Part 26 (1/2)

Contrasted with these groups of constant forms are three inconstant types which we now take up. They belong to two different groups, according to the cause of their inconstancy. In one species which I call _O. lata_, the question of stability or instability must remain wholly unsolved, as only pistillate flowers are produced, and no seed can be fertilized save by the use of the pollen of another form, and therefore by hybridization. The other head comprises two fertile forms, _O.

scintillans_ and _O. elliptica_, which may easily be fertilized with their own pollen, but which gave a progeny only partly similar to the parents.

The _Oenothera lata_ is a very distinct form [541] which was found more than once in the field, and recently (1902) in a luxuriant flowering specimen. It has likewise been raised from seeds collected in different years at the original station. It is also wholly pistillate. Apparently the anthers are robust, but they are dry, wrinkled and nearly devoid of contents. The inner wall of cells around the groups of pollen grow out instead of being resorbed, partly filling the cavity which is left free by the miscarriage of the pollen-grains. This miscarriage does not affect all the grains in the same degree, and under the microscope a few of them with an apparently normal structure may be seen. But the contents are not normally developed, and I have tried in vain to obtain fertilization with a large number of flowers. Only by cross-fertilization does _O. lata_ produce seeds, and then as freely as the other species when self-fertilized. Of course its chance of ever founding a wild type is precluded by this defect.

_O. lata_ is a low plant, with a limp stem, bent tips and branches, all very brittle, but with dense foliage and luxuriant growth. It has bright yellow flowers and thick flower-buds. But for an unknown reason the petals are apt to unfold only partially and to remain wrinkled throughout the flowering time. The stigmas are slightly divergent from the normal type, [542] also being partly united with one another, and laterally with the summit of the style, but without detriment to their function.

Young seedlings of _lata_ may be recognized by the very first leaves.

They have a nearly orbicular shape and are very sharply set off against their stalk. The surface is very uneven, with convexities and concavities on both sides. This difference is lessened in the later leaves, but remains visible throughout the whole life of the plant, even during the flowering season. Broad, sinuate leaves with rounded tips are a sure mark of _O. lata_. On the summits of the stems and branches they are crowded so as to form rosettes.

Concerning inheritance of these characteristics nothing can be directly a.s.serted because of the lack of pollen. The new type can only be perpetuated by crosses, either with the parent form or some other mutant. I have fertilized it, as a rule, with _lamarckiana_ pollen, but have often also used that from _nanella_ and others. In doing so, the _lata_ repeats its character in part of its offspring. This part seems to be independent of the nature of the pollen used, but is very variable according to external circ.u.mstances. On the average one-fourth of the offspring become _lata_, the others a.s.suming the type of the pollen-parent, if this was a _lamarckiana_ or [543] partly this type and partly that of any other of the new species derived from _lamarckiana_, that might have been used as the pollen-parent. This average seems to be a general rule, recurring in all experiments, and remaining unchanged through a long series of successive generations. The fluctuations around this mean go up to nearly 50% and down nearly to 1%, but, as in other cases, such extreme deviations from the average are met with only exceptionally.

The second category includes the inconstant but perfectly fertile species. I have already given the names of the only two forms, which deserve to be mentioned here.

One of them is called _scintillans_ or the s.h.i.+ny evening-primrose, because its leaves are of a deep green color with smooth surfaces, glistening in the suns.h.i.+ne. On the young rosettes these leaves are somewhat broader, and afterwards somewhat narrower than those of _O.

lamarckiana_ at the corresponding ages. The plants themselves always remain small, never reaching the stature of the ancestral type. They are likewise much less branched. They can easily be cultivated in annual generations, but then do not become as fully developed and as fertile, as when flowering in the second year. The flowers have the same structure as those of the _lamarckiana_, but are of a smaller size.

[544] Fertilizing the flowers artificially with their own pollen, excluding the visiting insects by means of paper bags, and saving and sowing the seed of each individual separately, furnishes all the requisites for the estimation of the degree of stability of this species. In the first few weeks the seed-pans do not show any unequality, and often the young plants must be replanted at wider intervals, before anything can be made out with certainty. But as soon as the rosettes begin to fill it becomes manifest that some of them are more backward than others in size. Soon the smaller ones show their deeper green and broader leaves, and thereby display the attributes of the _scintillans_. The other grow faster and stronger and exhibit all the characteristics of ordinary _lamarckiana_s.

