Obrázky stránek
PDF
ePub

er be

tree I will answer that nectarines have disputable trinity—and there can no longalso been produced from peach seeds. The

any

doubt as to the fundamental likeanswer to one answers the other. It is ness of the bud-variety and the seed-variety. true that bud-variations, if we use that Yet I must bring another proof of this term, as we logically must, to denote all likeness to your mind. It is well known variations between phytons, are commonly that the seedlings of plants become more less marked than seed-variations, but this variable as the species is cultivated ; and it is only because the conditions of origin and is also true that bud-varieties are more freenvironment of the phyton are less varied quent and more marked in cultivated than those of the seedling. The phytons plants. Note, for example, the tendency of originate from one parent, not from two; cultivated plants to bear variegated or cutand they all grow in very like conditions. leaved or weeping shoots, and the fact that But I am convinced that, when we consider the colors and doubleness of flowers often the plant individual in the light of evolu- vary greatly upon the same plant. Many of tion, the bugbear of bud-variation vanishes. our best known roses, carnations, chrysan

A good proof that bud-variation and seed- themums, violets and other garden plants variation are one in kind is afforded by the originated as bud-sports. This fact is so well fact that selection can be practiced for the known that critical gardeners are always on improvement of forms originating by either the alert for such variations. In any house means. Darwin was surprised, as he says, of 200 roses, all grown from cuttings, the to “hear from Mr. Salter that he brings the grower will expect to find more than one deprinciple of selection to bear on variegated parture from the type, either in color or freeplants propagated by buds, and has thus dom of bloom or in habit of plant. Every greatly improved and fixed several varie- gardener will recall the sporting' tendenties. He informs me that at first a branch cies of Perle des Jardins rose, and the fact often produces variegated leaves on one side that several commercial varieties have alone, and that the leaves are marked only sprung from it by bud-variation. As early with an irregular edging, or with a few lines as 1865 Carrière gave a descriptive list of of white and yellow. To improve and fix 154 named bud-varieties, and remarked at such varieties he finds it necessary to en- length upon their frequency amongst culticourage the buds at the bases of the most vated plants. This fact of greater buddistinctly marked leaves and to propagate variability under cultivation was fully from them alone. By following, with per- recognized by Darwin, and he regarded this severance, this plan during three or four as one of the strongest proofs that such vasuccessive seasons a distinct and fixed var- riation, like seed-variation, is the direct iety can generally be secured.” This prac result of the conditions of life to which tice, or similar ones, is not only well known the plant has been exposed." to gardeners, but we have seen that nature In order to extend the proofs of the esselects in the same manner, through the op- sential ontogenetic likeness of bud and semieration of the same struggle for subsistence nal variations, I will call to your remeniwhich Darwin so forcibly applied to all other brance the fact that the characters of the forms of modification. Once given the three two phytons may be united quite as comfundamental principles in the phylogeny of pletely by means of asexual or graft hybridthe phyton, the variation amongst them- ism as by sexual hybridism. I do not need selves, the struggle for existence, the capa- to pursue this subject, except to say that bility of perpetuating themselves an in- we now believe that graft-hybrids are rare

6

6

and exceptional chiefly because the sub- the same relation to transmission of charject has received little experimental at- acters by means of seedage, we can demontention. Certainly the list given by Focke, strate it equally well by the converse proand the anatomical researches of Macfar- position—that both bear the same relation lane, show that such hybrids may be ex- to the perpetuation of their features by pected in a wide variety of subjects and cuttings. Some seed-varieties will not with some frequency. It is now stated pos- come true' by cuttings, and there are also itively by Daniel, as the result of direct ex- some bud-sports which will not, as every periment, that the seeds of cions of certain gardener of experience knows. I will cite cultivated herbs which are grafted upon a a single case of sporting’ in bud offspring. wild plant give offspring which show a One winter a chance tomato plant came up marked return to the wild type. I should in one of my greenhouses. I let it grow, also add that the breaking up of seminal and it bore fruit quite unlike any other hybrids into the characters of either parent variety which I ever saw. There was no may take place, as Darwin has shown, other tomato plant in the house. I propathrough either seed- or bud-variation. You gated it both by seeds and cuttings. I are all no doubt aware that hybrids gener- had two generations of cuttings. Those ally tend to revert to the types from which taken directly from the parent plant, they sprung, and this sometimes occurs came true' or very nearly so; then a lot even in hybrid offspring which is propagated of cuttings from these cutting-grown plants exclusively by buds or cuttings.

