CRYPTOGAMS 



307 



colonies are formed by the daughter cells, which result from division, remaining 

 enclosed in a common gelatinous envelope, formed by the mucilaginous degeneration 

 of their cell walls. Thus, the four-cornered, tabular cell colonies of the genus 

 Meris7>wpedia, often found floating in the water, are formed by repeated cell division, 

 which is always in one plane and in two directions only. The cell colonies of 

 Gloeocapsa, whose different species form, for the most part, olive-green or blue- 

 green patches on damp walls and rocks, present a peculiar appearance, as shown in 

 Fig. 225. The walls of the cells are mucilaginous and swollen. When a cell 

 divides, the walls of the daughter cells also become mucilaginous, while at the 

 same time they remain enclosed within the walls of the mother cell. In this 

 manner, through division in three dimensions of space, a cubical or rounded colony 

 composed of 2, 4, 8 or more cells is produced which eventually breaks up into 

 daughter colonies. 



2. Nostocaceae. — The simplest forms of this family, in which are included the 



Fig. 225. — Gloeocapsa polyderviatica. A, 

 III process of division ; B, to the left, 

 shortly after division ; C, a later stage. 

 (X.540.) 



Fig. 226. — A, Oscillaria princeps ; a, terminal cell ; b, c, 

 portions from the middle of a filament. In c, a dead 

 cell is shown between the living cells. B, Oscillaria 

 FroclicMi; i, with granules along the partition 

 walls. (X 540.) 



most highly developed of the Fission-Algae, are merely filamentous rows of cells, 

 unbranched and without any distinction of base or apex. This is the case in the 

 genus Oscillaria (Fig. 226), whose single filaments are motile and exhibit peculiar 

 gliding movements. The filaments consist of disc-shaped, blue-green cells, with 

 numerous small'granules disposed in their peripheral protoplasm, which, as a rule, 

 appears to be especially accumulated along the transverse walls (Fig. 226, B). The 

 terminal cells of the filaments are usually rounded. By the rounding off and 

 separation of any two adjoining cells the whole filament may break up into short 

 germinal segments, termed hoemogonia, which then grow out again into long 

 filaments. In species in which the filaments are invested with thick sheathing 

 walls, the hormogonia creep out of the cell envelope, leaving only the empty 

 sheath remaining. The species of Oscillaria are found in tufts, either freely float- 

 ing or growing upon damp soil. 



While in the case of Oscillaria and in several other genera the cells are all 

 alike, many Nostocaceae not only develop special cells, termed heteeocysts, which 

 seem to be incapable of further development, but also thick-walled resting cells or 

 spoees. This is the habit of the genus Nostoc, which is found growing on damp 



