312 A TEXTBOOK OF GENERAL BOTANY 
and swim out of the gelatinous material. The colonial develop- 
ment just described may be regarded as an indication of a non- 
motile, vegetative development interpolated between motile states. 
In somewhat more advanced types motile reproductive cells give 
rise to vegetative bodies consisting of rows of cells (Fig. 327), 
and these in turn reproduce by means of motile cells which 
resemble the mature individuals of the primitive, unicellular, 
motile forms (Fig. 328). The vegetative body here also may be 
regarded as being a development of a nonmotile state interpo- 
lated between motile states. In the higher alge the interpolated 
vegetative body becomes very complex (Fig. 335), while the 
portions which bear gametes are highly specialized (Fig. 336). 
In the alge there has been more than a single development 
of sexuality; they have developed from the fusion of similar 
gametes to the fertilization of eggs by spermatozoids, and this 
development has taken place independently along a number of 
different lines (Figs. 348; 353). The development of a complex 
vegetative body has also occurred independently along different 
lines, with the result that there is great variation in structure 
among the alge (Figs. 325, 359, 342, 3844, 345, 348, 350, 
352, 354). 
CLASS CYANOPHYCEAE (BLUE-GREEN ALG) 
The members of this class are very simple plants. The indi- 
viduals consist of single cells, of colonies of cells held together 
by a gelatinous covering, or of chains of cells (Figs. 311-314). 
The popular name, blue-green alge, refers to the characteristic 
blue-green color of these plants. 
Cell structure. The Cyanophyceae contain chlorophyll and in 
addition the blue pigment phycocyanin. 
The cells are characterized by having a colorless central por- 
tion, the so-called central body, surrounded by a pigmented zone. 
The central body seems to represent an incipient nucleus, but is 
not separated from the remainder of the cell by a membrane, 
while its structure is certainly much more primitive than that 
