, was the repet.i.tion of the sowing process, by saving and sowing the seed which ripened on the introduced plants. These various methods have led to the discovery of over a dozen new types, never previously observed or described.

Leaving the physiologic side of the relations of these new forms for the next lecture, it would be profitable to give a short description of the several novelties. To this end they may be combined under five different heads, according to their systematic value. The first head includes those which are evidently to be considered as varieties, in the narrower sense of the word, as previously given. The second and third heads indicate the real progressive elementary species, first those which are as strong as the parent-species, and secondly a group of weaker types, apparently not destined to be successful. Under the fourth head I shall include some inconstant forms, and under the last head those that are organically incomplete.

Of varieties with a negative attribute, or real retrograde varieties, I have found three, all of them in a flowering condition in the field. I have given them the names of _laevifolia_, _brevistylis_ and _nanella_.

[527] The _laevifolia_, or smooth-leaved variety, was one of the very first deviating types found in the original field. This was in the summer of 1887, seventeen years ago. It formed a little group of plants growing at some distance from the main body, in the same field. I found some rosettes and some flowering stems and sowed some seed in the fall.

The variety has been quite constant in the field, neither increasing in number of individual plants nor changing its place, though now closely surrounded by other _Lamarckiana_s. In my garden it has proved to be constant from seed, never reverting to the original _lamarckiana_, provided intercrossing was excluded.

It is chiefly distinguished from Lamarck's evening-primrose by its smooth leaves, as the name indicates. The leaves of the original form show numerous sinuosities in their blades, not at the edge, but anywhere between the veins. The blade shows numbers of convexities on either surface, the whole surface being undulated in this manner; it lacks also the brightness of the ordinary evening-primrose or _Oenothera biennis_.

These undulations are lacking or at least very rare on the leaves of the new _laevifolia_. Ordinarily they are wholly wanting, but at times single leaves with slight manifestations of this [528] character may make their appearance. They warn us that the capacity for such sinuosities is not wholly lost, but only lies dormant in the new variety. It is reduced to a latent state, exactly as are the apparently lost characters of so many ordinary horticultural varieties.

Lacking the undulations, the _laevifolia_ leaves are smooth and bright.

They are a little narrower and more slender than those of the _lamarckiana_. The convexities and concavities of leaves are said to be useful in dry seasons, but during wet summers, such as those of the last few years, they must be considered as very harmful, as they retain some of the water which falls on the plants, prolonging the action of the water on the leaves. This is considered by some writers to be of some utility after slight showers, but was observed to be a source of weakness during wet weather in my garden, preventing the leaves from drying. Whether the _laevifolia_ would do better under such circ.u.mstances, remains to be tested.

The flowers of the _laevifolia_ are also in a slight degree different from those of _lamarckiana_. The yellow color is paler and the petals are smoother. Later, in the fall, on the weaker side branches these differences increase. The _laevifolia_ petals become smaller and are often not emarginated at the apex, becoming ovate [529] instead of obcordate. This shape is often the most easily recognized and most striking mark of the variety. In respect to the reproductive organs, the fertility and abundance of good seed, the _laevifolia_ is by no means inferior or superior to the original species.

_O. brevistylis_, or the short-styled evening primrose, is the most curious of all my new forms. It has very short styles, which bring the stigmas only up to the throat of the calyx tube, instead of upwards of the anthers. The stigmas themselves are of a different shape, more flattened and not cylindrical. The pollen falls from the anthers abundantly on them, and germinates in the ordinary manner.

The ovary which in _lamarckiana_ and in all other new forms is wholly underneath the calyx-tube, is here only partially so. This tube is inserted at some distance under its summit. The insertion divides the ovary into two parts: an upper and a lower one. The upper part is much reduced in breadth and somewhat attenuated, simulating a prolongation of the base of the style. The lower part is also reduced, but in another manner. At the time of flowering it is like the ovary of _lamarckiana_, neither smaller nor larger. But it is reached by only a very few pollen-tubes, and is therefore always incompletely fertilized. It does [530] not fall off after the fading away of the flower, as unfertilized ovaries usually do; neither does it grow out, nor a.s.sume the upright position of normal capsules. It is checked in its development, and at the time of ripening it is nearly of the same length as in the beginning. Many of them contain no good seeds at all; from others I have succeeded in saving only a hundred seeds from thousands of capsules.

These seeds, if purely pollinated, and with the exclusion of the visits of insects, reproduce the variety, entirely and without any reversion to the _lamarckiana_ type.

Correlated with the detailed structures is the form of the flower-buds.

They lack the high stigma placed above the anthers, which in the _lamarckiana_, by the vigorous growth of the style, extends the calyx and renders the flower bud thinner and more slender. Those of the _brevistylis_ are therefore broader and more swollen. It is quite easy to distinguish the individuals by this striking character alone, although it differs from the parent in other particulars.

