GROUP I. THALLOPHYTA : ALGM : CHLOROPHYCE^. 



245 



filaments composed of cylindrical cells, occur as floating green masses in ponds 

 and springs. Each cell contains a peripheral layer of protoplasm in close con- 

 tact with the cell-wall, en- 

 closing a large central 

 vacuole in which the nu- 

 cleus is situated in a mass 



of protoplasm connected x^Sms SfV 



with the peripheral layer 

 by several delicate proto- 

 plasmic filaments. The 

 chloroplastids are the 

 most conspicuous feature 

 of the cell ; in Spirogyra 

 (Fig. 17-3) and Sirogonium, 

 the chloroplastids, of which 

 there may be from one 

 to four, lie in the parietal 

 protoplasm; they are spir- 

 ally twisted in Spirogyra, but are nearly straight in Sirogonium ; like those 

 of the Desmids, these chloroplastids contain several pyrenoids with associated 

 starch-grains ; in Zygnema (Fig. 174 A) each cell contains two chloroplastids, 

 suspended in the middle line, each CDntaining a pyrenoid with starch-grains. 



The filaments elongate by the growth and division of all the constituent cells, 

 and readily break up into segments, consisting of one or more cells, which gtow 

 into new filaments. 



The sexual organs (gametangia) are quite undifferentiated ; any or all of the 

 cells of a filament may act as sexual organs, the whole of its contents being 

 converted into a single non-ciliate gamete. The sexual process (conjugation) 

 consists in the fusion of the gametes derived from two cells belong ng generally 

 to two filaments, but sometimes to the same filament. It is effected, in most 

 cases, by the development of a lateral outgrowth from the middle of each 



FIG. 174. A Fragment of a filam-ant of Zygnema ; 

 in each cell are two star-shaped chloroplastids con- 

 nected by a colourless mass of protoplasm in which lies 

 the nucleus. B Closterium. C Euastrum, two Desmids 

 with chloroplastids; in B there is a vacuole at each 

 end in which a number of granules may be seen in 

 motion. 



FIG. 175. Conjugation in Spirogyra (x 400). At A two cells of adjacent filaments are 

 about to conjugate, and are putting out protuberances (a) towards each other; cl chl&ro- 

 plastid; fc nucleus. At B the gamete p of the one cell is passing over and fusing with 

 that of the other (p). At C the process of conjugation is completed, the zygospore Z being 

 the product. 



gametangium ; the two outgrowths meet (Fig. 175) and their walls become 

 absorbed at the point of contact so that the cavities of the two gametangia are 



