Part 28 (1/2)
A critical study of the existing evidence seems to be of great importance in order to ascertain the best way of dealing with new facts, and of estimating the value of the factors concerned. [577] It is manifest that we must be very careful and conservative in dealing with new facts that are brought to our attention, and every effort should be made to bring additional evidence to light. Many vegetable anomalies are so rare that they are met with only by the purest chance, and are then believed to be wholly new. When a white variety of some common plant is met with for the first time we generally a.s.sume that it originated on that very spot and only a short time previously. The discovery of a second locality for the same variety at once raises the question as to a common origin in the two instances. Could not the plants of the second locality have arisen from seeds transported from the first?
White varieties of many species of blue-bells and gentians are found not rarely, white-flowering plants of heather, both of _Erica Tetralix_ and _Calluna vulgaris_ occur on European heaths; white flowers of _Brunella vulgaris_, _Ononis repens_, _Thymus vulgaris_ and others may be seen in many localities in the habitats of the colored species. Pelories of l.a.b.i.ates seem to occur often in Austria, but are rare in Holland; white bilberries (_Vaccinium Myrtillus_) have many known localities throughout Europe, and nearly all the berry-bearing species in the large heath family are recorded as having white varieties.
[578] Are we to a.s.sume a single origin for all the representatives of such a variety, as we have done customarily for all the representatives of a wild species? Or can the same mutation have been repeated at different times and in distant localities? If a distinct mutation from a given species is once possible, why should it not occur twice or thrice?
A variety which seems to be new to us may only appear so, because the spot where it grows had hitherto escaped observation. _Lychnis preslii_ is a smooth variety of _Lychnis diurna_ and was observed for the first time in the year 1842 by Sekera. It grew abundantly in a grove near Munchengratz in southern Hungary. It was accompanied by the ordinary hairy type of the species. Since then it has been observed to be quite constant in the same locality, and some specimens have been collected for me there lately by Dr. Nemec, of Prague. No other native localities of this variety have been discovered, and there can be no doubt that it must have arisen from the ordinary campion near the spot where it still grows. But this change may have taken place some years before the first discovery, or perhaps one or more centuries ago. This could only be known if it could be proved that the locality had been satisfactorily investigated previously, and that the variety had not [579] been met with. Even in this case only something would be discovered about the time of the change, but nothing about its real nature.
So it is in many cases. If a variety is observed in a number of specimens at the time of its first discovery, and at a locality not studied previously, it takes the aspect of an old form of limited distribution, and little can be learned as to the circ.u.mstances under which it arose. If on the contrary it occurs in very small numbers or perhaps even in a single individual, and if the spot where it is found is located so that it could hardly have escaped previous observation, then the presumption of a recent origin seems justified.
What has to be ascertained on such occasions to give them scientific value? Three points strike me as being of the highest importance. First, the constancy of the new type; secondly, the occurrence or lack of intermediates, and last, but not least, the direct observation of a repeated production.
The first two points are easily ascertained. Whether the new type is linked with its more common supposed ancestor by intermediate steps is a query which at once strikes the botanist. It is usually recorded in such cases, and we may state at once that the general result is, that such intermediates do not occur. This is [580] of the highest importance and admits of only two explanations. One is that intermediates may be a.s.sumed to have preceded the existent developed form, and to have died out afterwards. But why should they have done so, especially in cases of recent changes? On the other hand the intermediates may be lacking because they have never existed, the change having taken place by a sudden leap, such as the mutations described in our former lectures. It is manifest that the a.s.sumption of hypothetical intermediates could only gain some probability if they had been found in some instance. Since they do not occur, the hypothesis seems wholly unsupported.
The second point is the constancy of the new type. Seeds should be saved and sown. If the plant fertilizes itself without the aid of insects, as do some evening-primroses, the seed saved from the native locality may prove wholly pure, and if it does give rise to a uniform progeny the constancy of the race may be a.s.sumed to be proved, provided that repeated trials do not bring to light any exceptions. If the offspring shows more than one type, cross-fertilization is always to be looked to as the most probable cause, and should be excluded, in order to sow pure seeds. Garden-experiments of this kind, and repeated trials, should always be combined [581] with the discovery of a presumed mutation. In many instances the authors have realized the importance of this point, and new types have been found constant from the very beginning. Many cases are known which show no reversions and even no partial reversions.
This fact throws a distinct light on our first point, as it makes the hypothesis of a slow and gradual development still more improbable.
My third point is of quite another nature and has not as yet been dealt with. But as it appeals to me as the very soul of the problem, it seems necessary to describe it in some detail. It does not refer to the new type itself, nor to any of its morphologic or hereditary attributes, but directly concerns the presumed ancestors themselves.
