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 rearrange themselves only after contact of the cell with one of another filament of the plant.  This zygoblast only becomes free after the process of conjugation, as described below.

3.  The oosphere of characeae, mosses and liverworts, and vascular cryptogams, where in special structures produced by cell-divisions there arise single primordial cells, which divide into two portions, of which the upper portion dissolves or becomes mucilaginous, while the lower contracts and rearranges itself to form the oosphere.

4.  Spores of mosses and liverworts, of vascular cryptogams, and pollen cells of phanerogams, which are the analogue of the spores.

The type in all these cases is this:  A mother-cell produces by cell-division four daughter-cells.  This is so far vegetative.  Each daughter-cell contracts and becomes more or less rounded, secretes a wall of its own, and by the bursting or absorption of the wall of its mother-cell becomes free.  This is evidently a rejuvenescence.

Examples of reproductive cells formed by free-cell formation are: 

1.  The ascospores of fungi and algae.

2.  The zoospores or mobile spores of many algae and fungi.

3.  The germinal vesicles of phanerogams.

The next portion of my subject is the study of the methods by which these special cells reproduce the plant.

1st.  Asexual methods.

1.  Rejuvenescence gives rise to a swarm-spore or zoospore.  The whole of the protoplasm of a cell contracts, becomes rounded and rearranged, and escapes into the water, in which the plant floats as a mass of protoplasm, clear at one end and provided with cilia by which it is enabled to move, until after a time it comes to rest, and after secreting a wall forms a new plant by ordinary cell-division.  Example:  Oedogonium.

2.  Free-cell formation forms swarm-spores which behave as above.  Example:  Achlya.

3.  Free-cell formation forms the typical motionless spore of algae and fungi.  For instance, in the asci of lichens there are formed from a portion of the protoplasm four or more small ascospores, which secrete a cell-wall and lie loose in the ascus.  Occasionally these spores may consist of two or more cells.  They are set free by the rupture of the ascus, and germinate by putting out through their walls one or more filaments which branch and form the thallus of a new individual.  Various other spores formed in the same way are known as tetraspores, etc.