Still another vegetative method of reproduction is that by bulbils or gemmae.

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.

Gemmae 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 hyphae.  Under certain conditions single cells of the gonidia become surrounded with a dense felt of hyphae, 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.

If we broaden somewhat the definition of rejuvenescence and free-cell formation, and do not call the mother-cells of spores of mosses, higher cryptogams, and also the mother-cells of pollen-grains, reproductive cells, which strictly speaking they are not, but only producers of the spores or pollen-grains, then we may say that cell-division is confined to vegetative processes, rejuvenescence and free-cell formation are confined to reproductive processes.

Rejuvenescence may be defined as the rearrangement of the whole of the protoplasm of a cell into a new cell, which becomes free from the mother-cell, and may or may not secrete a cell-wall around it.

If instead of the whole protoplasm of the cell arranging itself into one mass, it divides into several, or if portions only of the protoplasm become marked out into new cells, in each case accompanied by rounding off and contraction, the new cells remaining free from one another, and usually each secreting a cell wall, then this process, whose relation to rejuvenescence is apparent, is called free-cell formation.