Species and Varieties, Their Origin by Mutation Part 20

This good fortune however, I did have with the wild teasel or _Dipsacus sylvestris_. [404] Stems of this and of allied species are often met with and have been described by several writers, but they were always considered as accidents and n.o.body had ever tried to cultivate them. In the summer of 1885 I saw among a lot of normal wild teasels, two nicely twisted stems in the botanical garden of Amsterdam. I at once proposed to ascertain whether they would yield a hereditary race and had all the normal individuals thrown away before the flowering time. My two plants flowered in this isolated condition and were richly pollinated by insects. Of course, at that time, I knew nothing of the dependency of monstrosities on external conditions, and made the mistake of sowing the seeds and cultivating the next generation in too great numbers on a small s.p.a.ce. But nevertheless the anomaly was repeated, and the aberrant individuals were once more isolated before flowering. The third generation repeated the second, but produced sixty twisted stems on some 1,600 individuals. The result was very striking and quite sufficient for all further researches, but the normal condition of the race was not reached. This was the case only after I had discovered the bad effects of growing too many plants in a limited s.p.a.ce. In the fourth generation I restricted my whole culture to about 100 individuals, and by this simple [405] means at once got up to 34% of twisted stems. This proportion has since remained practically the same. I have selected and isolated my plants during five succeeding generations, but without any further result, the percentage of twisted stems fluctuating between 30 and about 45 according to the size of the cultures and the favorableness or unfavorableness of the weather.

It is very interesting to note that all depends on the question whether one has the good fortune of finding a rich race or not, as this pedigree-culture shows. Afterwards everything depends on treatment and very little on selection. As soon as the treatment becomes adequate, the full strength of the race at once displays itself, but afterwards no selection is able to improve it to any appreciable amount. Of course, in the long run, the responses will be the same as those of the pistilloid poppies on the average, and some influence of selection will show itself on closer scrutiny.

Compared with the polycephalous poppies my race of twisted teasels is much richer in atavists. They are never absent, and always const.i.tute a large part of each generation and each bed, comprising somewhat more than half of the individuals. Intermediate stages between them and the wholly twisted stems are not wanting, [406] and a whole series of steps may easily be observed from sufficiently large cultures. But they are always relatively rare, and any lot of plants conveys the idea of a dimorphous race, the small twisted stems contrasting strongly with the tall straight ones.

A sharper contrast between good representatives of a race and their atavists is perhaps to be seen in no other instance. All the details contribute to the differentiation in appearance. The whole stature of the plants is affected by the varietal mark. The atavists are not, as in the case of the poppies, obviously allied with the type by a full range of intermediate steps, but quite distant from it by their rarity. There seems to be a gap in the same way as between the striped flowers of the snapdragon and their uniform red atavists, while with the poppies the atavists may be viewed as being only the extremes of a series of variations fluctuating around some average type.

From this reason it is as interesting to appreciate the hereditary position of the atavists of twisted varieties as it was for the red-flowered descendants of the striped flowers. In order to ascertain this relation it is only necessary to isolate some of them during the blooming-period. I made this experiment in the summer of 1900 with the eighth generation of my race, and contrived [407] to isolate three groups of plants by the use of parchment bags, covering them alternately, so the flowers of only one group were accessible to insects, at a time. I made three groups, because the atavists show two different types. Some specimens have decussate stems, others bear all their leaves in whorls of three, but in respect to the hereditary tendency of the twisting character this difference does not seem to be of any importance.

In this way I got three lots of seeds and sowed enough of them to have three groups of plants each containing about 150-200 well developed stems. Among these I counted the twisted individuals, and found nearly the same numbers for all three. The twisted parents gave as many as 41% twisted children, but the decussate atavists gave even somewhat more, viz., 44%, while the ternate specimens gave 37%. Obviously the divergences between these figures are too slight to be dwelt upon, but the fact that the atavists are as true or nearly as true inheritors of the twisted race as the best selected individuals is clearly proved by this experience.

