CHAPTER IV. — THE THALLOPHYTA. 



279 



or filaments. There are two orders, the Chroococcacae and the 

 Nostocacece. 



The Chroococcaceae occur either as isolated cells, or, more 

 commonly, in groups, imbedded in a gelatinous matrix. The 

 cells of the mass are, however, never closely connected. Glceo- 

 a b c capsa may be taken as an illustration of 



the group. Like many of its congeners, 

 it is found growing on damp rocks adja- 

 cent to springs, where they form slimy 

 masses. The cell-walls swell and become 

 converted into a stratified jelly, and in the 

 mean time the cells multiply by fission in 

 different planes within the jelly, forming 

 ** e masses of various sizes. The plant and 



Fig. 469. — Gloeocapsa. a, 



fully developed ceil with geia- the wav it multiplies are illustrated in Fig. 



tinous, greatly thickened and ' 



stratified ceii-waii : b, c, d. e, 469. Other forms, as Mensmopedia, pro- 



illustrate mode of multiplies- ..... . 



tion. Magnified about 150 di- duce by division m one plane symmetri- 

 cal, tabular families, consisting of four, 

 eight, sixteen, thirty-two or sixty-four rounded cells, held together 

 by a firm gelatinous matrix. 



It is believed by many that most of the forms of this group 

 are but stages in the development of higher forms. 



The Nostocaceae include a number of genera, the most im- 

 portant of which are the Nostocs, Oscilla- 

 torias, Scytonemas and Rivularias. 



The Xostocs occur as greenish or brown- 

 ish, tough gelatinous masres, some species 

 of which are as large as a hen's egg, or 

 even larger. They are common in ponds 

 or slow streams, or on the damp ground 

 bordering rivers, swamps and lakes. If a 

 section be made of one of these masses 

 and examined microscopically, it will be 

 seen to contain, imbedded in the jelly, 

 very numerous serpentine threads, com- 

 posed of spherical cells loosely attached 

 to each other in chains or moniliform rows- 

 At intervals in the chain of cells occur larger cells, called hetero- 

 cysts. Fig. 470 shows one of these threads highly magnified. 



Fig. 470. — Filament of Nos- 

 toc magnified about 800 diam- 

 eters, a, a heteroevst. 



