Chapter 37

Of late the same mutation has occurred in the garden of C.A. White at Washington. The parent form in this case was the "Acme," of the ordinary weak and spreading habit of growth. It is known as one of the best and most stable of the varieties and was grown by Mr. White for many years, and had not given any sign of a tendency towards change. Seeds from some of the best plants in 1899 were sown the following spring, and the young seedlings unexpectedly exhibited a marked difference from their parents. From the very outset they were more strong and erect, more compact and of a darker green than the "Acme." When they reached the fruiting stage they had developed into typical representatives of the _Lycopersicum solanopsis_ or upright division. The whole lot of plants comprised only some 30 specimens, and this number, of course, is too small to base far-reaching conclusions upon. But all of the lot showed this type, no true "Acme" being seen among them. The fruit differed in flavor, consistency and color from that of the parent, and it also ripened earlier than the latter. No seed was saved from [657] these plants, but the following year the "Acme" was sown again and found true to its type. Seeds saved from this generation in 1900 have, however, repeated the mutation, giving rise to exactly the same new upright form in 1901. This was called by its originator "The Washington." Seeds from this second mutation were kindly sent to me by Mr. White, and proved true to their type when sown in my garden.

Obviously it is to be assumed in the case of the tomatoes as well as in instances from other genera cited, that characters of ancestors, which are not displayed in their progeny, have not been entirely lost, but are still present, though in a latent condition. They may resume their activity unexpectedly, and at once develop all the features which they formerly had borne.

Latency, from this point of view, must be one of the most common things in nature. All organisms are to be considered as internally formed of a host of units, partly active and partly inactive. Extremely minute and almost inconceivably numerous, these units must have their material representatives within the most intimate parts of the cells.

[658] LECTURE XXIII

TAXONOMIC ANOMALIES

The theory of descent is founded mainly on comparative studies, which have the advantage of affording a broad base and the convincing effect of concurrent evidence brought together from widely different sources. The theory of mutation on the other hand rests directly upon experimental investigations, and facts concerning the actual descent of one form from another are as yet exceedingly rare. It is always difficult to estimate the validity of conclusions drawn from isolated instances selected from the whole range of contingent phenomena, and this is especially true of the present case. Systematic and physiologic facts seem to indicate the existence of universal laws, and it is not probable that the process of production of new species would be different in the various parts of the animal and vegetable kingdoms. Moreover the principle of unit-characters, the preeminent significance of which has come to be more fully recognized of late, is in full harmony [659] with the theory of sudden mutations. Together these two conceptions go to strengthen the probability of the sudden origin of all specific characters.

Experimental researches are limited in their extent, and the number of cases of direct observation of the process of mutation will probably never become large enough to cover the whole field of the theory of descent. Therefore it will always be necessary to show that the similarity between observed and other cases is such as to lift above all doubt the assertion of their resulting from the same causes.

Besides the direct comparison of the mutations described in our former lectures, with the analogous cases of the horticultural and natural production of species and varieties at large, another way is open to obtain the required proof. It is the study of the phenomena, designated by Casimir de Candolle by the name of taxonomic anomalies. It is the assertion that characters, which are specific in one case, may be observed to arise as anomalies or as varieties in other instances. If they can be shown to be identical or nearly so in both, it is obviously allowable to assume the same origin for the specific character and for the anomaly. In other terms, the specific marks may be considered as having originated according to the laws [660] that govern the production of anomalies, and we may assume them to lie within reach of our experiments. The experimental treatment of the origin of species may also be looked upon as a method within our grasp.

The validity and the significance of these considerations will at once become clear, if we choose a definite example. The broadest and most convincing one appears to me to be afforded by the cohesion of the petals in gamopetalous flowers. According to the current views the families with the petals of their flowers united are regarded as one or two main branches of the whole pedigree of the vegetable kingdom. Eichler and others assume them to constitute one branch, and therefore one large subdivision of the system. Bessey, on the other hand, has shown the probability of a separate origin for those groups which have inferior ovaries. Apart from such divergencies the connation of the petals is universally recognized as one of the most important systematic characters.

How may this character have originated? The heath-family or the Ericaceae and their nearest allies are usually considered to be the lowest of the gamopetalous plants. In them the cohesion of the petals is still subject to reversionary exceptions. Such cases of atavism may [661] be observed either as specific marks, or in the way of anomalies. _Ledum_, _Monotropa_ and _Pyrola_, or the Labrador tea, the Indian pipe and wintergreen are instances of reversionary gamopetalism with free petals. In heaths (_Erica Tetralix_) and in rhododendrons the same deviation is observed to occur from time to time as an anomaly, and even the common _Rhododendron ponticum_ of our gardens has a variety in which the corolla is more or less split. Sometimes it exhibits five free petals, while at other times only one or two are entirely free, the remaining four being incompletely loosened.