It is evident that here we have a double race, including two types, which may be combined in different degrees. These combinations determine a wide range of changes in the stature of the plants, and it seems hardly right to use the [408] same term for such changes as for common variations. It is more a contention of opposite characters than a true phenomenon of simple variability. Or perhaps we might say that it is the effect of the cooperation of a very variable mark, the twisting, with a scarcely varying attribute of the normal structure of the stem. Between the two types an endless diversity prevails, but outwardly there are limits which are never transgressed. The double race is as permanent, and in this sense as constant, as any ordinary simple variety, both in external form, and in its intimate hereditary qualities.

I have succeeded in discovering some other rich races of twisted plants.

One of them is the Sweet William (_Dianthus barbatus_), which yielded, after isolation, in the second generation, 25% of individuals with twisted stems, and as each individual produces often 10 and more stems, I had a harvest of more than half a thousand of instances of this curious, and ordinarily very rare anomaly. My other race is a twisted variety of _Viscaria oculata_, which is still in cultivation, as it has the very consistent quality of being an annual. It yielded last summer (1903) as high a percentage as 65 of twisted individuals, many of them repeating the monstrosity on several branches. After some occasional observations _Gypsophila paniculata_ [409] seems to promise similar results. On the other hand I have sowed in vain the seeds of twisted specimens of the soapwort and the cleavewort (_Saponaria officinalis_ and _Galium Aparine_). These and some others seem to belong to the same group as the valerian and to const.i.tute only poor or so-called half-races.

Next to the torsions come the fasciated stems. This is one of the most common of all malformations, and consists, in its ordinary form, of a flat ribbon-like expansion of the stems or branches. Below they are cylindrical, but they gradually lose this form and a.s.sume a flattened condition. Sometimes the rate of growth is unequal on different portions or on the opposite sides of the ribbon, and curvatures are produced and these often give to the fasciation a form that might be compared with a shepherd's crook. It is a common thing for fasciated branches and stems to divide at the summit into a number of subdivisions, and ordinarily this splitting occurs in the lower part, sometimes dividing the entire fasciated portion. In biennial species the rosette of the root-leaves of the first year may become changed by the monstrosity, the heart stretching in a transverse direction so as to become linear. In the next year this line becomes the base from which the stem grows. In such cases the fasciated stems [410] are broadened and flattened from the very beginning, and often retain the incipient breadth throughout their further development. Species of primroses (_Primula j.a.ponica_ and others), of b.u.t.tercups (_Ranunculus bulbosus_), the rough hawksbeard (_Crepis biennis_), the Aster _Tripolium_, and many others could be given as instances.

Some of these are so rare as to be considered as poor races, and in cultural trials do not produce the anomaly except in a very few instances. Heads of rye are found in a cleft condition from time to time, single at their base and double at the top, but this anomaly is only exceptionally repeated from seed. Flattened stems of _Rubia tinctorum_ are not unfrequently met with on the fields, but they seem to have as little hereditary tendency as the split rye (_Secale Cereale_).

Many other instances could be given. Both in the native localities and in pedigree-cultures such ribboned stems are only seen from time to time, in successive years, in annual and biennial as well as in perennial species. The purple pedicularis (_Pedicularis pal.u.s.tris_) in the wild state, and the sunflower among cultivated plants, may be cited instead of giving a long list of a.n.a.logous instances.

On the other hand rich races of flattened stems are not entirely lacking. They easily betray [411] themselves by the frequency of the anomaly, and therefore may be found, and tried in the garden. Under adequate cultivation they are here as rich in aberrant individuals as the twisted races quoted above, producing in good years from 30-40% and often more instances. I have cultivated such rich races of the dandelion (_Taraxac.u.m officinale_), of _Thrincia hirta_, of the dame's violet (_Hesperis matronalis_), of the hawkweed (_Picris hieracioides_), of the rough hawksbeard (_Crepis biennis_), and others.

Respecting the hereditary tendencies these rich varieties with flattened stems may be put in the same category with the twisted races. Two points however, seem to be of especial interest and to deserve a separate treatment.

The common c.o.c.ks...o...b..or _Celosia cristata_, one of the oldest and most widely cultivated fasciated varieties may be used to ill.u.s.trate the first point. In beds it is often to be seen in quite uniform lots of large and beautiful crests, but this uniformity is only secured by careful culture and selection of the best individuals. In experimental trials such selection must be avoided, and in doing so a wide range of variability at once shows itself. Tall, branched stems with fan-shaped tops arise, const.i.tuting a series of steps towards complete atavism.

