Varieties: Their Origin by Mutation (1906)
Hugo de Vries
Every one knows the "four-leaved" clover. It is occasionally found on lawns, in pastures and by the roadsides. Specimens with five leaflets may be found now and then in the same place, or on the same plant, but these are rarer. I have often seen isolated plants with quaternate leaves, but only rarely have I observed individuals with more than one such leaf.
The two cases are essentially dissimilar. They may appear to differ but little morphologically, but from the point of view of heredity they are quite different. Isolated quaternate leaves are of but little interest, while the occurrence of many on the same individual indicates a distinct variety. In making experiments upon this point it is necessary to transplant the divergent individuals to a garden in order to furnish them proper cultural conditions and to keep them under constant observation. When a plant bearing a quaternate leaf is thus transplanted however, it rarely repeats the anomaly. But when plants with two or more quaternate leaves on the same individual are chosen it indicates that it belongs to a definite race, which under suitable conditions may prove to become very rich in the anomalies in question.
Obviously it is not always easy to decide definitely whether a given individual belongs to such a race or not. Many trials may be necessary to secure the special race. I had the good fortune to find two plants of clover, bearing one quinate and several quaternate leaves, on an excursion in the neighborhood of Loosdrecht in Holland. After transplanting them into my garden, I cultivated them during three years and observed a slowly increasing number of anomalous leaves. This number in one summer amounted to 46 quaternate and 16 quinate leaves, and it was evident that I had secured an instance of the rare "five-leaved" race which I am about to describe.
Before doing so it seems desirable to look somewhat closer into the morphological features of the problem. Pinnate and palmate leaves often vary in the number of their parts. This variability is generally of the nature of a common fluctuation, the deviations grouping themselves around an average type in the ordinary way. Ash leaves bear five pairs, and the mountain-ash (Sorbus Aucuparia) has six pairs of leaflets in addition to the terminal one. But this number varies slightly, the weaker leaves having less, the stronger more pairs than the average. Such however, is not the case with ternate leaves, which seem to be quite constant. Four leaflets occur so very rarely that one seems justified in regarding them rather as an anomaly than as a fluctuation. And this is confirmed by the almost universal absence of two-bladed clover-leaves.
Considering the deviation as an anomaly, we may look into its nature. Such an inquiry shows that the supernumerary leaflets owe their origin to a splitting of one or more of the normal ones. This splitting is not terminal, as is often the case with other species, and as it may be seen sometimes in the clover. It is for the most part lateral. One of the lateral nerves grows out becoming a median nerve of the new leaflet. Intermediate steps are not wanting, though rare, and they show a gradual separation of some lateral part of a leaflet, until this division reaches the base and divides the leaflet into two almost equal parts. If this splitting occurs in one leaflet we get the "four-leaved" clover, if it occurs in two there will be five leaflets. And if, besides this, the terminal leaflet produces a derivative on one or both of its sides, we obtain a crown of six or seven leaflets on one stalk. Such were often met with in the race I had under cultivation, but as a rule it did not exceed this limit.
The same phenomenon of a lateral doubling of leaflets may of course be met with in other instances. The common laburnum has a variety which often produces quaternate and quinate leaves, and in strawberries I have also seen instances of this abnormality. It occurs also in pinnate leaves, and complete sets of all the intermediate links may often be found on the false or bastard-acacia (Robinia Pseud-Acacia).
Opposed to this increase of the number of leaflets, and still more rare and more curious is the occurrence of "single-leaved" varieties among trees and herbs with pinnate or ternate leaves. Only very few instances have been described, and are cultivated in gardens. The ashes and the bastard-acacia may be quoted among trees, and the "one-leaved" strawberry among herbs. Here it seems that several leaflets have been combined into one, since this one is, as a rule, much larger than the terminal leaflet of an ordinary leaf of the same species. These monophyllous varieties are interesting also on account of their continuous but often incomplete reversion to the normal type.
Pinnate and palmate leaves are no doubt derivative types. They must have originated from the ordinary simple leaf. The monophylly may therefore be considered as a reversion to a more primitive state and the monophyllous varieties may be called atavistic.
