218 BOTANY. 



masses. Each filament has a basal cell (which is spherical 

 and thick walled), and sometimes interstitial ones ; the prin- 

 cipal cells of the filaments are usually cylindrical and often 

 much elongated ; at the outer end they become attenuated 

 into long slender hyaline hairs. Special reproductive bod- 

 ies, called resting spores, are formed before the close of the 

 growing season ; these appear just above the basal cells, one 

 on each filament, and are much larger and thicker walled 

 than the remaining cells. Upon the death of the mass of 

 filaments the resting spores remain, and from these upon the 

 advent of favorable conditions new filaments are developed. 



Seven genera are known in the United States, the principal ones 

 being Rivularia, Zonotrichia, and Mastigonema ; their species are 

 found in water or wet places everywhere ; they also constitute the 

 so-called gonidia of lichens. 



286. Order Scytonemacese. In this order the differen- 

 tiation becomes so great that the filaments may be said to 

 attain a distinct individuality ; they branch here and there, 

 and are furnished with thick- walled heterocysts, which are 

 basal or interstitial. In this order there is also a well-de- 

 veloped sheath surrounding each filament, which may be 

 compared with the poorly defined one of the preceding 

 orders. The filaments form little masses or mats, growing 

 in the water or on wet ground, or even on the moist bark of 

 trees. 



We have tliree genera, the principal one of which is Scytonema, 

 wliich contains eleven species. Some of these are the "gonidia" of 

 lichens. 



287. Closely related to the foregoing orders, but not 

 falling within the class Cyanophyceae, is the doubtful order 

 Palmellacece. The cells are single or in colonies, and im- 

 bedded in a gelatinous matter, much as in the Chroococcaceae ; 

 but the cells are destitute of phycocyanine or phycoxanthine, 

 containing only chlorophyll. This, however, is hardly a 

 sufficient character for separating them. It is, moreover, 

 not certainly known whether the forms included in this 

 order are autonomous species ; it seems probable that at 

 least a portion of them are only early stages of other plants. 