was taken, making the second asexual genStill another proof of the similarity of eration from the original seedling. While bud-varieties and seed-varieties is the fact most of the seeds' came true,' few of these that the seeds of bud-varieties are quite as second cuttings did, and, moreover, they likely to reproduce the variety as the seeds 'sported into several very unlike forms— of seed-varieties are to reproduce their so much unlike that I had both red and parents. Darwin and others have recorded yellow fruits from them. In respect to this seminal transmission of bud-sports. transmission of characters, then, bud- and “Notwithstanding the sudden production seed-varieties are alike, because either class of bud-varieties," Darwin writes, "the char

may or may not transmit its marks either acters thus acquired are sometimes capable by seeds or buds. of transmission by seminal reproduction. Finally, let me say, in proof of the further Mr. Rivers has found that moss-roses similarity of bud- and seed-variations, that [which are bud-varieties] generally repro- each class follows the incidental laws of duce themselves by seed; and the mossy external resemblance which pertain to the character has been transferred by crossing other class. For instance, there are analofrom one species to another.” This general gous variations in each, giving rise to the fact that bud-sports may reproduce many same kinds of variegation, the same anomaof their essential acquired characters by lies of cut and colored foliage, of weeping seeds is so well grounded in the minds of branches, party-colored fruits and the like; gardeners that. the most critical of them and the number of similar variations may make no distinction, in this respect, be- be as great for any ameliorated plant in the tween varieties of bud and seed origin when one class as in the other.

The most expert selecting parents for making crosses. And observer is not able to distinguish between if we can prove the similarity of bud and bud-varieties and seed-varieties ; the only seed variations by showing that both bear way of distinguishing the two is by means of the records of their origins, and because specific names of organisms founded upon such records of any varieties are few we so fragmentary and scant material as those have come to overlook the frequency of applied to fossil plants; and yet I cannot bud-variation and to ascribe all progressive help feeling that some of these contemporavariability in the vegetable kingdom to neous variations are reversions to very old seeds or sex.

types. I was first led to this opinion by a Whilst it is not my purpose to discuss study of the sports in ginkgo leaves, and the original sources of bud-variations, I finding them suggestive of Mesozoic types. cannot forbear to touch upon one very re

“ This variation in leaf characters,” I wrote markable fact concerning reversions. It is at the time, "recalls the geologic history of a common notion that all bud-varieties are the ginkgo, for it appears to be true that atavistic, but this position is untenable if leaves upon the young and vigorous shoots one accepts the hypothesis, which I have of trees are more like their ancestors than here outlined, of the ontogenetic individual- are the leaves upon old plants or less vigority of the phyton, and if he holds, at the ous shoots, as if there is some such geneasame time, to the transforming influence of logical record in leaves as there is in the deenvironment. It is also held by some that velopment of embryos in animals." Subse. bud-varieties are the effects of previous quent observation has strengthened my becrossing, but this is controverted by Dar- lief in the atavistic origin of many of these win in the statement that characters some- abnormal forms, and this explanation of times appear in bud-varieties which do not them is exactly in line with the characters pertain to any known living or extinct spe- of reversions in animals and in cultivated cies; and the observations which I am plants. It would, of course, be futile to atabout to recite also indicate the improba- tempt any discussion of the merits of the bility of such influence in a large class of specific types proposed by palæobotanists,

The instances to which I call your but in those cases, like the ginkgo, where attention are, I think, true reversions to the geologic types are fairly well marked, ancestral types. Those of you who have constant and frequent, and where the similar observed the young non-blooming shoots of contemporaneous variations are rare, there tulip-tree, sassafras and some other trees is apparently good reason for regarding will have noticed that the leaves upon them contemporaneous forms as fitful recollecoften assume unusual shapes. Thus the tions of an ancient state; and this supposileaves of sassafras often vary from the typ- tion finds additional support in the ginkgo, ical oval form to three-lobed and mitten- because the species is becoming extinct, a shaped upon the strong shoots.

There are fact which also applies to the tulip-tree, the most various forms on many tulip-trees, which is now much restricted in its distrithe leaves ranging from almost circular and bution. I am further reinforced in this view merely emarginate to long-ovate and vari- by Ward's excellent study of the evolution ously lobed; all of them have been most of the plane-tree, for, in this instance, it admirably illustrated and discussed recently seems to be well determined that the geoby Holm in the proceedings of the National logic type has fairly well marked specific Museum. Holm considers the various characters, and the auricular or peltate base forms of these liriodendron leaves to be so upon contemporaneous leaves, which remany proofs of the invalidity of the fossil cords the connection between the two, is species which very closely resemble them. sufficiently rare to escape comment. VaThis may be true, for there are probably no rious writers have remarked upon the

cases.

ves

similarities of these occasional leaves to the common seedless 'top' onion, and I geologic types, but, so far as I recall, have been able, by treatment, to vary the they regard them as remnants or root of the horse-radish, a plant which tiges of the ancient types rather than as rarely, if ever, produces viable seeds in this reversions to them. There is this impor- climate ; and there are variable seedless tant difference between a remnant and a re- plants in our greenhouses. I might also version. A remnant or rudiment is more cite the fact that most fungi are sexless, so or less uniformly present under normal far as we know, and yet they have varied conditions, and it should give evidence of into innumerable species. You will be inbeing slowly on the decline ; whilst a rever- terested in a concrete case of the apple. sion is a reappearance of wholly lost char- The Newtown Pippin, which originated acters under unusual or local conditions. upon Long Island, New York, has been Now, my chief reasons for considering these widely disseminated by graftage. In Virsports to be reversions is the fact that they ginia it has varied into a form known as occur upon the sterile and verdurous shoots, the Albemarle Pippin, and a New York the very shoots which are most likely to apple exporter tells me that it is a poorer vary and to revert because they receive the shipper than the Northern Newtown and is greatest amount of food supply, as Darwin not so long-keeping. In the extreme Northhas shown to be the case with independent western States the Newtown, while it has plants. And I am therefore able to make not been rechristened there, is markedly still another analogy between phytons and unlike the Eastern fruit, being much longer plants, and to illustrate again the essential and bearing distinct ridges about the apex. sameness of bud-variations and seed-varia- Finally, in New South Wales, the ridges are tions.

