The products of the sexual cells show us clearly enough that out of each kind of egg only its own species of organism can be developed. Certainly it is not so plain that, besides their visible microscopical characters, the tissues and organic parts are in possession of more general characters, identical in all the differently-specialised tissues of a single organism; but we may infer the existence of such latent characters, at least partly, from the results obtained, in the case of plants, by grafting, in the case of animals, by transplantation and transfusion.
In the case of plants one may graft a twig cut from one tree upon the stem or lower part of another tree of the same kind, and so bring about a firm and lasting union between the two. In a short time the corresponding tissues of the parts brought into connection quietly unite. Thus from two different individuals a single living organism may be produced artificially.
One would expect, therefore, that a twig and stem, chosen from two closely allied species, such as, for instance, the pear and the apple, would unite when the suitable tissues were put together. But this does not happen. Successful grafting depends far less on the conjunction of obviously appropriate parts than upon characters unrecognisable by us, such as deep-seated kinship between the parts, and the specific characters of their cells; while in the case of individuals of the same species two pieces will unite even if they are not brought together in appropriate conjunction, or when they belong to different parts of the organism, as, for instance, to the root and the leaf; yet in the absence of deep-seated kinship union will not take place.
Generally this kinship, which has been called vegetative affinity, depends, like sexual affinity, upon the degree of systematic relationship. It appears that the same condition of things occurs as when, in ordinary fertilisation, sexual cells from different varieties, or species, are united. In both cases it happens, on the average, that union is the more to be expected the more closely the plants concerned are akin, in a natural system of classification.
But in grafting, as in cross-fertilisation, unexpected exceptions to this rule occur. Relying upon these, Naegeli thought that the external distinguishing tokens do not always indicate correctly the intrinsic constitutional differences. Frequently union will not take place between plants most near akin in classification, most alike in external characters; while it will occur between plants most different in outward aspect and belonging to different genera or even families. In other words, external characters give no certain index to the degree of vegetative affinity or of sexual affinity between two kinds of plants.
As an example of this, Vöchting, in his treatise upon transplantation of plant-tissues, takes the tribes of pear-trees. Grafting between these and apple-trees takes place only with difficulty, although the apple is a close kinsman and belongs to the same genus. On the other hand, most of them graft easily upon the quince, although that belongs to a different genus. In this case, also, there is no sexual affinity between the pollen and the ova. Hybrids are not formed between the pear and the apple.
It seems probable to me, although as yet I cannot get complete proof of it, that sexual and vegetative affinity, that is to say, the relationship between the egg-cell and the pollen of two species, and the relation between twig and stem, depend upon the same intrinsic qualities of that elementary organism the cell.
Vöchting distinguishes as harmonic or disharmonic the modes of union between twig and stem, according to whether or no they reach the formation of functional unity. Among cases of disharmony there are several interesting gradations. Generally speaking, in the case of plants not adapted to each other, no attempt at union occurs, and the grafted twig speedily perishes; sometimes even the stem dies, as if it had been poisoned by the graft. In other cases the disharmony is not shown so strongly. The twig and the stem begin to unite, but, sooner or later, disturbances occur, and complete destruction results. According to Vöchting, in the case of some Cruciferæ the disturbances are as follows: the twig begins to form roots at its lower end, and these grow into the stem of the host. Through them the twig uses as food the juices and salts of the stem, refusing to unite with the stem so as to form a single individual. As Vöchting says, this formation of roots simply is an attempt on the part of the twig to complete its own individuality. Instead of growing into corporate union with the stem, the twig attempts to become a parasite upon it. A further consequence often is, that the stem, too, begins to respond to the unadaptive stranger's influence. Thus, when Vöchting grafted a Rhipsalis paradoxa on an Opuntia labouretiana, he found that round the roots of the graft the tissues of the host threw out a protective sheath of cork, or turned in places to a gelatinous mass.
In some cases experimenters have overcome disharmony between two species, A and B, by making use of a third species, C, with a vegetative affinity for both A and B. Thus, an intermediary between the two disharmonic forms is made, and by such an arrangement a single functional individual is produced from pieces of three different species. Thus, upon A, as stock, a shoot of C is grafted, while upon this shoot of C, as stock, a shoot of B in turn is grafted.
In the matter of these different grades of disharmony, a comparison may be made between sexual and vegetative affinities. In many cases the spermatozoa of one species will not impregnate the eggs of another species. In other cases, the alien spermatozoon may penetrate the egg and unite with its nucleus, making, however, an unsatisfactory combination in various degrees of infertility. Sometimes the fertilised egg divides only a few times and then dies; sometimes development proceeds to the stage of the blastula, the gastrula, or even further; but it then comes to an end, through intrinsic causes beyond our ken, and, finally, complete destruction follows.