Such cases of atavism make it probable that the coherence of the petals has originally arisen by the same method, but by action in the opposite direction. The direct proof of this conclusion is afforded by a curious observation, made by Vilmorin upon the bright and large-flowered garden-poppy, _Papaver bracteatum_. Like all poppies it has four petals, which are free from one another. In the fields of Messrs. Vilmorin, where it is largely cultivated for its seeds, individuals occur from time to time which are anomalous in this respect. They exhibit a tendency to produce connate petals. Their flowers become monopetalous, and the whole strain is designated by the name of _Papaver_ [662] _bracteatum monopetalum_. Henry de Vilmorin had the kindness to send me some of these plants, and they have flowered in my garden during several years. The anomaly is highly variable. Some flowers are quite normal, exhibiting no sign of connation; others are wholly gamopetalous, the four petals being united from their base to the very margin of the cup formed. In consequence of the broadness of the petals however, this cup is so wide as to be very shallow.

Intermediate states occur, and not infrequently. Sometimes only two or three petals are united, or the connation does not extend the entire length of the petals. These cases are quite analogous to the imperfect splitting of the corolla of the rhododendron. Giving free rein to our imagination, we may for a moment assume the possibility of a new subdivision of the vegetable kingdom, arising from Vilmorin's poppy and having gamopetalous flowers for its chief character. If the character became fixed, so as to lose its present state of variability, such a group of supposititious gamopetalous plants might be quite analogous to the corresponding real gamopetalous families. Hence there can be no objection to the view, that the heaths have arisen in an analogous manner from their polypetalous ancestors. Other species of [663] the same genus have also been recorded to produce gamopetalous flowers, as for instance, _Papaver hybridum_, by Hoffmann. Poppies are not the sole example of accidental gamopetaly. Linnaeus observed the same deviation long ago for _Saponaria officinalis_, and since, it has been seen in _Clematis Vitalba_ by Jaeger, in _Peltaria alliacea_ by Schimper, in _Silene annulata_ by Boreau and in other instances. No doubt it is not at all of rare occurrence, and the origin of the present gamopetalous families is to be considered as nothing extraordinary. It is, as a matter of fact, remarkable that it has not taken place in more numerous instances, and the mallows show that such opportunities have been available at least more than once.

Other instances of taxonomic anomalies are afforded by leaves. Many genera, the species of which mainly bear pinnate or palmate leaves, have stray types with undivided leaves. Among the brambles, _Rubus odoratus_ and _R. flexuosus_ may be cited, among the aralias, _Aralia crassifolia_ and _A. papyrifera_, and among the jasmines, the deliciously scented sambac (_Jasminum Sambac_). But the most curious instance is that of the telegraph-plant, or _Desmodium gyrans_, each complete leaf of which consists of a large terminal leaflet and two little lateral ones. These latter keep up, [664] night and day, an irregular jerking movement, which has been compared to the movements of a semaphore. _Desmodium_ is a papilionaceous plant and closely allied to the genus _Hedysarum_, which has pinnate leaves with numerous pairs of leaflets. Its place in the system leaves no doubt concerning its origin from pinnate-leaved ancestors. At the time of its origination its leaves must have become reduced as to the number of the blades, while the size of the terminal leaflet was correspondingly increased.

It might seem difficult to imagine this great change taking place suddenly. However, we are compelled to familiarize ourselves with such hypothetical assumptions. Strange as they may seem to those who are accustomed to the conception of continuous slow improvements, they are nevertheless in complete agreement with what really occurs. Fortunately the direct proof of this assertion can be given, and in a case which is narrowly related, and quite parallel to that of the _Desmodium_, since it affects a plant of the same family. It is the case of the monophyllous variety of the bastard-acacia or _Robinia Pseud-Acacia_. In a previous lecture we have seen that it originated suddenly in a French nursery in the year 1855. It can be propagated by seed, and exhibits a curious degree [665] of variability of its leaves. In some instances these are one-bladed, the blade reaching a length of 15 cm., and hardly resembling those of the common bastard-acacia. Other leaves produce one or two small leaflets at the base of the large terminal one, and by this contrivance are seen to be very similar to those of the _Desmodium_, repeating its chief characters nearly exactly, and only differing somewhat in the relative size of the various parts. Lastly real intermediates are seen between the monophyllous and the pinnate types. As far as I have been able to ascertain, these are produced on weak twigs under unfavorable conditions; the size of the terminal leaflet decreases and the number of the lateral blades increases, showing thereby the presence of the original pinnate type in a latent condition.

