ChlorochytriedB 



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peripheral portions of the thalli of various larger marine Alga^. Reproduction 

 takes place by the formation of zoogonidia or isoplanogametes, or both. 

 The contents of the cell become broken up by successive divisions, in a 

 manner similar to that which occurs in Characium, into a large number of 

 small zoogonidia or gametes. These are liberated either by a perforation in 

 the wall of the mother-cell or by the gelatinization of the inner layer of the 

 wall, which then becomes protruded as a large vesicle in which the motile 

 cells 'swarm' for a short time. The biciliated zoogonidia or the quadriciliated 

 ' zygozoospores ' come to rest on the epidermis of the host-plant and germinate 

 at once. Should the new plant be an endophyte, it penetrates the host either 

 through a stoma or by forcing itself between two epidermal cells. Some of 

 the vegetative cells become akinetes and pass the winter in that condition. 



Fig. 138. A, young vegetative cell of Chlorochytmum Leinixe Cohn, x 475. />/), Centrosph&ra 

 FacciolfB Borzi ; B and C, x 475 ; D, showing escape of zoogonidia (after Borzi), x 410. 



The cell-wall is invariably thick and strong in the adult cells of all the 

 members of this group. There is one extensive parietal chloroplast, usually 

 lining the whole of the cell-wall and often with inwardly extending lobes, and 

 containing one or many pyrenoids. 



In Chlorococciim the cells, which live on damp ground or in water, are 

 gregarious and often form a stratum. They grow much in size, become 

 coenocytic, and often angular by compression. From 8 to 32 zoogonidia arise 

 in each mother-cell. The species of this genus are for the most part very 

 imperfectly known. Centrosphsera is another gregarious form occurring as a 

 rule unattached among various Algte in the mud of pools and ponds. The 

 cells are ovoid, with a thick lamellose cell-wall, provided with an asymmetrical 

 button-like excrescence at one side (fig. 138 B and C). One species of this 

 genus has recently been found as an epiphyte on the hairs of the leaves of 

 Callitriche. Reproduction takes place by minute zoogonidia set free in large 

 numbers from the mother-cells. 



Phyllobium (fig. 137 C) is another endophytic genus in the leaves of 

 moisture-loving plants such as A jug a and Lysimachia, and one species 

 (P. sphagnicola, G. S. W., '08) occurs on the leaves of Sphagnum. The plant- 

 body is a branched coanocyte, the branches traversing the intercellular spaces 



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