Next, there is the process by which one plant becomes two by the dying off of some connecting portion between two growing parts.
Take, for instance, the case of the liverworts. In these there is a thallus which starts from a central point and continually divides in a forked or dichotomous manner. Now, if the central portion dies away, it is obvious that there will be as many plants as there were forkings, and the further the dying of the old end proceeds, the more young plants will there be.
Take again, among higher plants, the cases of suckers, runners, stolons, offsets, etc. Here, by a process of growth but little removed from the normal, portions of stems develop adventitious roots, and by the dying away of the connecting links may become independent plants.
Still another vegetative method of reproduction is that by bulbils or gemmæ.
A bulbil is a bud which becomes an independent plant before it commences to elongate; it is generally fleshy, somewhat after the manner of a bulb, hence its name. Examples occur in the axillary buds of Lilium bulbiferum, in some Alliums, etc.
The gemma is found most frequently in the liverworts and mosses, and is highly characteristic of these plants, in which indeed vegetative reproduction maybe said to reach its fullest and most varied extent.
Gemmæ are here formed in a sort of flat cup, by division of superficial cells of the thallus or of the stem, and they consist when mature of flattened masses of cells, which lie loose in the cup, so that wind or wet will carry them away on to soil or rock, when, either by direct growth from apical cells, as with those of the liverworts, or with previous emission of thread-like cells forming a "protonema," in the case of the mosses, the young plant is produced from them.
The lichens have a very peculiar method of gemmation. The lichen-thallus is composed of chains or groups of round chlorophyl-containing cells, called "gonidia," and masses of interwoven rows of elongated cells which constitute the hyphæ. Under certain conditions single cells of the gonidia become surrounded with a dense felt of hyphæ, these accumulate in numbers below the surface of the thallus, until at last they break out, are blown or washed away, and start germination by ordinary cell division, and thus at once reproduce a fresh lichen-thallus. These masses of cells are called soredia.
Artificial budding and grafting do not enter into the scope of this paper.
As in the general growth and the vegetative reproduction of plants cell-division is the chief method of cell formation, so in the reproduction of plants by special cells the great feature is the part played by cells which are produced not by the ordinary method of cell division, but by one or the other processes of cell formation, namely, free-cell formation or rejuvenescence.