The sudden origin of this "one-leaved" acacia in a nursery may be taken as a prototype of the ancient origin of _Desmodium_. Of course the comparison only relates to a single character, and the movements of the leaflets are not affected by it. But the monophylly, or rather the size of the terminal blade and the reduction of the lateral ones, may be held to be sufficiently illustrated by the bastard-acacia. It is worth while to state, that analogous varieties have also arisen in other genera. The "one-leaved" [666] strawberry has already been referred to. It originated from the ordinary type in Norway and at Paris. The walnut likewise, has its monophyllous variety. It was mentioned for the first time as a cultivated tree about 1864, but its origin is unknown. A similar variety of the walnut, with "one-bladed" leaves but of varying shapes, was found wild in a forest near Dieppe in France some years ago, and appeared to be due to a sudden mutation.

Something more is known concerning the "one-bladed" ashes, varieties of which are often seen in our parks and gardens. The common form has broad and deeply serrate leaves, which are far more rounded than the leaflets of the ordinary ash. The majority of the leaves are simple, but some produce one or two smaller leaflets at their base, closely corresponding in this respect to the variations of the "one-bladed" bastard-acacia, and evidently indicating the same latent and atavistic character. In some instances this analogy goes still further, and incompletely pinnate leaves are produced with two or more pairs of leaflets. Besides this variable type another has been described by Willdenow. It has single leaves exclusively, never producing smaller lateral leaflets, and it is said to be absolutely constant from seed, while the more variable types [667] seem to be also more inconstant when propagated sexually. The difference is so striking and affords such a reliable feature that Koch proposed to make two distinct varieties of them, calling the pure type _Fraxinus excelsior monophylla_, and the varying trees _F. excelsior exheterophylla_. Some writers, and among them Willdenow, have preferred to separate the "one-leaved" forms from the species, and to call them _Fraxinus simplicifolia_.

According to Smith and to Loudon, the "one-leaved" ashes are found wild in different districts in-England. Intermediate forms have not been recorded from these localities. This mode of origin is that already detailed for the laciniate varieties of alders and so many other trees. Hence it may be assumed that the "one-leaved" ashes have sprung suddenly but frequently from the original pinnate species. The pure type of Willdenow should, in this case, be considered as due to a slightly different mutation, perhaps as a pure retrograde variety, while the varying strains may only be eversporting forms. This would likewise explain part of their observed inconstancy.

In this respect the historic dates, as collected by Korshinsky, are not very convincing. Vicinism has of course, almost never been excluded, and part of the multiformity of the offspring [668] must obviously be due to this most universal agency. Indirect vicinism also plays some part, and probably affords the explanation of some reputed mutative productions of the variety. So, for instance, in the case of Sinning, who after sowing the seeds of the common ash, got as large a proportion as 2% of monophyllous trees in a culture of some thousand plants. It is probable that his seeds were taken partly from normal plants, and partly from hybrids between the normal and the "one-bladed" type, assuming that these hybrids have pinnate leaves like their specific parent, and bear the characters of the other parent only in a latent condition.

Our third example relates to peltate leaves. They have the stalk inserted in the middle of the blade, a contrivance produced by the connation of the two basal lobes. The water-lilies are a well known instance, exhibiting sagittate leaves in the juvenile stage and changing in many species, into nearly circular peltate forms, of which _Victoria regia_ is a very good example, although its younger stages do not always excite all the interest they deserve. The Indian cress (_Tropaeolum_), the marsh pennywort or _Hydrocotyle_, and many other instances could be quoted. Sometimes the peltate leaves are not at all orbicular, but are elongated, oblong or elliptic, and with only the lobes [669] at the base united. The lemon-scented _Eucalyptus citriodora_ is one of the most widely known cases. In other instances the peltate leaves become more or less hollow, constituting broad ascidia as in the case of the crassulaceous genus _Umbilicus_.