The leaves of the _O. brevistylis_ are more rounded at the tip, but the difference is only p.r.o.nounced at times, slightly in the adult rosettes, but more clearly on the growing summits of the stems and branches. By this character, the plants [531] may be discerned among the others, some weeks before the flowers begin to show themselves. But the character by which the plants may be most easily recognized from a distance in the field is the failure of the fruits. They were found there nearly every year in varying, but always small numbers.

Leaving the short-styled primrose, we come now to the last of our group of retrograde varieties. This is the _O. nanella_, or the dwarf, and is a most attractive little plant. It is very short of stature, reaching often a height of only 20-30 cm., or less than one-fourth of that of the parent. It commences flowering at a height of 10-15 cm., while the parent-form often measures nearly a meter at this stage of its development. Being so very dwarfed the large flowers are all the more striking. They are hardly inferior to those of the _lamarckiana_, and agree with them in structure. When they fade away the spike is rapidly lengthened, and often becomes much longer than the lower or vegetative part of the stem.

The dwarfs are one of the most common mutations in my garden, and were observed in the native locality and also grown from seeds saved there.

Once produced they are absolutely constant. I have tried many thousands of seeds from various dwarf mutants, and never observed [532] any trace of reversion to the _lamarckiana_ type. I have also cultivated them in successive generations with the same result. In a former lecture we have seen that contrary to the general run of horticultural belief, varieties are as constant as the best species, if kept free from hybrid admixtures. This is a general rule, and the exceptions, or cases of atavism are extremely rare. In this respect it is of great interest to observe that this constancy is not an acquired quality, but is to be considered as innate, because it is already fully developed at the very moment when the original mutation takes place.

From its first leaves to the rosette period, and through this to the lengthening of the stem, the dwarfs are easily distinguished from any other of their congeners. The most remarkable feature is the shape of the leaves. They are broader and shorter, and especially at the base they are broadened in such a way as to become apparently sessile. The stalk is very brittle, and any rough treatment may cause the leaves to break off. The young seedlings are recognizable by the shape of the first two or three leaves, and when more of them are produced, the rosettes become dense and strikingly different from others. Later leaves are more nearly like the parent-type, but the petioles remain short. The bases of the blades are frequently [533] almost cordate, the laminae themselves varying from oblong-ovate to ovate in outline. The stems are often quite unbranched, or branched only at the base of the spike.

Strong secondary stems are a striking attribute of the _lamarckiana_ parent, but they are lacking, or almost so in the dwarfs. The stem is straight and short, and this, combined with the large crown of bright flowers, makes the dwarfs eminently suitable for bed or border plants.

Unfortunately they are very sensitive, especially to wet weather.

_Oenothera gigas_ and _O. rubrinervis_, or the giant, and the red-veined evening-primroses, are the names given to two robust and stout species, which seem to be equal in vigor to the parent-plant, while diverging from it in striking characters. Both are true elementary species, differentiated from _lamarckiana_ in nearly all their organs and qualities, but not showing any preponderating character of a retrograde nature. Their differences may be compared with those of the elementary species of other genera, as for instance, of _Draba_, or of violets, as will be seen by their description.

The giant evening-primrose, though not taller in stature than _O.

lamarckiana_, deserves its name because it is so much stouter in all respects. [534] The stems are robust, often with twice the diameter of _lamarckiana_ throughout. The internodes are shorter, and the leaves more numerous, covering the stems with a denser foliage. This shortness of the internodes extends itself to the spike, and for this reason the flowers and fruits grow closer together than on the parent-plant. Hence the crown of bright flowers, opening each evening, is more dense and more strikingly brilliant, so much the more so as the individual flowers are markedly larger than those of the parents. In connection with these characters, the flower-buds are seen to be much stouter than those of _lamarckiana_. The fruits attain only half the normal size, but are broader and contain fewer, but larger seeds.

The _rubrinervis_ is in many respects a counterpart to the _gigasv, but its stature is more slender. The spikes and flowers are those of the _lamarckianav, but the bracts are narrower. Red veins and red streaks on the fruits afford a striking differentiating mark, though they are not absolutely lacking in the parent-species. A red hue may be seen on the calyx, and even the yellow color of the petals is somewhat deepened in the same way. Young plants are often marked by the pale red tinge of the mid-veins, but in adult rosettes, or from lack of suns.h.i.+ne, this hue is often very faint.

[535] The leaves are narrow, and a curious feature of this species is the great brittleness of the leaves and stems, especially in annual individuals, especially in those that make their stem and flowers in the first year. High turgidity and weak development of the mechanical and supporting tissues are the anatomical cause of this deficiency, the bast-fibers showing thinner walls than those of the parent-type under the microscope. Young stems of _rubrinervis_ may be broken off by a sharp stroke, and show a smooth rupture across all the tissues, while those of _lamarckiana_ are very tough and strong.