On the other hand we have seen that these atavistic varieties may revert to their nearest progenitors, and this leads to the curious conception of positive and negative atavism. For if the change of compound leaves into single ones is a retrograde or negative step, the conversion of single or ternate leaves into pinnate and palmate ones must evidently be considered in this case as positive atavism.
This discussion seems to throw some light on the increase of leaflets in the clover. The pea-family, or the group of papilionaceous plants, has pinnate leaves ordinarily, which, according to our premises, must be considered as a derivative type. In the clovers and their allies this type reverts halfway to the single form, producing only three leaflets on each stalk. If now the clover increases its number of leaflets, this may be considered as a reversion to its nearest progenitors, the papilionaceous plants with pinnate leaves. Hence a halfway returning and therefore positive atavism. And as I have already mentioned in a former lecture, pinnate leaves are also sometimes produced by my new race of clover.
Returning to the original plants of this race, it is evidently impossible to decide whether they were really the beginning of a new strain, and had originated themselves by some sudden change from the common type, or whether they belonged to an old variety, which had propagated itself perhaps during centuries, unobserved by man. But the same difficulty generally arises when new varieties are discovered. Even the behavior of the plants themselves or of their progeny does not afford any means of deciding the question. The simplest way of stating the matter therefore, is to say that I accidentally found two individuals of the '' five-leaved'' race. By transplanting them into my garden, I have isolated them and kept them free from cross-fertilization with the ordinary type. Moreover, I have brought them under such conditions as are necessary for the full development of their characters. And last but not least, I have tried to improve this character as far as possible by a very rigid and careful selection.
The result of all this effort has been a rapid improvement of my strain. I saved the seed of the original plants in 1889 and cultivated the second generation in the following year. It showed some increase of the anomaly, but not to a very remarkable degree. In the flowering period I selected four plants with the largest number of quaternate and quinate leaves and destroyed all the others. I counted in the average 25 anomalous organs on each of them. From their seed I raised the third generation of my culture in the year 1891.
This generation included some 300 plants, on which above 8000 leaves were counted. More than 1000 were quaternate or quinate, the ternate leaves being still in the majority. But the experiment clearly showed that'' four-leaved '' clovers may be produced in any desired quantity, provided that the seed of the variety is available. In the summer only three, four and five leaflets on one stalk were seen, but towards the fall, and after the selection of the best individuals, this number increased and came up to six and seven in some rare instances.
The selection in this year was by no means easy. Nearly all the individuals produced at least some quaternate leaves, and thereby showed the variety to be quite pure. I counted the abnormal organs on a large group of the best plants, and selected 20 excellent specimens from them, with more than one-third of all their leaves changed in the desired manner.
Having brought my race up to this point, I was able to introduce a new and far more easy mark, afforded by the seedlings, for my selections. This mark has since remained constant, and has brought about a rapid continuance of the improvement, without necessitating such large cultures.
This seedling in the various species of clover usually begins with a first leaf above the cotyledons of a different structure from those that follow. It has only one blade instead of three. But in my variety the increase of the number of the leaflets may extend to these primary organs, and make them binate or even ternate. Now it is obvious that an individual, which begins with a divided primary leaf, will have a greater tendency to produce a large number of supernumerary leaflets than a plant which commences in the ordinary way. Or in other words, the primary leaves afford a sure criterion for the selection, and this selection may be made in the seed-pans. In consequence, no young individual with an undivided primary leaf was planted out. Choosing the 20 or 30 best specimens in the seed-pan, no further selection was required, and the whole lot could be left to cross-fertilization by insects.
The observation of this distinguishing mark in the young seedlings has led to the discovery of another quality as a starting-point for further selection. According to the general rule of pedigree-culture, the seeds of each individual plant are always saved and sowed separately. This is done even with such species as the clover, which are infertile when self-pollinated, and which are incapable of artificial pollination on the required scale, since each flower produces only one seed. My clover was always left free to be pollinated by insects. Obviously this must have led to a diminution of the differentiating characters of the individual plants. But this does not go far enough to obliterate the differences, and the selection made among the seedlings will always throw out at least a large part of those that have suffered from the cross.