The numerical proportion of these two groups has been found different on different occasions. Some plants give about one-third _scintillans_ and two-thirds _lamarckiana_, while the progeny of individuals of another strain show exactly the reverse proportion.

Two points deserve to be noticed. First the progeny of the _scintillans_ appears to be mutable in a large degree, exceeding even the _lamarckiana_. The same forms that are produced most often by the parent-family are also most ordinarily [545] met with among the offspring of the s.h.i.+ny evening-primrose. They are _oblonga_, _lata_ and _nanella_. _Oblonga_ was observed at times to const.i.tute as much as 1% or more of the sowings of _scintillans_, while _lata_ and _nanella_ were commonly seen only in a few scattering individuals, although seldom lacking in experiments of a sufficient size.

Secondly the instability seems to be a constant quality, although the words themselves are at first sight, contradictory. I mean to convey the conception that the degree of instability remains unchanged during successive generations. This is a very curious fact, and strongly reminds us of the hereditary conditions of striped-flower varieties.

But, on the contrary, the atavists, which are here the individuals with the stature and the characteristics of the _lamarckiana_, have become _lamarckiana_s in their hereditary qualities, too. If their seed is saved and sown, their progeny does not contain any _scintillans_, or at least no more than might arise by ordinary mutations.

One other inconstant new species is to be noted, but as it was very rare both in the field and in my cultures, and as it was difficult of cultivation, little can as yet be said about it. It is the _Oenothera elliptica_, with narrow elliptical leaves and also with elliptical petals. It repeats [546] its type only in a very small proportion of its seed.

All in all we thus have a group of a dozen new types, springing from an original form in one restricted locality, and seen to grow there, or arising in the garden from seeds collected from the original locality.

Without any doubt the germs of the new types are fully developed within the seed, ready to be evolved at the time of germination. More favorable conditions in the field would no doubt allow all of the described new species to unfold their attributes there, and to come into compet.i.tion with each other and with the common parents. But obviously this is only of secondary importance, and has no influence on the fact that a number of new types, a.n.a.logous to the older swarms of _Draba_, _Viola_ and of many other polymorphous species, have been seen to arise directly in the wild state.

[547]

LECTURE XIX

EXPERIMENTAL PEDIGREE-CULTURES

The observation of the production of mutants in the field at Hilversum, and the subsequent cultivation of the new types in the garden at Amsterdam, gives ample proof of the mutability of plants. Furthermore it furnishes an a.n.a.logy with the hypothetical origin of the swarms of species of _Draba_ and _Viola_. Last but not least important it affords material for a complete systematic and morphologic study of the newly arisen group of forms.

The physiologic laws, however, which govern this process are only very imperfectly revealed by such a study. The instances are too few.

Moreover the seeds from which the mutants spring, escape observation. It is simply impossible to tell from which individual plants they have been derived. The laevifolia and the brevistylis have been found almost every year, the first always recurring on the same spot, the second on various parts of the original field. It is therefore allowable to a.s.sume a common [548] origin for all the observed individuals of either strain.

But whether, besides this, similar strains are produced anew by the old _lamarckiana_ group, it is impossible to decide on the sole ground of these field-observations.

The same holds good with the other novelties. Even if one of them should germinate repeatedly, without ever opening its flowers, the possibility could not be excluded that the seeds might have come originally from the same capsule but lain dormant in the earth during periods of unequal length.

Other objections might be cited that can only be met by direct and fully controlled experiments. Next to the native locality comes the experimental garden. Here the rule prevails that every plant must be fertilized with pollen of its own, or with pollen of other individuals of known and recorded origin. The visits of insects must be guarded against, and no seeds should be saved from flowers which have been allowed to open without this precaution. Then the seeds of each individual must be saved and sown separately, so as to admit of an appreciation, and if necessary, a numerical determination of the nature of its progeny. And last but not least the experiments should be conducted in a similar manner during a series of successive years.

[549] I have made four such experiments, each comprising the handling of many thousands of individual plants, and lasting through five to nine generations. At the beginning the plants were biennial, as in the native locality, but later I learned to cultivate them in annual generations.

They have been started from different plants and seeds, introduced from the original field into my garden at Amsterdam.