This last [412] however, is not to be reached easily. It often requires several successive generations grown from seed collected from the most atavistic specimens. And even such selected strains are always reverting to the crested type. There is no transgression, no springing over into a purely atavistic form, such as may be supposed to have once been the ancestor of the present c.o.c.ks...o...b.. The variety includes crests and atavists, and may be perpetuated from both. Obviously every gardener would select the seeds of the brightest crests, but with care the full crests may be recovered, even from the worst reversionists in two or three generations. It is a double race of quite the same const.i.tution as the twisted teasels.

My second point is a direct proof of this a.s.sertion, but made with a fasciated variety of a wild species. I took for my experiment the rough hawksbeard. In the summer of 1895 I isolated some atavists of the fifth generation of my race, which, by ordinary selection, gave in the average from 20-40% of fasciated stems. My isolated atavists bore abundant fruit, and from these I had the next year a set of some 350 plants, out of which about 20% had broadened and linear rosettes. This proportion corresponds with the degree of inheritance which is shown in many years by the largest and strongest [413] fasciated stems. It strengthens our conclusion as to the innermost const.i.tution of the double races or ever-sporting varieties.

Twisted stems and fasciations are very striking monstrosities. But they are not very good for further investigation. They require too much s.p.a.ce and too much care. The calculation of a single percentage requires the counting of some hundreds of individuals, taking many square meters for their cultivation, and this, as my best races are biennial, during two years. For this reason the countings must always be very limited, and selection is restrained to the most perfect specimens.

Now the question arises, whether this mark is the best upon which to found selection. This seems to be quite doubtful. In the experiments on the heredity of the atavists, we have seen that they are, at least often, in no manner inferior to even the best inheritors of the race.

This suggests the idea that it is not at all certain that the visible characters of a given individual are a trustworthy measure of its value as to the transmission of the same character to the offspring. In other words, we are confronted with the existence of two widely different groups of characters in estimating the hereditary tendency. One is the visible quality of the individuals and the other is the direct observation [414] of the degree in which the attribute is transmitted.

These are by no means parallel, and seem in some sense to be nearly independent of each other. The fact that the worst atavists may have the highest percentage of varietal units seems to leave no room for another explanation.

Developing this line of thought, we gradually arrive at the conclusion that the visible attribute of a varying individual is perhaps the most untrustworthy and the most unreliable character for selection, even if it seems in many cases practically to be the only available one. The direct determination of the degree of heredity itself is obviously preferable by far. This degree is expressed by the proportion of its inheritors among the offspring, and this figure therefore should be elevated to the highest rank, as a measure of the hereditary qualities.

Henceforward we will designate it by the name of hereditary percentage.

In scientific experiments this figure must be determined for every plant of a pedigree-culture singly, and the selection should be founded exclusively or at least mainly on it. It is easily seen that this method requires large numbers of individuals to be grown and counted. Some two or three hundred progeny of one plant are needed to give the decisive figure for this one [415] individual, and selection requires the comparison of at least fifty or more individuals. This brings the total amount of specimens to be counted up to some tens of thousands. In practice, where important interests depend upon the experiments, such numbers are usually employed and often exceeded, but for the culture of monstrosities, other methods are to be sought in order to avoid these difficulties.

The idea suggests itself here that the younger the plants are, when showing their distinguis.h.i.+ng marks, the more of them may be grown on a small s.p.a.ce. Hence the best way is to choose such attributes, as may already be seen in the young seedlings, in the very first few weeks of their lives. Fortunately the seed-leaves themselves afford such distinctive marks, and by this means the plants may be counted in the pans, requiring no culture at all in the garden. Only the selected individuals need be grown to ripen their seeds, and the whole selection may be made in the spring, in the gla.s.shouse. Instead of being very troublesome, the determination of the hereditary percentages becomes a definite reduction of the size of the experiments. Moreover it may easily be effected by any one who cares for experimental studies, but has not the means required for cultures on a larger scale. And lastly, there are [416] a number of questions about heredity, periodicity, dependency on nourishment and other life conditions, and even about hybridizing, which may be answered by this new method.

