Species and Varieties, Their Origin by Mutation Part 19

[382] Any inequality from this cause will increase the difference in the size of the young leaves, augment the inequality of their production of organic matter and for this reason go on in an ever increasing rate.

Rain and spraying, or on the other hand dryness of the soil, have still greater consequences. The slightest unevenness of the surface will cause some spots to dry rapidly and others to retain moisture during hours and even sometimes during days.

Seeds, germinating in such little moist depressions grow regularly and rapidly, while those on the dryer elevations may be r.e.t.a.r.ded for hours and days, before fully unfurling their seed-leaves. After heavy rains these differences may be observed to increase continually, and in some instances I found that plants were produced only on the wet spots, while the dry places remained perfectly bare. From this the wet spots seem to be the most favorable, but on the other hand, seeds may come to germinate there too numerously and so closely that the young plants will be crowded together and find neither s.p.a.ce nor light enough, for a free and perfect development. The advantage may change to disadvantage in this way unless the superfluous individuals are weeded out in due time.

[383] From all these and other reasons some plants will be favored by the external conditions from the beginning, while others will be r.e.t.a.r.ded, and the effects will gradually increase until at last they become sufficient to account for a considerable amount of individual variability. There is no doubt that the difference in the strength of the plant and in the size of the capsules, going from 5-10 grams for a single fruit, are for the most part due to these unavoidable circ.u.mstances. I have tried all conceivable means to find remedies for these difficulties, but only by sowing my seeds in pans in a gla.s.s-house have I been able to reach more constant and equal conditions. But unfortunately such a method requires the planting out of the young seedlings in the beginning of the summer, and this operation is not without danger for opium-poppies, and especially not without important influence on the monstrosity of the pistilloid variety. Consequently my sowings of this plant have nearly always been made in the beds.

In order to show how great the influence of all these little things may become, we only have to make two sowings on neighboring beds and under conditions which have carefully been made as equal as possible. If we use for these controlling experiments seeds from one and the same capsule, it will soon become evident that [384] no exact similarity between the two lots may be expected. Such differences as may be seen in these cases are therefore never to be considered of value when comparing two lots of seeds of different origin, or under varying conditions. No amount of accuracy in the estimation of the results of a trial, or in the counting out of the several degrees of the anomaly, is adequate to overcome the inaccuracy resulting from these differences.

It is certainly of great importance to have a correct conception in regard to the influence of the surrounding conditions on the growth of a plant and on the development of the attribute we are to deal with. No less important is the question of the sensibility of the plants to these factors. Obviously this sensibility must not be expected to remain the same during the entire life-period, and periods of stronger and of weaker responses may be discerned.

In the first place it is evident that external or inner influences are able to change the direction of the development of an organ only so long as this development is not yet fully finished. In the young flower-bud of the pistilloid poppy there must evidently be some moment in which it is definitely decided whether the young stamens will grow out normally or become metamorphosed into secondary pistils. From this [385] moment no further change of external conditions is able to produce a corresponding change in the degree of the anomaly. The individual strength of the whole plant may still be affected in a more or less manifest degree, but the number of converted stamens of the flower has been definitely fixed. The sensitive period has terminated.

In order to determine the exact moment of this termination of the period of sensibility, I have followed the development of the flower buds during the first weeks of the life of the young plants. The terminal flower may already be seen in young plants only seven weeks old, with a stem not exceeding 5-6 cm. in height and a flower-bud with a diameter of nearly 1 mm., in which the stamens and secondary pistils are already discernible, but still in the condition of small rounded protuberances on the thalamus. Though it is not possible at that time to observe any difference between the future normal and converted stamens, it does not seem doubtful that the development is so far advanced, that in the inner tissues the decision has already definitely been taken. In the next few days this decision rapidly becomes visible, and the different parts of the normal stamens and the metamorphosed carpels soon become apparent.

From this observation it [386] can be inferred that the sensitive period of the anomaly is limited for the terminal flower-head, to the first few weeks of the life of the young plants. The secondary heads manifestly leave this period at a somewhat later stage.