more marked and other characters appear,

and the variety is there known as the FiveIII.

crowned Pippin. This is not an isolated I now wish to recall your attention more case. Most Northeastern varieties of apples specifically to the subject of asexual varia- tend to take on this elongated form in the tion. I have shown that no two branches Pacific Northwest, to become heavy-grained are alike any more than are any two plants. and coarse-striped in the Mississippi Valley I have also cited the frequent occurrence of and the Plains, and to take other characterdifferences so marked that they are called istic forms in the higher lands of the South bud-varieties or sports.

Carrière enumer- Atlantic States. This asexual variation is ated over 150 of them of commercial im- sometimes very rapid. An illustration came portance in France, and, as nearly as I can directly under my own observation (and estimate, there are no fewer than 200 named upon which I have once reported) in the horticultural varieties grown at the present case of the Chilian strawberry. Within moment in this country which had a like two years this plant, growing in my garden, origin. It is also known that there are a varied or departed from its wild type so number of species in which seeds are prac- widely as to be indistinguishable from the tically unknown, and yet which run into common garden strawberry, which has been many varieties, as the pineapple, banana regarded by many botanists to be specifiand bread-fruit; and note, if you will, the cally distinct from the Chilian berry. This great variations in weeping willows, a tree remarkable departure, which has enabled which never fruits in this country. In our me, as I believe, to reconstruct the evolugardens there are three or four varieties of tion of the garden strawberry, was one in which no seedling plants were concerned. parts as by seeds. Now all these vegetative If all the common garden strawberries owe parts, when established as independent their origin to a like source as I cannot plants, produce flowers and good seeds, and doubt—then we have here a most instruc- these seeds often perpetuate the very chartive case of sexless evolution, but one in acters which have originated in the asexual which the subsequent generations reproduce generations, as we have seen in the case of these characters of sexless origin by means many bud-varieties; and it should also be of seeds.

remarked that these phytons usually transThis asexual modification is not confined mit almost perfectly the characters acquired to domesticated plants. Any plant which by the plant from which they sprung. Or, is widely distributed by man by means of to put the whole matter in a convenient cuttings or other vegetative parts may be ex- phrase, there may be, and is, a progressive pected to vary in the same manner,

as much

evolution of plants without the aid of sex. experiment shows; and if they behave in Now, where is Weismann's germ-plasm? this way when disseminated by man they One of the properties of this material—if an must undergo similar modification when assumption can receive such designationsimilarly disseminated by nature herself. is its localization in the reproductive organs I need only cite a few instances of habitual or parts. But the phyton has no reproducasexual distribution of wild plants to recall to tive parts; or, if it has them, they are deyour attention the fact that such means of veloped after the phyton has lived a perdistribution is common in nature, and that in fectly sexless life, and possibly after generasome cases the dispersion over wide areas is tions of such life, in which it and its progeny quite as rapid as by means of seeds; and may either have remained comparatively some plants, as various potamogetons, cera- stable or may have varied widely, as the tophyllums and other aquatics, are more circumstances may have determined. If productive of detachable winter buds and any flower, therefore, contains germ-plasm other separable vegetable organs than they it must have derived it out of the asexual are of seeds. The brittle willows drop their or vegetative or soma-plasm. And I will twigs when injured by storms of ice or ask where the germ-plasm is in ferns. wind, or by animals, and many of these cut- These plants are fertilized in the prothallic tings take root in the moist soil, and they stage, and one brief sexual state is all that may be carried far down streams or distrib- the plant enjoys, after which the sex-organs uted along lake shores; the may-apple and

die and wholly disappear. The fern, as the a host of rhizomatous plants march onward layman knows the plant, is wholly asexual, from the original starting point ; the bry- and the spores are as sexless as buds; yet ophyllum easily drops its thick leaves, each these spores germinate and give rise to anone of which may establish a new colony of other brief prothallic or sexual stage, and if plants; the leaves of the lake-cress (Nas- there is any germ-plasm at all in these turtium lacustre) float down the streams fleeting sexual organs it must have come and develop a new plant while they travel; from the sexless spores. It is interesting the house-leeks surround themselves with to note, in this connection, this bud-variacolonies of off-shoots, the black raspberry tion is as frequent in ferns as in other travels by looping stolons, and the straw plants. Or, if the Weismannians can locate berry by long runners; the tiger-lily scat- the germ-plasm in all these instances, pray ters its bulb-like buds, and all bulbiferous tell us where it is in the myriads of sexless plants spread quite as easily by their fleshy fungi! There is no such thing as continu

« PředchozíPokračovat »