286 PAET III. THE CLASSlFfCATION OF PLANTS. 



on Algae, or on aquatic Fungi, but sometimes on Phanerogams, and on aquatic 

 animals. 



In accordance with the form and structure of the body, the Chytridiaceaa 

 may be sub-divided into two groups ; the Myxochytridineae (including such 

 genera as Sphaerita, Olpidium, Olpidiopsis, Synchytrium, etc.) where the body 

 is a single spherical or oval cell, destitute, for a time at least, of a cell-wall ; 

 and the Mycochytridineee where the body has a cell-wall from the first, and is 

 usually more or less mycelioid, and is commonly branched. 



The asexual organs of reproduction are in all cases unicellular, and present 

 two distinct forms ; a thin- walled sporangium (gonidangium in the sexual forms) 

 which at once gives rise to zoospores ; a thick-walled sporangium (sometimes 

 termed a resting-spore), formed by a process of encystment in the asexual 

 form?, by a sexual process in the sexual forms where it represents the sporo- 

 phyte, which only gives rise to zoospores after a longer or shorter period of 

 quiescence. 



The simpler forms (MyxochytriJineae) are holocarpic ; that is, the whole cell 

 becomes a sporangium, being invested by a cell-wall. The other forms are 

 eucarpic, that is, a part only of the body goes to form the sporangium ; some of 

 them (Sporochytrieae) are monocarpic, that is, each produces but one sporan- 

 gium; whilst others are polycarpic (Hypochytriese), that is, each forms several 

 sporangia. In any case the formation of reproductive organs closes the life of 

 the individual. 



The zoospores (and zoogonidia) vary somewhat in form, but are generally 

 spherical or ovoid, with either one or two cilia. They are generally formed 

 directly from the contents of the sporangium (or gonidangium), but in Synchy- 

 trium the contents undergo division, and escape from the sporangium, surrounded 

 by a delicate membrane, as a group of cells (sorus) each of which gives rise to 

 a number of zoospores. When the sporangium is, as is frequently the case, 

 formed within the cells of a host-plant, it puts out a tubular outgrowth which 

 reaches the surface, and thus the zoospores are set free, and, in their turn, 

 make their way into the tissues of a host. 



A sexual process, of the nature of conjugation, has been observed in some 

 forms, both of the Myxochytridinae and of the Mycochytridineae, but the 

 differentiation of the sexual organs as male and female is only rudimentary. 

 In the former case (e.g. Olpidiopsis) two unicellular individuals of unequal size 

 fuse together, the contents of the smaller (probably the male) passing over into 

 the larger cell. In Polyphagus, one of the simpler Mycochytridineae, one in- 

 dividual extrudes its protoplasmic contents as a naked sphere to which another 

 individual applies one of its hyphae through which its contents are conveyed 

 into the sphere. The product of conjugation is a zygospore, which, like the 

 thick-walled sporangia, gives rise to a number of zoospores on germinating after 

 a period of quiescence. 



An indication of an alternation of generations may be traced in the life- 

 history of the sexual members of the order. The plant is the gametophyte, 

 reproducing itself by zoogonidia formed in thin-walled gonidangia ; the zygo- 

 spore is the sporophyte, giving rise to zoospores, each of which, on germination, 

 gives rise to a gametophyte. 



This order is one of great systematic interest, inasmuch as it shows affinity 