In order to prove the accuracy of this conclusion I have tried to injure the anomalies after the expiration of the first six or seven weeks. I deprived them of their leaves, and damaged them in different ways. I succeeded in making them very weak and slender, without being able to diminish the number of the supernumerary carpels. The proportionality of the size of the central fruit and the development of the surrounding crown can often be modified or even destroyed by this means, and the apparent exceptions from this rule, which are often observed, may find their explanation in this way.

In the second place I have tried to change the development of the anomaly during the period of sensibility, and even in the last part of it. This experiment succeeded fully when carried out within the fifth or sixth week after the beginning of the germination. As means of injury I transplanted the young plants. To this end I sowed my seeds in pans in unmanured soil, planted them out in little pots with richly prepared earth, grew them in these during a few weeks and afterwards transferred them to the [387] beds, taking care that the pats were removed, but the b.a.l.l.s of earth not broken.

In consequence of this treatment the plants became very large and strong, with luxuriant foliage and relatively numerous large flowers and fruits. But almost without exception they were poor in anomalous stamens, at least so on the terminal heads. On a lot of some 70 plants more than 50 had less than half a crown of secondary capsules, while from the same packet of seed the control-plants gave in an equal number more than half of filled crowns on all plants with the exception of five weak specimens.

It is curious to compare such artificially injured plants with the ordinary cultures. Strong stems and heavy fruits, which otherwise are always indicative of showy crowns, now bear fruits wholly or nearly dest.i.tute of any anomalous change. The commonly prevailing rule seems to be reversed, showing thereby the possibility of abolis.h.i.+ng the correlation between individual strength and anomaly by an artificial encroachment upon the normal conditions.

Aside from these considerations the experiments clearly give proof of the existence of a period of sensibility limited to the first weeks of the life of the plant for the terminal flower. This knowledge enables us to explain many apparent [388] parent abnormalities, which may occur in the experiments.

We now may take a broader view of the period of sensibility. Evidently the response to external influences will be greater the younger the organ. Sensibility will gradually diminish, and the phenomena observed in the last part of this period may be considered as the last remainder of a reaction which previously must have been much stronger and much readier, providing that it would be possible to isolate them from, and contrast them with, the other responses of the same plant.

With the light thus cast upon the question, we may conclude that the sensitive period commences not only at the beginning of the germination, but must also be considered to include the life of the seed itself. From the moment of fertilization and the formation of the young embryo the development must be subjected to the influence of external agencies which determine the direction it will take and the degree of development it will finally be able to acquire. Probably the time of growth of the embryo and of the ripening of the seed correspond exactly to the period of highest sensibility. This period is only interrupted during the resting stage of the seed, to be repeated in germination. Afterwards the sensibility [389] slowly and gradually decreases, to end with the definite decision of all further growth sometime before the outer form of the organ becomes visible under the microscope. The last period of life includes only an expansion of the tissues, which may still have some influence on their final size, but not on their form. This has been definitely arrested before the end of the sensitive period, and ordinarily before the commencement of that rapid development, which is usually designated by the name of growth, as contrasted with evolution.

Within the seed the evolution of the young plant manifestly depends upon the qualities and life-conditions of the parent-plant. The stronger this is, and the more favorable circ.u.mstances it is placed under, the more food will be available for the seed, and the healthier will be the development of the embryo. Only well-nourished plants give well-nourished seeds, and the qualities of each plant are for this reason at least, partly dependent on the properties of its parents and even of its grandparents.

From these considerations the inference is forced upon us that the apparently hereditary differences, which are observed to exist among the seeds of a species or a variety and even of a single strain or a single parent-plant, may for a large part, and perhaps wholly, be the result [390] of the life-conditions of their parents and grandparents. Within the race all ssvariability would in this way be reduced to the effects of external circ.u.mstances. Among these nourishment is no doubt the most momentous, and this to such a degree that older writers designated the external conditions by the term nourishment. According to Knight nutrition reigns supreme in the whole realm of variability, the kind of food and the method of nourishment coming into consideration only in a secondary way. The amount of useful nutrition is the all-important factor.

If this is so, and if nutrition decides the degree of deviation of any given character, the widest deviating individuals are the best nourished ones. The best nourished not only during the period of sensibility of the attribute under consideration, but also in the broadest sense of the word.