Leaving this discussion, we may inquire closer into the nature of the new criterion afforded by the seedlings. Two methods present themselves. First, the choice of the best seedlings. In the second place it becomes possible to compare the parent-plants by counting the number of deviating seedlings. This leads to the establishment of a percentage for every single parent, and gives data for comparisons. Two or three hundreds of seeds from a parent may easily be grown in one pan, and in this way a sufficiently high degree of accuracy may be reached. Only those parents that give the highest percentage are chosen, and among their progeny only the seedlings with trifoliolate primary leaves are planted out. The whole procedure of the selection is by this means confined to the glasshouse during the spring, and the beds need not be large, nor do they require any special care during the summer.
By this method I brought my strain within two years up to an average of nearly 90% of the seedlings with a divided primary leaf. Around this average the real numbers fluctuated between the maximum of 99% and the minimum of 70% or thereabouts. This condition was reached by the sixth generation in the year 1894, and has since proved to be the limit, the group of figures remaining practically the same during all the succeeding generations.
Such selected plants are very rich in leaves with four, five and six blades. Excluding the small leaves at the tops of the branches, and those on the numerous weaker side-branches, these three groups include the large majority of all the stronger leaves. In summer the range is wider, and besides many trifoliolate leaves the curiously shaped seven-bladed ones are not at all rare. In the fall and in the winter the range of variability is narrowed, and at first sight the plants often seem to bear only quinquefoliolate leaves.
I have cultivated a new generation of this race nearly every year since 1894, using always the strictest selection. This has led to a uniform type, but has not been adequate to produce any further improvement. Obviously the extreme limit, under the conditions of climate and soil, has been reached. This extreme type is always dependent upon repeated selection. No constant variety, in the older sense, has been obtained, nor was any indication afforded that such a type might ever be produced. On the contrary, it is manifest that the new form belongs to the group of ever-sporting varieties. It is never quite free from the old atavistic type of the trifoliolate leaves, and invariably, when external conditions become less favorable, this atavistic form is apt to gain dominion over the more refined varietal character. Reversions always occur, both partial and individual.
Some instances of these reversions may now be given. They are not of such a striking character as those of the snapdragon. Intermediate steps are always occurring, both in the leaves themselves, and in the percentages of deviating seedlings of the several parent plants.
On normal plants of my variety the quinquefoliolate leaves usually compose the majority, when there are no weak lateral branches, or when they are left out of consideration. Next to these come the fours and the sixes, while the trifoliolate and seven-bladed types are nearly equal in number. But out of a lot of plants, grown from seed of the same parent, it is often possible to choose some in which one extreme prevails, and others with a preponderating number of leaves with the other extreme number of leaflets. If seed from these extremes are saved separately, one strain, that with numerous seven-bladed leaves will remain true to the type, but the other will diverge more or less, producing leaves with a varying number of subdivisions.
Very few generations of such opposite selection are required to reduce the race to an utterly poor one. In three years I was able to nearly obliterate the type of my variety. I chose the seedlings with an undivided primary leaf, cultivated them and counted their offspring separately after the sowing. I found some parents with only 2-3% of seedlings with divided primary leaves. And by a repeated selection in this retrograde direction I succeeded in getting a great number of plants, which during the whole summer made only very few leaves with more than three blades. But an absolute reversion could no more be reached in this direction than in the normal one. Any sowing without selection would be liable to reduce the strain to an average condition.
The production of varietal and of atavistic leaves is dependent to a high degree on external conditions. It agrees with the general rule, that favorable circumstances strengthen the varietal peculiarities, while unfavorable conditions increase the number of the parts with the atavistic attribute. These influences may be seen to have their effect on the single individuals, as well as on the generations growing from their seed. I cannot cite here all the experimental material, but a single illustrative example may be given. I divided a strong individual into two parts, planted one in rich soil and the other in poor sand, and had both pollinated by bees with the pollen of some normal individuals of my variety growing between them. The seeds of both were saved and sown separately, and the two lots of offspring cultivated close to each other under the same external conditions. In the beginning no difference was seen, but as soon as the young plants had unfolded three or four leaves, the progeny of the better nourished half of the parent-plant showed a manifest advance. This difference increased rapidly and was easily seen in the beds, even before the flowering period.