Seed-leaves show many deviations from the ordinary shape, especially in dicotyledonous plants. A very common aberration is the multiplication of their number, and three seed-leaves in a whorl are not rarely met with.

The whorl may even consist of four, and in rare cases of five or more cotyledons. Cleft cotyledons are also to be met with, and the fissure may extend varying distances from the tips. Often all these deviations may be seen among the seedlings of one lot, and then it is obvious that together they const.i.tute a scale of cleavages, the ternate and quaternate whorls being only cases where the cleaving has reached its greatest development. All in all it is manifest that here we are met by one type of monstrosity, but that this type allows of a wide range of fluctuating variability. For brevity's sake all these cleft and ternate, double cleft and quaternate cotyledons and even the higher grades are combined under one common name and indicated as tricotyls.

A second aberration of young seed-plants is exactly opposite to this. It consists of the union of the two seed-leaves into a single organ. This ordinarily betrays its origin by [417] having two separate apices, but not always. Such seedlings are called syncotyledonous or syncotyls.

Other monstrosities have been observed from time to time, but need not be mentioned here.

It is evident that the determination of the hereditary percentage is very easy in tricotylous or syncotylous cultures. The parent plants must be carefully isolated while blooming. Many species pollinate themselves in the absence of bees; from these the insects are to be excluded.

Others have the stamens and stigmas widely separated and have to be pollinated artificially. Still others do not lend themselves to such operations, but have to be left free to the visits of bees and of humble-bees, this being the only means of securing seed from every plant. At the time of the harvest the seeds should be gathered separately from each plant, and this precaution should also be observed in studies of the hereditary percentage at large, and in all scientific pedigree-cultures. Every lot of seeds is to be sown in a separate pan, and care must be taken to sow such quant.i.ties the three to four hundred seedlings will arise from each. As soon as they display their cotyledons, they are counted, and the number is the criterion of the parent-plant. Only parent-plants with the highest percentages are selected, and out of [418] their seedlings some fifty or a hundred of the best ones are chosen to furnish the seeds for the next generation.

This description of the method shows that the selection is a double one.

The first feature is the hereditary percentage. But then not all the seedlings of the selected parents can be planted out, and a choice has to be made. This second selection may favor the finest tricotyls, or the strongest individuals, or rely on some other character, but is unavoidable.

We now come to the description of the cultures. Starting points are the stray tricotyls which are occasionally found in ordinary sowings. In order to increase the chance of finding them, thousands of seeds of the same species must be inspected, and the range of species must be widened as much as possible.

Material for beginning such experiments is easily obtained, and almost any large sample of seeds will be found suitable. Some tricotyls will be found among every thousand seedlings in many species, while in others ten or a hundred times, as many plants must be examined to secure them, but species with absolutely pure dicotylous seeds are very rare.

The second phase of the experiment, however, is not so promising. Some species are rich, and others are poor in this anomaly. This difference [419] often indicates what can be expected from further culture. Stray tricotyls point to poor species or half-races, while more frequent deviations suggest rich or double-races. In both cases however, the trial must be made, and this requires the isolation of the aberrant individuals and the determination of their hereditary percentage.

In some instances the degree of their inheritance is only a very small one. The isolated tricotyls yield 1 or 2% of inheritors, in some cases even less, or upwards up to 3 or 4%. If the experiment is repeated, no amelioration is observed, and this result remains the same during a series of successive generations. In the case of _Polygonum convolvulus_, the Black bindweed, I have tried as many as six generations without ever obtaining more than 3%. With other species I have limited myself to four successive years with the same negative result, as with spinage, the Moldavian dragon-head, (_Dracocephalum moldavic.u.m_), and two species of corn catch-fly (_Silene conica_ and _S.

conoidea_).

Such poor races hardly afford a desirable material for further inquiries. Happily the rich races, though rare, may be discovered also from time to time. They seem to be more common among cultivated plants and horticultural as well as agricultural species may be used. Hemp [420] and mercury (_Mercurialis annua_) among the first, snapdragon, poppies, _Phacelia_, _Helichrysum_, and _Clarkia_ among garden-flowers may be given as instances of species containing the rich tricotylous double races.