This discussion casts a curious light upon the whole question of selection. Not of course upon the choice of elementary species or varieties out of the original motley a.s.sembly which nature and old cultures offer us, but upon the selection of the best individuals for isolation and for the improvement of the race. These are, according to my views, only the best nourished ones. Their external conditions have been the [391] most favorable, not only from the beginning of their own life in the field, but also during their embryonic stages, and even during the preparation of these latter in the life of their parents and perhaps even their grandparents. Selection then, would only be the choice of the best nourished individuals.

In connection with the foregoing arguments I have tried to separate the choicest of the poppies with the largest crown of pistilloid stamens, from the most vigorous individuals. As we have already seen, these two attributes are as a rule proportional to one another. Exceptions occur, but they may be explained by some later changes in the external circ.u.mstances, as I have also pointed out. As a rule, these exceptions are large fruits with comparatively too few converted stamens; they are exactly the contrary from what is required for a selection. Or plants, which from the beginning were robust, may have become crowded together by further growth, and for these reasons become weaker than their congeners, though retaining the full development of the staminodal crown, which was fixed during the sensitive period and before the crowding. I have searched my beds yearly for several years in vain to find individuals which might recommend themselves for selection without having the stamp of permanent, [392] or at least temporarily better, nourishment. No starting-point for such an independent selection has ever been met with.

Summing up the consequences of this somewhat extended discussion, we may state it as a rule that a general proportion between the individual strength and the degree of development of the anomaly exists. And from this point of view it is easy to see that all external causes which are known to affect the one, must be expected to influence the other also.

It will therefore hardly be necessary to give a full description of all my experiments on the relations of the monstrosity to external conditions. A hasty survey will suffice.

This survey is not only intended to convey an idea of the relations of pistilloid poppies to their environment, but may serve as an example of the principle involved. According to my experience with a large range of other anomalies, the same rule prevails everywhere. And this rule is so simple that exact knowledge of one instance may be considered as sufficient to enable us to calculate from a.n.a.logy what is to be expected from a given treatment of any other anomaly. Our appreciation of observed facts and the conditions to be chosen for intended cultures are largely dependent on such calculations. What I am now going to describe [393] is to be considered therefore as an experimental basis for such expectations.

First of all comes the question how many individuals are to be grown in a given place. When sowing plants for experimental purposes it is always best to sow in rows, and to give as few seeds to each row as possible, so as to insure all necessary s.p.a.ce to the young plants. On the other hand the seeds do not all germinate, and after sowing too thinly, gaps may appear in the rows. This would cause not only a loss of s.p.a.ce, but an inequality between the plants in later life, as those nearest the gaps would have more s.p.a.ce and more light, and a larger area for their roots than those growing in the unbroken rows. Hence the necessity of using large quant.i.ties of seed and of weeding out a majority of young plants on the spots where the greatest numbers germinate.

Crowded cultures as a rule, will give weak plants with thin stems, mostly unbranched and bearing only small capsules. According to the rule, these will produce imperfect crowns of secondary pistils. The result of any culture will thus be dependent to a high degree on the number of individuals per square meter. I have sown two similar and neighboring beds with the thoroughly mixed seeds of parent-plants of the same strain and culture, using as much [394] as 2.5 cu. cm. per square meter. On one of the beds I left all the germinating plants untouched and nearly 500 of them flowered, but among them 360 were almost without pistillody, and only 10 had full crowns. In the other bed I weeded away more than half of the young plants, leaving only some 150 individuals and got 32 with a full crown, nearly 100 with half crowns and only 25 apparently without monstrosity.

These figures are very striking. From the same quant.i.ty of seed, in equal s.p.a.ces, by similar exposure and treatment I got 10 fully developed instances in one and 32 in the other case. The weeding out of supernumerary individuals had not only increased the percentage of bright crowns, but also their absolute number per square meter. So the greatest number of anomalies upon a given s.p.a.ce may be obtained by taking care that not too many plants are grown upon it: any increase of the number beyond a certain limit will diminish the probability of obtaining these structures. The most successful cultures may be made after the maximum number of individuals per unit of area has been determined. A control-experiment was made under the same conditions and with the same seed, but allowing much less for the same s.p.a.ce. I sowed only 1 cu. cm. on my bed of 2 square meters, and thereby avoided [395]

nearly all weeding out. I got 120 plants, and among them 30 with full crowns of converted stamens, practically the same number as after the weeding out in the first experiment. This shows that smaller quant.i.ties of seed give an equal chance for a greater number of large crowns, and should therefore always be preferred, as it saves both seed and labor.

