Microthamniede 



301 



Pleurothamnion, which is a very close ally of Gongrosira, the thallus is 

 encrusted with lime. In Microthamnion the branches are blunt without 

 the slightest trace of attenuation, and all the species of this genus appear 

 to be invariably free-floating and unattached. The greatest reduction of 

 branches is seen in Glwoplax, which is epiphytic on the leaves of Sphagnum. 



Each cell contains one parietal lobed chloroplast (rarely subdivided?), 

 with or without a pyrenoid. In Gongrosira and Endoderma the chloroplast 

 may sometimes contain from one to three pyrenoids. 



The zoogonidangia may be developed from almost any cell of the thallus, 

 and in most cases are clearly differentiated by their larger size (fig. 194 (7; 



Fig. 194. A (7, Endoderma WittrocJcii (Wille) Lagerh. A, young plant consisting of germinating 

 zoogonidium which has penetrated the cell-wall of a species of Ectocarpus ; B, later stage 

 showing growth of Endoderma within the wall of the host ; C, older plant with zoogonidangia 

 (zg). All x 425 (after Wille). D, E. Pithophorss G. S. West on the upper part of a spore and 

 adjacent vegetative cell of Pithophora Clevei Wittr. ; three cells show the massive chloro- 

 plast with a single pyrenoid, the rest in outline only. E, E. polymorpha G. S. West on the 

 vegetative cells of Pithophora Clevei Wittr., outline of cells only. D and E, x 460. 



fig. 196 D and E). The zoogonidia are ovoid or ellipsoid, biciliated in some 

 genera (Microthamnion, Gongrosira, Chloroclonium, Leptosira, etc.), but 

 quadriciliated in others (Sporocladus, Trichophilus, Endoderma and Pseuden- 

 doclonium). Only one zoogonidium arises in the mother-cell in Glceoplaac, 

 2 8 in Endoderma, 4 8 in Microthamnion and Pleurothamnion, and many 

 in Gongrosira. In Microthamnion the zoogonidia have no pigment-spot. 

 The germination of the zoogonidium is generally direct, but in Leptosira 

 a small Gharacium-\ike plant is first formed the contents of which divide 

 to form 4 aplanospores, which are set free by the dissolution of the wall of 

 the mother-cell, each growing into a new plant. 