The peloric toad-flax in my experiment was seen to arise thrice from the same strain. Three different individuals of my original race showed a tendency to produce peloric mutations, and they did so in a number of their seeds, exactly as the mutations of the evening-primroses were repeated nearly every year. Hence the inference, that whenever we find a novelty which is really of very recent date, the parent-strain which has produced it might still be in existence on the same spot. In the case of shrubs or perennials the very parents might yet be found. [582] But it seems probable, and is especially proved in the case of the evening-primroses, that all or the majority of the representatives of the whole strain have the same tendency to mutate. If this were a general rule, it would suffice to take some pure seeds from specimens of the presumed parents and to sow and multiply the individuals to such an extent that the mutation might have a chance to be repeated.
Unfortunately, this has not as yet been done, but in my opinion it should be the first effort of any one who has the good luck to discover a new wild mutation. Specimens of the parents should be transplanted into a garden and fertilized under isolated conditions. Seeds saved from the wild plant would have little worth, as they might have been partly fertilized by the new type itself.
After this somewhat lengthy discussion of the value of observations surrounding the discovery of new wild mutations, we now come to the description of some of the more interesting cases. As a first example, I will take the globular fruited shepherd's purse, described by Solms Laubach as _Capsella heegeri_. Professor Heeger discovered one plant with deviating fruits, in a group of common shepherd's purses in the market-place near Landau in Germany, in the fall of 1897. They were nearly spherical, [583] instead of flat and purse-shaped. Their valves were thick and fleshy, while those of the ordinary form are membranaceous and dry. The capsules hardly opened and therefore differed in this point from the shepherd's purse, which readily loosens both its valves as soon as it is ripe.
Only one plant was observed; whence it came could not be determined, nor whether it had arisen from the neighboring stock of C_apsella_ or not.
The discoverer took some seed to his garden and sent some to the botanical garden at Stra.s.sburg, of which Solms-Laubach is the director.
The majority of the seeds of course were sowed naturally on the original spot. The following year some of the seeds germinated and repeated the novelty. The leaves, stems and flowers were those of the common shepherd's purse, but no decision could be reached concerning the type of this generation before the first flowers had faded and the rounded capsules had developed. Then it was seen that the _heegeri_ came true from seed. It did so both in the gardens and on the market-place, where it was observed to have multiplied and spread in some small measure. The same was noted the following year, but then the place was covered with gravel and all the plants destroyed. It is not recorded to have been seen wild since.
[584] Intermediate forms have not been met with. Some slight reversions may occur in the autumn on the smallest and weakest lateral branches.
Such reversions, however, seem to be very rare, as I have tried in vain to produce them on large and richly branched individuals, by applying all possible inducements in the form of manure and of cutting, to stimulate the production of successive generations of weaker side branches.
This constancy was proved by the experiments of Solms-Laubach, which I have repeated in my own garden during several years with seed received from him. No atavists or deviating specimens have been found among many hundreds of flowering plants.
It is important to note that within the family of the crucifers the form of the capsule and the attributes of the valves and seeds are usually considered to furnish the characteristics of genera, and this point has been elucidated at some length by Solms-Laubach. There is, however, no sufficient reason to construe a new genus on the ground of Heeger's globular fruited shepherd's purse; but as a true elementary species, and even as a good systematic species it has proved itself, and as such it is described by Solms-Laubach, who named it in honor of its discoverer.
Exactly a.n.a.logous discoveries have been [586] instead of displaying a bright yellow cup. _O. cruciata_ grows in the Adirondack Mountains, in the states of New York and Vermont, and seems to be abundant there. It has been introduced into botanical gardens and yielded a number of hybrids, especially with _O. biennis and _O. lamarckiana_, and the narrow petals of the parent-species may be met with in combination with the stature and vegetative characteristics of these last named species.
_O. cruciata_ has a purple foliage, while _biennis_ and _lamarckiana_ are green, and many of the hybrids may instantly be recognized by their purple color.
The curious attribute of the petals is not to be considered simply as a reduction in size. On anatomical inquiry it has been found that these narrow petals bear some characteristics which, on the normal plants, are limited to the calyx. Stomata and hairs, and the whole structure of the surface and inner tissues on some parts of these petals are exactly similar to those of the calyx, while on others they have retained the characteristics of petals. Sometimes there may even be seen by the naked eye green longitudinal stripes of calyx-like structure alternating with bright yellow petaloid parts. For these reasons the cruciata character may be considered as a case of sepalody of the petals, or of the petals being partly converted into sepals.
[587] It is worth while to note that as a monstrosity this occurrence is extremely rare throughout the whole vegetable kingdom, and only very few instances have been recorded.
Two cases of sudden mutations have come to my knowledge, producing this same anomaly in allied species. One has been already alluded to; it pertains to the common evening-primrose or _Oenothera biennis_, and one is a species belonging to another genus of the same family, the great hairy willow-herb or _Epilobium _hirsutum_. I propose to designate both new forms by the varietal name of _cruciata_, or _cruciatum_.
_Oenothera biennis cruciata_ was found in a native locality of the _O.
biennis itself. It consisted of only one plant, showing in all its flowers the _cruciata_ marks. In all other respects it resembled wholly the _biennis_, especially in the pure green color of its foliage, which at once excluded all suspicion of hybrid origin with the purple _O.
cruciata_. Moreover in our country this last occurs only in the cultivated state in botanical gardens.