ivision, 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.

The only case of purely vegetative cell-formation which takes place by
either of these processes is that of the formation of endosperm in
Selaginella and phanerogams, which is a process of free-cell formation.

On the other hand, the universal contraction and rounding off of the
protoplasm, and the formation by either rejuvenescence or free-cell
formation, distinctly mark out the special or true reproductive cell.

Examples of reproductive cells formed by rejuvenescence are:

1. The swarm spores of many algae, as Stigeoclonium (figured in Sachs'
"Botany"). Here the contents of the cell contract, rearrange themselves,
and burst the side of the containing wall, becoming free as a
reproductive cell.

2. The zygoblasts of conjugating algae, as in Spirogyra. Here the
contents of a cell contract and rearran