This experience probably gives an explanation why the quinquefoliolate variety is so seldom met with in the wild state. For even if it did occur more often, the plants would hardly find circumstances favorable enough for the full development of their varietal character. They must often be so poor in anomalous leaves as to be overlooked, or to be taken for instances of the commonly occurring quadrifoliolate leaves and therefore as not indicating the true variety.
In the beginning of my discussion I have asserted the existence of two different races of "four-leaved" clovers, a poor one and a rich one, and have insisted on a sharp distinction between them. This distinction partly depends on experiments with clover, but in great part on tests with other plants. The previously mentioned circumstance, that clover cannot be pollinated on a sufficiently large scale otherwise than by insects, prevents trials in more than one direction at the same time and in the same garden. For this reason I have chosen another species of clover to be able to give proof or disproof of the assertion quoted.
This species is the Italian, or crimson clover, which is sometimes also called scarlet clover (Trifolium incarnatum). It is commonly used in Europe as a crop on less fertile soils than are required by the red clover. It is annual and erect and more or less hairy, and has stouter leaves than other kinds of clover. It has oblong or cylindrical heads with bright crimson flowers, and may be considered as one of the most showy types. As an annual it has some manifest advantages over the perennial species, especially in giving its harvest of hay at other seasons of the year.
I found some stray quaternate leaves of this plant some years ago, and tried to win from them, through culture and selection, a race that would be as rich in these anomalies as the red clover. But the utmost care and the most rigid selection, and all the attention I could afford, failed to produce any result. It is now ten years since I commenced this experiment, and more than once I have been willing to give it up. Last year (1903) I cultivated some hundreds of selected plants, but though they yielded a few more instances of the desired anomaly than in the beginning, no trace of a truly rich race could be discovered. The experimental evidence of this failure shows at least that stray "four-leaves" may occur, which do not indicate the existence of a true "four-" or "five-leaved" variety.
This conception seems destined to become of great value in the appreciation of anomalies, as they are usually found, either in the wild state or in gardens. And before describing the details of my unsuccessful pedigree-culture, it may be as well to give some more instances of what occurs in nature.
Stray anomalies are of course rare, but not so rare that they might not be found in large numbers when perseveringly sought for. Pitcher-like leaves may be found on many trees and shrubs and herbs, but ordinarily one or only two of them are seen in the course of many years on the same plant, or in the same strain. In some few instances they occur annually or nearly so, as in some individuals of the European lime-tree (Tilia parvifolia) and of the common magnolia (Magnolia obovata). Many of our older cultivated plants are very rich in anomalies of all kinds, and Cyclamen, Fuchsia, Pelargonium and some others are notorious sources of teratologic phenomena. Deviations in flowers may often be seen, consisting of changes in the normal number of the several organs, or alterations in their shape and color. Leaves may have two tips, instead of one, the mid-vein being split near the apex, and the fissure extending more or less towards the base. Rays of the umbels of umbelliferous plants may grow together and become united in groups of two or more, and in the same way the fruits of the composites may be united into groups. Many other instances could easily be given.