It is very interesting to note how strong the difference is between such cases and those which only yield poor races. The rich type at once betrays itself. No repeated selection is required. The stray tricotyls themselves, that are sought out from among the original samples, give hereditary percentages of a much higher type after isolation than those quoted above. They come up to 10-20% and in some cases even to 40%. As may be expected, individual differences occur, and it must even be supposed that some of the original tricotyls may not be pure, but hybrids between tricotylous and dicotylous parents. These are at once eliminated by selection, and if only the tricotyls which have the highest percentages are chosen for the continuance of the new race, the second generation comes up with equal numbers of dicotyls and tricotyls among the seedlings. The figures have been observed to range from 51-58% in the majority of the cases, and average 55%, rarely diverging somewhat more from this average.

Here we have the true type of an ever-sporting variety. Every year it produces in the [421] same way heirs and atavists. Every plant, if fertilized with its own pollen, gives rise to both types. The parent itself may be tricotylous or dicotylous, or show any amount of multiplication and cleavage in its seed-leaves, but it always gives the entire range among its progeny of the variation. One may even select the atavists, pollinate them purely and repeat this in a succeeding generation without any chance of changing the result. On an average the atavists may give lower hereditary figures, but the difference will be only slight.

Such tricotylous double races offer highly interesting material for inquiries into questions of heredity, as they have such a wide range of variability. There is little danger in a.s.serting that they go upwards to nearly 100%, and downwards to 0%, diverging symmetrically on both sides of their average (50-55%). These limits they obviously cannot transgress, and are not even able to reach them. Samples of seed consisting only of tricotyls are very rare, and when they are met with the presumption is that they are too few to betray the rare aberrants they might otherwise contain. Experimental evidence can only be reached by the culture of a succeeding generation, and this always discloses the hidden qualities, showing that the double [422] type was only temporarily lost, but bound to return as soon as new trials are made.

This wide range of variability between definite limits is coupled with a high degree of sensibility and adequateness to the most diverging experiments. Our tricotylous double races are perhaps more sensitive to selection than any other variety, and equally dependent on outer circ.u.mstances. Here, however, I will limit myself to a discussion of the former point.

In the second generation after the isolation of stray tricotylous seedlings the average condition of the race is usually reached, but only by some of the strongest individuals, and if we continue the race, sowing or planting only from their offspring, the next generation will show the ordinary type of variability, going upwards in some and downwards in other instances. With the _Phacelia_ and the mercury and some others I had the good luck in this one generation to reach as high as nearly 90% of tricotylous seedlings, a figure indicating that the normal dicotylous type had already become rare in the race. In other cases 80% or nearly 80% was easily attained. Any further divergence from the average would have required very much larger sowings, the effect of selection between a limited number of parents being only to retain the high degree once [423] reached; so for instance with the mercury, I had three succeeding generations of selection after reaching the average of 55%, but their extremes gave no increasing advance, remaining at 86, 92 and 91%.

If we compare these results with the effects of selection in twisted and fasciated races, we observe a marked contrast. Here they reached their height at 30-40%, and no number of generations had the power of making any further improvement. The tricotyls come up in two generations to a proportion of about 54%, which shows itself to correspond to the average type. And as soon as this is reached, only one generation is required to obtain a very considerable improvement, going up to 80 or even 90%.

It is evident that the cause of this difference does not lie in the nature of the monstrosity, but is due to the criterion upon which the selection is made. Selection of the apparently best individuals is one method, and it gives admirable results. Selection on the ground of the hereditary percentages is another method and gives results which are far more advantageous than the former.

In the lecture on the pistillody of the poppies we limited ourselves to the selection of the finest individuals and showed that there is always a manifest correlation between the individual [424] strength of the plant and the degree of development of its anomaly. The same holds good with other monstrosities, and badly nourished specimens of rich races with twisted or fasciated stems always tend to reversion. This reversion, however, is not necessarily correlated with the hereditary percentage and therefore does not always indicate a lessening of the degree of inheritance. This shows that even in those cases an improvement may be expected, if only the means can be found to subject the twisted and the fasciated races to the same sharp test as the tricotylous varieties.

