584 MICROSCOPIC FORMS OF VEGETABLE LIFE THALLOPHYTES 



such filament it is obvious that the two oldest segments are found at 

 its opposite extremities, and that each subdivision of the inter- 

 mediate cells must carry them farther and farther from each other. 

 This is a very different mode of increase from that of the Confervaceas, 

 in which commonly the terminal cell alone undergoes subdivision, 

 and is consequently the one last formed. 



The sexual generative process in the Desmidiacece, which occurs 

 but rarely compared with that of binary division, always consists of 

 an act of ' conjugation.' It commences with the dehiscence of the 

 firm external envelope of each of the conjugating cells, so as to 

 separate it into two valves (fig. 438, 0, D ; fig. 439, C). The 

 contents of each cell thus set free without any distinct investment 

 blend with those of the other ; and a zygospore is formed by their 

 union, which soon acquires a truly cellulose envelope. 1 This enve- 

 lope is at first very delicate, and 

 is filled with green and granular 

 contents ; by degrees the envelope 

 acquires increased thickness, and 

 its contents become brown or red. 

 Ultimately the envelope becomes 

 differentiated into three layers, of 

 which the innermost and outer- 

 most are colourless, while the 

 middle one is firmer and brown. 

 The outer surface is sometimes 

 smooth, as in Closterium and its 

 allies (fig. 439 ; Plate IX, fig. 8) ; 

 but in Cosmarium it becomes 

 granular, tuberculated, or spiiious 

 (fig. 438, D; Plate VIII, figs. 1, 

 4), the spines being sometimes 

 simple and sometimes forked at 

 their extremities. The mode in 

 which conjugation takes place in 

 the filamentous species constitut- 

 ing the Desmidiece proper is, how- 

 The filaments first separate into 

 their component joints, and when two cells approach in conjugation, 

 the outer cell-wall of each splits or gapes at that part which adjoins 

 the other cell, and a new growth takes place which forms a sort of con- 

 necting-tube that unites the cavities of the two cells (fig. 440, 1), E). 

 Through this tube the entire endochrome of one cell passes over 

 into the cavity of the other (D) ; and the two are commingled so as 

 to form a single mass (E), as is the case in many of the Conjugate. 

 The joint which contains the zygospore can scarcely be distinguished 

 at first (after the separation of the empty cell), save by the greater 

 density of its contents; but the proper coats of the zygospore 

 gradually become more distinct, and the enveloping cell-wall disap- 

 pears. 



1 In certain species of Closterium, as in many of the Diatomacece, the act of 

 conjugation gives origin to tivo zygospores. 



FIG. 438. Conjugation of Cosmarium 

 botrytis: A, mature cell; B, empty 

 cell-envelope ; C, transverse view ; 

 D, zygospore with empty cell enve- 

 lopes. 



ever, in many respects different. 



