318 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 oftert 

 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. 



Five genera are known in the United States, the principal ones- 

 being Rimila/ria, Calofhrix, and Mastigonema ; their species are found, 

 in water or wet places everywhere ; they also constitute the so-called. 

 gonidia of lichens. 



287. — Closely related to the foregoing orders, but not 

 falling within the class Cyanophyceffi, is the doubtful order 

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

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

 but the cells are destitute of phycocyanine or phyeoxanthine, 

 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. 



We have five genera, the principal one of which is Seytonema, 

 which contains nineteen species. Some of these are the "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. 