If we select some of these anomalies for breeding-experiments, our results will not agree throughout, but will tend to group themselves under two heads. In some cases the isolation of the deviating individuals will at once show the existence of a distinct variety, which is capable of producing the anomaly in any desired number of instances, only dependent on a favorable treatment and a judicious selection. In other cases no treatment and no selection are adequate to give a similar result, and the anomaly remains refractory despite all our endeavors to breed it. The cockscomb and the peloric fox-glove are widely known instances of permanent anomalies, and others will be dealt with in future lectures. On the other hand I have often tried in vain to win an anomalous race from an accidental deviation, or to isolate a teratologic variety out of more common aberrations. Two illustrative examples may be quoted. In our next lecture we shall deal with a curious phenomenon in poppies, consisting in the change of the stamens into pistils and giving rise to a bright crown of secondary capsules around the central one. Similar anomalies may be occasionally met with in other species of the same genus. But they are rare, and may show the conversion of only a single stamen in the described manner. I observed this anomaly in a poppy called Papaver commutatum, and subjected it during several years to a rigid selection of the richest individuals. No amelioration was to be gained and the culture had to be given up. In the same way I found on the bulbous buttercup (Ranunculus bulbosus) a strain varying largely in the number of the petals, amounting often to 6-8, and in some flowers even yet to higher figures. During five succeeding years I cultivated five generations, often in large numbers, selecting always those which had the highest number of petals, throwing out the remainder and saving the seed only from the very best plants. I got a strain of selected plants with an average number of nine petals in every flower, and found among 4000 flowers four having 20 petals or more, coming up even to 31 in one instance. But such rare instances had no influence whatever on the selection, since they were not indicative of individual qualities, but occurred quite accidentally on flowers of plants having only the average number of petals. Now double flowers are widely known to occur in other species of the buttercups, both in the cultivated varieties and in some wild forms. For this reason it might be expected that through a continuous selection of the individuals with the largest numbers a tendency to become double would be evolved. Such, however, was not the case. No propensity to vary in any definite direction could be observed. Quite on the contrary, an average condition was quickly reached, and then remained constant, strongly counteracting all selection.
Such experiences clearly show that the same anomaly may occur in different species, and no doubt in strains of the same species from different localities, according to at least two different standards. The one is to be called the poor, and the other the rich variety. The first always produces relatively few instances of the deviation, the last is apt to give as many of them as desired. The first is only half-way a variety, and therefore would deserve the name of a half-race ; the second is not yet a full constant variety, but always fluctuates to and fro between the varietal and the specific mark, ever-sporting in both directions. It holds a middle position between a half-race and a variety, and therefore might be called a "middle-race." But the term ever-sporting variety seems more adequate to convey a right idea of the nature of this curious type of inheritance.
From this discussion it will be seen that the behavior of the crimson clover is not to be considered as an exception, but as a widely occurring type of phenomenon, occurring perhaps in all sorts of teratologic deviations, and in wide ranges of species and genera. Hence it may be considered worth while to give some more details of this extended experiment.
Ten years ago (1894-5) I bought and sowed about a pound of seed of the crimson clover. Among many thousands of normal seedlings I found two with three and one with four cotyledons. Trusting to the empirical rules of correlation, I transplanted these three individuals in order to isolate them in the flowering period. One of them produced during the ensuing summer one four-bladed and one five-bladed leaf. The seeds were saved separately and sown the following spring and the expected result could soon be seen. Among some 250 individual plants I counted 22 with one or two deviations, and 10 with from three to nine four- or five-bladed leaves. Proportions nearly similar have been observed repeatedly. Better nourished individuals have produced more deviating leaves on one plant, partly owing to the larger number of stems and branches, and poor or average specimens have mostly been without any aberration or with only one or two abnormal leaves. No further improvement could be attained. Quadrifoliolate leaves were always rare, never attaining a number that would put its stamp on a whole bed. I have endeavored to get some six- and seven-bladed crimson clover leaves, but in vain; selection, culture of many hundreds of individuals, manure, and the best possible treatment has not been adequate to produce them. Of course I am quite convinced that a repetition of my experiment on a far larger scale would yield the desired types, but then only in such rare instances that they would have no influence whatever on the average, or on the improvement of the race. The eighth generation in the year 1903 has not been noticeably better than the second and third generations after the first selection.
In comparing this statement with the results gained in the experiment with the red clover, the difference is at once striking. In one case a rich variety was isolated, and, by better treatment and sharp methods of selection, was brought up in a few years to its highest pitch of development. In the other case a very weak race was shown to exist, and no amount of work and perseverance was adequate to improve it to any noticeable degree.