Both the giant and the red-veined species are easily recognized in the rosette-stage. Even the very young seedlings of the latter are clearly differentiated from the _lamarckiana_, but often a dozen leaves are required, before the difference may be seen. Under such circ.u.mstances the young plants must reach an age of about two months before it is possible to discern their characters, or at least before these characters have become reliable enough to enable us to judge of each individual without doubt. But the divergencies rapidly become greater.

The leaves of _O. gigas_ are broader, of a deeper green, the blade more sharply set off against the stalk, all the rosettes [536] becoming stout and crowded with leaves. Those of _O. rubrinervis_ on the contrary are thin, of a paler green and with a silvery white surface; the blades are elliptic, often being only 2 cm. or less in width. They are acute at the apex and gradually narrowed into the petiole.

It is quite evident that such pale narrow leaves must produce smaller quant.i.ties of organic food than the darker green and broad organs of the _gigas_. Perhaps this fact is accountable partly, at least, for the more robust growth of the giant in the second year. Perhaps also some relation exists between this difference in chemical activity and the tendency to become annual or biennial. The _gigas_, as a rule, produces far more, and the _rubrinervis_ far less biennial plants than the _lamarckiana_. Annual culture for the one is as unreliable as biennial culture for the other. _Rubrinervis_ may be annual in apparently all specimens, in sunny seasons, but _gigas_ will ordinarily remain in the state of rosettes during the entire first summer. It would be very interesting to obtain a fuller insight into the relation of the length of life to other qualities, but as yet the facts can only be detailed as they stand.

Both of these stout species have been found [537] quite constant from the very first moment of their appearance. I have cultivated them from seed in large numbers, and they have never reverted to the _lamarckiana_. From this they have inherited the mutability or the capacity of producing at their turn new mutants. But they seem to have done so incompletely, changing in the direction of more absolute constancy. This was especially observed in the case of _rubrinervis_, which is not of such rare occurrence as _O. gigas_, and which it has been possible to study in large numbers of individuals. So for instance, the "red-veins" have never produced any dwarfs, notwithstanding they are produced very often by the parent-type. And in crossing experiments also the red-veins gave proof of the absence of a mutative capacity for their production.

Leaving the robust novelties, we may now take up a couple of forms, which are equally constants and differentiated from the parent species in exactly the same manner, though by other characters, but which are so obviously weak as to have no manifest chance of self maintenance in the wild state. These are the whitish and the oblong-leaved evening-primroses or the _Oenothera albida_ and _oblonga_.

_Oenothera albida_ is a very weak species, with whitish, narrow leaves, which are evidently incapable [538] of producing sufficient quant.i.ties of organic food. The young seedling-plants are soon seen to lag behind, and if no care is taken of them they are overgrown by their neighbors.

It is necessary to take them out, to transplant them into pots with richly manured soil, and to give them all the care that should be given to weak and sickly plants. If this is done fully grown rosettes may be produced, which are strong enough to keep through the winter. In this case the individual leaves become stronger and broader, with oblong blades and long stalks, but retain their characteristic whitish color.

In the second year the stems become relatively stout. Not that they become equal to those of _lamarckiana_, but they become taller than might have been expected from the weakness of the plants in the previous stages. The flowers and racemes are nearly as large as those of the parent-form, the fruits only a little thinner and containing a smaller quant.i.ty of seed. From these seeds I have grown a second and a third generation, and observed that the plants remain true to their type.

_O. oblonga_ may be grown either as an annual, or as a biennial. In the first case it is very slender and weak, bearing only small fruits and few seeds. In the alternative case however, it [539] becomes densely branched, bearing flowers on quite a number of racemes and yielding a full harvest of seeds. But it always remains a small plant, reaching about half the height of that of _lamarckiana_.

When very young it has broader leaves, but in the adult rosettes the leaves become very narrow, but fleshy and of a bright green color. They are so crowded as to leave no s.p.a.ce between them unoccupied. The flowering spikes of the second year bear long leaf-like bracts under the first few flowers, but those arising later are much shorter. Numerous little capsules cover the axis of the spike after the fading away of the petals, const.i.tuting a very striking differentiating mark. This species also was found to be quite constant, if grown from pure seed.

We have now given the descriptions of seven new forms, which diverge in different ways from the parent-type. All were absolutely constant from seed. Hundreds or thousands of seedlings may have arisen, but they always come true and never revert to the original _O. lamarckiana_ type.

From this they have inherited the condition of mutability, either completely or partly, and according to this they may be able to produce new forms themselves. But this occurs only rarely, and combinations of more than one [540] type in one single plant seem to be limited to the admixture of the dwarf stature with the characters of the other new species.

These seven novelties do not comprise the whole range of the new productions of my _O. lamarckiana_. But they are the most interesting ones. Others, as the _O. semilata_ and the _O. leptocarpa_ are quite as constant and quite as distinct, but have no special claims for a closer description. Others again were sterile, or too weak to reach the adult stage and to yield seeds, and no reliable description or appreciation can be given on the ground of the appearance of a single individual.