1.84 



Tetrasporinese 



is central and star-shaped (Scherffel, '08 A), and the two contractile vacuoles 

 and the pigment-spot of the Chlamydomonad are retained. There is a de- 

 finite wall to each cell, but in most cases the mass of jelly is due to the 

 conversion into mucilage of its outer layers. In many instances this is so 

 complete that the jelly is structureless, but in others the cells retain to a 

 variable degree their individual envelopes (Palmodictyon ; fig. 1155). In 



the dominant vegetative state of Schizo- 

 chlamys the outer envelopes are more or 

 less persistent and the ecdysis of the wall 

 is by fragmentation (fig. 113 B). Division 

 of the cells occurs transversely or obliquely 

 in two or three directions, the daughter- 

 cells rapidly attaining their full size and 

 undergoing further divisions. In Chloran- 

 gium, Prasinocladus (fig. 110 A) and Hor- 

 wofo7a(fig.H5 C) the mucilage is secreted 

 in a unilateral manner and becomes tough, 

 branched colonies of cells on gelatinous 

 stalks being gradually built up. 



The colonies frequently become dis- 

 membered into smaller portions by the 

 disorganization of parts of the jelly, each 

 portion increasing to form a new colony. 



Asexual reproduction takes place by 

 biciliated zoogonidia of the Chlamydo- 

 monad type. They arise either by the 

 transformation of a vegetative cell into a 

 zoogonidangium in which several (4 or 8) 

 zoogonidia arise (Apiocystis; Schizochla- 

 mys, fig. 113 C and D), or by the as- 

 sumption by the ordinary vegetative cell 

 of the motile Chlamydomonadine con- 

 dition (Tetraspora). The last-mentioned 

 fact is a valuable piece of evidence in 

 favour of the view that the Palmellacese 

 have originated by the intercalation of a 



simple though well-marked vegetative condition between two formerly suc- 

 cessive motile phases. 



Gamogenesis has been observed in some of the genera by the fusion 

 of isogametes, either similar in all- respects to the zoogonidia and produced 

 singly in a gametangium, or much smaller than the zoogonidia and produced 

 in numbers from a gametangium (Palmetto). 



Fig. 110. A C, Prasinocladus lubricus 

 Kuck. A, part of a small colony, 

 x 470 ; B, a single cell showing the 

 chromatophores, x 960 ; C, zoogoni- 



Physocytium confervicola Borzi, x 600 

 (after Borzi). D, young colonies at- 

 tached to a filament of Spirogyra; E, 

 formation of zoogonidia; F, zoogoni- 

 dium ; G, gamete ; H, germination of 

 zygote. 