I wish to point out that the decision of what is to be expected from deviating specimens may become manifest within one or two generations. Even the generation grown from the seeds of the first observed aberrant individuals, if gathered after sufficient isolation during the period of blossoming, may show which type of inheritance is present, whether it is an unpromising half-race, or a richly endowed sporting variety. I have kept such strains repeatedly after the first isolation, and a special case, that of cotyledoneous aberrations, will be dealt with later. The first generation always gave a final decision, provided that a suitable method of cultivation for the species under observation was found at the beginning. This however, is a condition, which it is not at all easy to comply with, when new sorts are introduced into a garden. Especially so when they had been collected in the wild state. Often one or two years, sometimes more, are necessary to find the proper method of sowing, manuring, transplanting and other cultural methods satisfactory to the plants. Many wild species require more care and more manure in gardens than the finest garden flowers. And a large number are known to be dependent on very particular conditions of soil.
One of the most curious features of anomalies, which has been learned from accumulated instances, is the fact that they obey definite laws as to their occurrence on the different parts of the plant. Obviously such laws are not apparent as long as each plant produces only one or two, or, at most, a few instances of the same deviation. On the contrary, any existing regularity must betray itself, as soon as a larger number of instances is produced. A rule of periodicity becomes most clearly manifest in such cases.
This rule is shown by no other race in a more undoubted and evident manner than by the "five-leaved" clover. Evidently the several degrees of deviation, going from three to seven leaflets, may be regarded as responses to different degrees of variation, and their distribution over the stems and branches, or over the whole plant, may be considered as the manifestation of the ever-changing internal tendency to vary.
Considered from this point of view, my plants always showed a definite periodicity in this distribution, which is the same for the whole plant. Each of them, and each of the larger branches, begin with atavistic leaves or with slight deviations. These are succeeded by greater deviations, but only the strongest axes show as many as seven leaflets on a stalk. This ordinarily does not occur before the height of development is reached, and often only towards its close. Then the deviation diminishes rapidly, returning often to atavistic leaves at the summit of the stem or branch. I give the numbers of the leaves of a branch, in their order from the base to the top. They were as follows:
3. 4. 5. 6. 7. 5. 5. 4.
But this is a selected case, and such regular examples of the expected periodicity are rarely found. Often one or more of the various steps are lacking, or even leaves with smaller numbers may be interspersed among those with larger numbers of leaflets. But while the regularity of the periodicity is in some degree diminished by such occurrences, yet the rule always holds good, when taken broadly. It may be expressed by stating that the bases and apices have on the average fewer leaflets on each leaf than the middle parts of the stem and branches, and that the number of leaflets gradually increases from the base toward a maximum, which is reached in organs on the middle or upper part of the axis, and then diminishes from this toward the apex.
This periodicity is not limited to the stems and branches, considered singly, but also holds good in a comparison made between the branches of a single stem, in regard to their relative places on that stem. So it is also for the whole plant. The first stems, produced by the subterranean axis, ordinarily show only a low maximum deviation: the next succeeding being more divergent and the last ones returning to less differentiated forms.
It is evident that on a given stem the group of deviating leaves will be extended upward and downward, with the increase of the number of these organs. This shows that a stem, or even a plant, promises a higher degree of differentiation if it commences with its aberration earlier. Hence it becomes possible to discern the most promising individuals in early youth, and this conclusion leads to a very easy and reliable method of selection, which may be expressed simply as follows: the seedlings which commence earliest with the production of four- and five-foliolate leaves are the best and should be selected for the continuance of the race. And it is easily seen that this rule agrees with that given above, and which was followed in my pedigree-culture.
Furthermore it is seen that there is a complete agreement between the law of periodicity and the responses of the deviations to nourishment and other conditions of life. Weak plants only produce low degrees of deviation, the stronger the individual becomes, the higher it reaches in the scale of differentiation, and the more often it develops leaves with five or more blades. Whether weakness or strength are derived from outer causes, or from the internal succession of the periods of life, is evidently of no consequence, and in this way the law of periodicity may be regarded as a special instance of the more general law of response to external conditions.
