GERM-CELLS AND SOMA: SEXUAL AND ASEXUAL REPRODUCTION
WHEN multiplying, cells may separate out, individual ones breaking away, or perhaps remaining joined together in clusters and increasing continually in volume through the division of individual component cells. In some cases this increase in the total quantity of cells goes on indefinitely. In others, however, the species has a characteristic limit, namely the colony. Transition is possible from single-celled organism to polyplastid. Such multiplication of cells may occur in one dimension, as in the filamentary systems; in two dimensions, as in membranes; or in three dimensions, i.e. volume. In the more primitive forms of coexistence, no special cells seem specifically earmarked to reproduction, i.e. there are no germ-cells as such. All the cellular elements composing them may be equally divided, and it is from such individual division that growth and reproduction of the whole is attained. This is observable among bacteria in general, in certain ascomycete fungi or yeasts, and in certain unicellular algae such as the genus Protococcus, etc.
As a contrast, in other algae, some cells become functionally distinct from the remainder, because their protoplasm and nuclear matter are divided into protoplasmic formations constituting spore. When the ripening thereof is completed, the cell membrane is broken and the spores stream abroad. Each spore on finding a suitable environment will give rise to the development of a fresh individual. This is a type of specialization which causes a particular cell to take upon itself the functions of reproduction, and occurs in single-celled algae like Sphaerella or in multicellular filamentary algae like Ulothrix.
However, alongside reproduction by spores, a form of sexual reproduction also develops in various species. Certain cells form protoplasts from which issue cellular elements, smaller than spores, namely the gametes. These are similarly released by fracture of the membrane of the cell where they are generated and they have to unite two by two