THALLOPHYTA: ALGAE 11 



These granules usually lie in the outer part of the cell, in many fila- 

 mentous forms being commonly grouped along the cross walls. 



Cell division is accomplished by a ring-like wall that develops from 

 the outside toward the center, finally cutting the cell in half (Fig. 2). 

 At the same time the chromatin separates into two approximately equal 

 masses without the formation of chromosomes or other features of 

 mitosis. 



Reproduction. Because the Cyanophyceae are unicellular plants, cell 

 division results in reproduction, a method called fission. The division 

 of a cell to form two new individuals directly is the simplest method of 

 reproduction in the plant kingdom. In most of the Cyanophyceae the 

 cell walls break down to form abundant mucilage. Generally, as in 

 Gloeocapsa, this holds together a group of cells derived from a single cell 

 by repeated division, thus forming a colony (Fig. L4). Here the mucilage 

 surrounding the cells is in concentric layers; but in many other genera it 

 is in a continuous mass made up of the confluent sheaths of the individual 

 cells. 



In some of the filamentous types, such as Lynghya, a firm mucilaginous 

 sheath is present around the whole colony, but in Oscillatoria, a related 

 genus, the cell walls are more resistant and no sheath is formed (Fig. \D). 

 In both genera the cells are compactly arranged in the colony, each cell, 

 except the terminal one, being shortly cylindrical. That their shape 

 results from mutual pressure is shown by the fact that the free surface of 

 the end cell is convex. This is also true of cells adjacent to a dead cell 

 in the filament. 



The type of colony produced depends on the way in which the cells 

 divide. In a filament all the divisions occur in one plane. Where the 

 cells divide in two planes, a plate or a hollow sphere one layer thick is 

 produced. Divisions in three planes usually result in a somewhat 

 massive type of colony. 



In most of the filamentous forms, with the exception of Oscillatoria 

 and its relatives, differentiated cells, called heterocysts, appear in the 

 colony. They may be seen in such common genera as Nostoc and 

 Anabaena (Fig. IC). A heterocyst is an enlarged vegetative cell that 

 becomes thick- walled and transparent. Heterocysts usually occur singly 

 but at rather frequent intervals, thus dividing the filament into segments 

 called hormogonia. These become detached and move away from one 

 another to form new colonies. A hormogonium is merely an isolated 

 portion of the original filament. In Oscillatoria and related genera 

 hormogonia are formed by the death of unmodified cells here and there 

 in the colony (Fig. ID). 



Although none of the Cyanophyceae produces zoospores or gametes, 

 most of the filamentous members form nonmotile resting spores. One 