Much remains to be done, and the principle of the selection of parents according to the average const.i.tution of their progeny seems to be one of the most promising in the whole realm of variability.

Besides tricotylous, the syncotylous seedlings may be used in the same way. They are more rarely met with, and in most instances seem to belong only to the unpromising half-races. The black bindweed (_Polygonum Convolvulus_), the jointed charlock (Rapha.n.u.s Raphanistrum), the glaucous evening-primrose (_Oenothera glauca_) and many other plants seem to contain such half-races. On the other hand I found a plant of _Centranthus macrosiphon_ yielding as much as 55% of syncotylous children [425] and thereby evidently betraying the nature of a rich or double race. Likewise the mercury was rich in such deviations. But the best of all was the Russian sunflower, and this was chosen for closer experiments.

In the year of 1888 I had the good luck to isolate some syncotylous seedlings and of finding among them one with 19% of inheritors among its seeds. The following generation at once surpa.s.sed the ordinary average and came up in three individuals to 76, 81 and even 89%. My race was at once isolated and ameliorated by selection. I have tried to improve it further and selected the parents with the highest percentages during seven more generations; but without any remarkable result. I got figures of 90% and above, coming even in one instance up to the apparent purity of 100%. These, however, always remained extremes, the averages fluctuating yearly between 80-90% or thereabouts, and the other extremes going nearly every year downwards to 50%, the value which would be attained, if no selection were made.

Contra-selection is as easily made as normal selection. According to our present principle it means the choice of the parents with the smallest hereditary percentage. One might easily imagine that by this means the dicotylous seedlings could be rendered pure. This, however, [426] is not at all the case. It is easy to return from so highly selected figures as for instance 95% to the average about of 50%, as regression to mediocrity is always an easy matter. But to transgress this average on the lower side seems to be as difficult as it is on the upper side. I continued the experiment during four succeeding generations, but was not able to go lower than about 10%, and could not even exclude the high figures from my strain. Parents with 65-75% of syncotylous seedlings returned in each generation, notwithstanding the most careful contra-selection. The attribute is inherent in the race, and is not to be eliminated by so simple a means as selection, nor even by a selection on the ground of hereditary percentages.

We have dealt with torsions and fasciations and with seedling variations at some length, in order to point out the phases needing investigation according to recent views. It would be quite superfluous to consider other anomalies in a similar manner, as they all obey the same laws. A hasty survey may suffice to show what prospects they offer to the student of nature.

First of all come the variegated leaves. They are perhaps the most variable of all variations. They are evidently dependent on external circ.u.mstances, and by adequate nutrition the leaves may even become absolutely white or [427] yellowish, with only scarcely perceptible traces of green along the veins. Some are very old cultivated varieties, as the wintercress, or _Barbarea vulgaris_. They continuously sport into green, or return from this normal color, both by seeds and by buds.

Sports of this kind are very often seen on shrubs or low trees, and they may remain there and develop during a long series of years. Bud-sports of variegated holly, elms, chestnuts, beeches and others might be cited.

One-sided variegation on leaves or twigs with the opposite side wholly green are by no means rare. It is very curious to note that variegation is perhaps the most universally known anomaly, while its hereditary tendencies are least known.

Cristate and plumose ferns are another instance. Half races or rare accidental cleavages seem to be as common with ferns as cultivated double races, which are very rich in beautiful crests. But much depends on cultivation. It seems that the spores of crested leaves are more apt to reproduce the variety than those of normal leaves, or even of normal parts of the same leaves. But the experiments on which this a.s.sertion is made are old and should be repeated. Other cases of cleft leaves should also be tested. Ascidia are far more common than is usually believed.

Rare instances point [428] to poor races, but the magnolias and lime-trees are often so productive of ascidia as to suggest the idea of ever-sporting varieties. I have seen many hundred ascidia on one lime-tree, and far above a hundred on the magnolia. They differ widely in size and shape, including in some cases two leaves instead of one, or are composed of only half a leaf or of even still a smaller part of the summit. Rich ascidia-bearing varieties seem to offer notable opportunities for scientific pedigree-cultures.