Weeding out is a somewhat dangerous operation in a comparative trial.

Any one who has done it often, knows that there is a strong propensity to root out the weaker plants and to spare the stronger ones. Obviously this is the best way for ordinary purposes, but for comparisons evidently one should not discriminate. This rule is very difficult in practice, and for this reason one should never sow more than is absolutely required to meet all requirements.

Our second point is the manuring of the soil. This is always of the highest importance, both for normal and for anomalous attributes. The conversion of the stamens into pistils is in a large measure dependent upon the conditions of the soil. I made a trial with some 800 flowering plants, using one sample of seed, but sowing one-third on richly manured soil, one-third on an unprepared bed of my garden, and one-third on nearly pure sand. In all other respects the three groups were treated in the same way. Of [396] the manured plants one-half gave full crowns, of the non-manured only one-fifth, and on the sandy soil a still smaller proportion. Other trials led to the same results. I have often made use of steamed and ground horn, which is a manure very rich in nitrogenous substances. One-eighth of a kilo per square meter is an ample amount.

And its effect was to increase the number of full crowns to an exceptional degree.

In the controlling trial and under ordinary circ.u.mstances this figure reached some 50%, but with ground horn it came up as high as 90%. We may state this result by the very striking a.s.sertion that the number of large crowns in a given culture may be nearly doubled by rich manure.

All other external conditions act in a similar manner. The best treatment is required to attain the best result. A sunny exposure is one of the most essential requisites, and in some attempts to cultivate my poppies in the shade, I found the pistillody strongly reduced, not a single full crown being found in the whole lot. Often the weather may be hurtful, especially during the earlier stages of the plants. I protected my beds during several trials by covering them with gla.s.s for a few weeks, until the young plants reached the gla.s.s covering. I got a normal number of full crowns, some 55%, at a time [397] when the weather was so bad as to reduce the number in the control experiments to 10%.

It would be quite superfluous to give more details or to describe additional experiments. Suffice to say, that the results all point in the same direction, and that pistillody of the poppies always clearly responds to the treatment, especially to external conditions during the first few weeks, that is, during the period of sensitiveness. The healthier and the stronger the plants the more fully they will develop their anomaly.

In conclusion something is to be said about the choice of the seed.

Obviously it is possible to compare seeds of different origin by sowing and treating them in the same way, giving attention to all the points above mentioned. In doing so the first question will be, whether there is a difference between the seeds of strong plants with a bright crown around the head and those of weaker individuals with lesser development of the anomaly. It is evident that such a difference must be expected, since the nutrition of the seed takes place during the period of the greatest sensitiveness.

But the experiments will show whether this effect holds good against the influences which tend to change the direction of the development of the anomaly during the time of germination. [398] The result of my attempt has shown that the choice of the seeds has a manifest influence upon the ultimate development of the monstrosity, but that this influence is not strong enough to overwhelm all other factors.

The choice of the fullest or smallest crowns may be repeated during succeeding generations, and each time compared with a culture under average conditions. By this means we come to true selection-experiments, and these result in a notable and rapid change of the whole strain. By selecting the brightest crowns I have come up in three years from 40 to 90 and ultimately to 120 converted stamens in the best flower of my culture, and in selecting the smallest crowns I was able in three years to exclude nearly all good crowns, and to make cultures in which heads with less than half-filled crowns const.i.tuted the majority. But such selected strains always remain very sensitive to treatment, and by changing the conditions the effect may be wholly lost in a single year, or even turned in the contrary direction. In other words, the anomaly is more dependent on external conditions during the germinating period than on the choice of the seeds, providing these belong to the pistilloid variety and have not deteriorated by some crossing with other sorts.

At the beginning of this lecture I stated that [399] no selection is adequate to produce either a pure strain of brightly crowned flower-heads without atavism, or to conduce to an absolute and permanent loss of the anomaly. During a series of years I have tested my plants in both directions, but without the least effect. Limits are soon reached on both sides, and to transgress these seems quite impossible.