The validity of this law of periodicity is of course not limited to our "five-leaved" clover. Quite on the contrary it is universal in ever-sporting varieties. Moreover it may be ascertained and studied in connection with the most widely different morphologic abnormalities, and therefore affords easily accessible material for statistical inquiry. I will now give some further instances, but wish to insist first upon the necessity of an inquiry on a far larger scale, as the evidence as yet is very scanty.
The great celandine (Chelidonium majus) has a very curious double variety. Its flowers are simpler and much more variable than in ordinary garden-varieties. The process of doubling consists mainly in a change of stamens into petals. This change is dependent on the season. On each stem the earliest flowers are single. These are succeeded by blossoms with one or two converted stamens, and towards the summer this number increases gradually, attaining 10-11 and in some instances even more altered filaments. Each year the same succession may be seen repeating itself on the stems of the old roots. Double tuberous begonias are ordinarily absolutely sterile throughout the summer, but towards autumn the new flowers become less and less altered, producing some normal stamens and pistils among the majority of metamorphosed organs. From these flowers the seeds are saved. Sometimes similar flowers occur at the beginning of the flowering-period. Double garden-camomiles (Chrysanthemum inodorum plenissimum) and many other double varieties of garden-plants among the great family of the composites are very sensitive to external agencies, and their flower-heads are fuller the more favorable the external conditions. Towards the autumn many of them produce fewer and fewer converted heads and often only these are fertile and yield seeds.
Ascidia afford another instance of this periodicity, though ordinarily they are by far too rare to show any regularity in their distribution. However, it is easy to observe that on lime-trees they prefer the lower parts of each twig, while on magnolias the terminal leaves of the branches are often pitcher-bearing. Ascidia of the white clover have been found in numbers, in my own experiment-garden, but always in the springtime. The thick-leaved saxifrage (Saxifraga crassifolia) is often very productive of ascidia, especially in the latter part of the season, and as these organs may be developed to very different degrees, they afford fine material for the study of the law of periodicity. On a garden-cytisus (Cytisus candicans attleyanus) I once had the good fortune to observe a branch with ascidia, which ordinarily are very rare in this species. It had produced seven ascidia in all, each formed by the conversion of one leaflet on the trifoliolate leaves. The first six leaves were destitute of this malformation and were quite normal. Then followed a group of five leaves, constituting the maximum of the period. The first bore one small pitcher-like blade, the second and third, each one highly modified organ, the fourth, two ascidia, and the last, one leaflet with slightly connate margins. The whole upper part of the branch was normal, with the exception of the seventeenth leaf, which showed a slight change in the same direction. All in all, the tendency to produce ascidia increased from the beginning to the tenth leaf, and decreased from this upward.
The European Venus' looking-glass was observed in my garden to produce some quaternate and some quinate flowers on the same specimens. The quinate were placed at the end of the branches, those with four petals and sepals lower down. The peloric fox-glove shows the highest degree of metamorphy in the terminal flowers of the stem itself, the weaker branches having but little tendency towards the formation of the anomaly. The European pine or Pinus sylvestris ordinarily has two needles in each sheath, but trifoliolate sheaths occur on the stems and stronger branches, where they prefer, as a rule, the upper parts of the single annual shoots. Camellia japonica is often striped in the fall and during the winter, but when flowering in the spring it returns to the monochromatic type.
Peloric flowers are terminal in some cases, but occur in the lower parts of the flower-spikes in others. Some varieties of gladiolus commence on each spike with more-or-less double flowers, which, higher up, are replaced by single ones. A wide range of bulbs and perennial garden-plants develop their varietal characters only partly when grown from seed and flowering for the first time. The annual garden-forget-me-not of the Azores (Myosotis azorica) has a variety with curiously enlarged flowers, often producing 20 or more corolla-segments in one flower. But this number gradually diminishes as the season advances. It would be quite superfluous to give further proof of the general validity of the law of periodicity in ever-sporting varieties.