This connation of the basal lobes is universally considered as a good and normal specific character. Nevertheless it has its manifest analogy in the realm of the anomalies. This is the pitcher or ascidium. On some trees it is of quite common occurrence, as on the lime-tree (_Tilia parvifolia_) and the magnolia (_Magnolia obovata_ and its hybrids). It is probable that both these forms have varieties with, and others without, ascidia. Of the lime-tree, instances are known of single trees which produce hundreds of such anomalous leaves yearly, and one such a tree is growing in the neighborhood of Amsterdam at Lage Vuursche. I have alluded to these cases more than once, but on this occasion a closer inspection of the structure of the ascidium is required. For this purpose we may take the lime-tree as an example. Take the shape of the normal leaves in the first place. These are cordate at their base and mainly inequilateral, but the general shape varies to a considerable extent. This variation is closely related to the position of the leaves on the twigs, and shows [670] distinct indications of complying with the general law of periodicity. The first leaves are smaller, with more rounded lobes, the subsequent leaves attain a larger size, and their lobes slightly change their forms. In the first leaves the lobes are so broad as to touch one another along a large part of their margins, but in organs formed later this contact gradually diminishes and the typical leaves have the lobes widely separated. Now it is easily understood that the contact or the separation of the lobes must play a part in the construction of the ascidia, as soon as the margins grow together. Leaves which touch one-another, may be affected by the connation without any further malformation. They remain flat, become peltate and exhibit a shape which in some way holds a middle position between the pennyworts and the lemon-scented eucalyptus. Here we have the repetition of the specific characters of these plants by the anomaly of another. Whenever the margins are not in contact, and become connate, notwithstanding their separation, the blade must be folded together in some slight degree, in order to produce the required contact. This is the origin of the ascidium. It is quite superfluous to insist upon the fact that their width or narrowness must depend upon the corresponding normal form. The more distant the [671] lobes, the deeper the ascidium will become. It should be added that this explanation of the different shapes of ascidia is of general validity.

Ascidia of the snake-plantain or _Plantago lanceolata_ are narrow tubes, because the leaves are oblong or lanceolate, while those of the broad leaved species of arrowhead, as for instance, the _Sagittaria japonica_, are of a conical shape.

From the evidence of the lime-tree we may conclude that normal peltate leaves may have originated in the same way. And from the fact that pitchers are one of the most frequent anomalies, we may conclude that the chance of producing peltate leaves must have been a very great one, and wholly sufficient to account for all observed cases. In every instance the previously existing shape of the leaf must have decided whether peltate or pitcher-like leaves would be formed. As far as we can judge peltate anomalies are quite uninjurious, while ascidia are forms which must impede the effect of the light on the leaf, as they conceal quite an important part of the upper surface. In this way it is easily conceivable that peltate leaves are a frequent specific character, while ascidia are not, as they only appear in the special cases of limited adaptation, as in the instances of the so called pitcher-plants. The genera _Nepenthes_, [672] _Sarracenia_ and some others are very well known and perhaps even the bladderworts or _Utricularia_ might be included here.

The reproduction of specific characters by anomalous ascidia is not at all limited to the general case as described above. More minute details may be seen to be duplicated in the same way. Proofs are afforded on one side by incomplete ascidia, and on the other by the double cups.

Incomplete ascidia are those of the _Nepenthes_. The leaf is divided into three parts, a blade, a tendril and the pitcher. Or in other words, the limb produces a tendril at its summit, by means of which the plant is enabled to fasten itself to surrounding shrubs and to climb between their branches. But the end of this tendril bears a well-formed urn, which however, is produced only after the revolving and grasping movements of the tendril have been made. Some species have more rounded and some more elongated ascidia and often the shape is seen to change with the development of the stem. The mouth of the urn is strengthened by a thick rim and covered with a lid. Numerous curious contrivances in these structures to catch ants and other insects have been described, but as they have no relation to our present discussion, we shall abstain from dealing with them. [673] Likewise we must refrain from a consideration of the physiologic qualities of the tendril, and confine our attention to the combination of a limb, a naked midvein and an ascidium. This combination is to be the basis of our discussion. It is liable to be produced all of a sudden. This assertion is proved by its occurrence as a varietal mark in one of our most ordinary cultivated plants. It is the group known as _Croton_, belonging to the genus _Codiaeum_. A variety is called _interruptum_ and another _appendiculatum_, and these names both relate to the interruption of the leaves by a naked midvein. The leaves are seen to be built up of three parts. The lower half retains the aspect of a limb; it is crowned by a vein without lateral nerves or blade-like expansions, and this stalk in its turn bears a short limb on its summit. The base of this apical limb exhibits two connate lobes, forming together a wide cup or ascidium. It should be stated that these _interruptum_ varieties are highly variable, especially in the relative size of the three principal parts of the leaf. Though it is of course conceded that the ascidium of _Nepenthes_ has many secondary devices which are lacking in _Croton_, it seems hardly allowable to deny the possibility of an analogous origin for both. Those of the _Croton_, according to our knowledge regarding similar cases, must [674] have arisen at once, and hence the conclusion that the ascidia of _Nepenthes_ are also originally due to a sudden mutation. Interrupted leaves, with an ascidium on a naked prolongation of the midvein, are by no means limited to the _Croton_ varieties. As stray anomalies they have often been observed, and I myself had the opportunity of collecting them on magnolia, on clover and on some other species. They are additional evidence in support of the explanation given above.