Taking these limits as the marks of the variety, and considering all fluctuations between them as responses to external influences working during the life of the individual or governing the ripening of the seeds, we get a clear picture of a permanent ever-sporting type. The limits are absolutely permanent during the whole existence of this already old variety. They never change. But they include so wide a range of variability, that the extremes may be said to sport into one another, so much the more so as one of the extremes is to be considered morphologically as the type of the variation, while the other extreme can hardly be distinguished from the normal form of the species.

[400]

LECTURE XIV

MONSTROSITIES

I have previously dealt with the question of the hereditary tendencies that cause monstrosities. These tendencies are not always identical for the same anomaly. Two different types may generally, be distinguished.

One of them const.i.tutes a poor variety, the other a rich one. But this latter is abundant and the first one is poor in instances of exactly the same conformation. Therefore the difference only lies in the frequency of the anomaly, and not in its visible features. In discovering an instance of any anomaly it is therefore impossible to tell whether it belongs to a poor or to a rich race. This important question can only be answered by direct sowing-experiments to determine the degree of heredity.

Monstrosities are often considered as accidents, and rightfully so, at least as long as they are considered from a morphological point of view.

Physiology of course excludes all accidentality. And in our present ease it shows [401] that some internal hereditary quality is present, though often latent, and that the observed anomalies are to be regarded as responses of this innate tendency to external conditions. Our two types differ in the frequency of these responses. Rare in the poor race, they are numerous in the rich variety. The external conditions being the same for both, the hereditary factor must be different. The tendency is weak in the one and strong in the other. In both cases, according to my experience, it may be weakened or strengthened by selection and by treatment. Often to a very remarkable degree, but not so far as to transgress the limits between the two races. Such transgression may apparently be met with from time to time, but then the next generation generally shows the fallacy of the conclusion, as it returns more or less directly to the type from which the strain had been derived.

Monstrosities should always be studied by physiologists from this point of view. Poor and rich strains of the same anomaly seem at first sight to be so nearly allied that it might be thought to be very easy to change the one into the other. Nevertheless such changes are not on record, and although I have made several attempts in this line, I never succeeded in pa.s.sing the limit. I am quite convinced that sometime [402]

a method will be discovered of arbitrarily producing such conversions, and perhaps the easiest way to attain artificial mutations may lie concealed here. But as yet not the slightest indication of this possibility is to be found, save the fallacious conclusions drawn from too superficial observations.

Unfortunately the poor strains are not very interesting. Their chance of producing beautiful instances of the anomaly for which they are cultivated is too small. Exceptions to this rule are only afforded by those curious and rare anomalies, which command general attention, and of which, therefore, instances are always welcome. In such cases they are searched for with perseverance, and the fact of their rarity impresses itself strongly on our mind.

Twisted stems are selected as a first example. This monstrosity, called _biastrepsis_, consists of strongly marked torsions as are seen in many species with decussate leaves, though as a rule it is very rare. Two instances are the most generally known, those of the wild valerian (_Valeriana officinalis_) and those of cultivated and wild sorts of teasels (_Dipsacus fullonum_, _D. sylvestris_, and others). Both of these I have cultivated during upwards of fifteen years, but with contradictory results. The valerian is a perennial herb, multiplying itself yearly by [403] slender rootstocks or runners producing at their tips new rosettes of leaves and in the center of these the flowering stem. My original plant has since been propagated in this manner, and during several years I preserved large beds with hundreds of stems, in others I was compelled to keep my culture within more restricted limits.

This plant has produced twisted stems of the curious shape, with a nearly straight flag of leaves on one side, described by De Candolle and other observers, nearly every year. But only one or two instances of abnormal stems occurred in each year, and no treatment has been found that proved adequate to increase this number in any appreciable manner.

I have sown the seeds of this plant repeatedly, either from normal or from twisted stems, but without better results. It was highly desirable to be able to offer instances of this rare and interesting peculiarity to other universities and museums, but no improvement of the race could be reached and I have been constrained to give it up. My twisted valerian is a poor race, and hardly anything can be done with it.

Perhaps, in other countries the corresponding rich race may be hidden somewhere, but I have never had the good fortune of finding it.