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 algæ.
2. The zoospores or mobile spores of many algæ 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 algæ 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.