THALLOPHYTA: FUNGI 



109 



Hum. The external part of the fundus then becomes transformed into a 

 sporangium, its protoplast undergoing cleavage into many zoospores 

 (Fig. S5A). A zoospore may penetrate a zygote of the host and, by the 

 secretion of a thick wall, become a resting spore. When the zygote 

 germinates, the resting spore of the fungus sends out tubes that give rise 

 to terminal sporangia. 



Olpidium. This fungus grows on many different hosts, some of which 

 are fresh-water algae. One species, Olpidium brassicae, attacks young 



A B 



Fig. 85. Chytridiales. A, sporangium of Chytridium olla attached to zygote of Oedo- 

 gonium; B, sporangia of Olpidium brassicae in root of cabbage seedling; also two zoospores of 

 same to the left. {A, after Campbell; B, after Woronin.) 



cabbage plants. A uniciliate zoospore comes to rest on the host, with- 

 draws its cilium, and secretes a cell wall. It sends a short tube into the 

 host and the protoplast enters one of the cells. At first the protoplast is 

 naked and amoeboid. It enlarges and becomes multinucleate, finally 

 occupying the whole cell cavity. Then it forms a cell wall and becomes 

 a sporangium. A tube is sent to the surface of the host and numerous 

 uniciliate zoospores escape through it (Fig. 855). Sexual reproduction 

 occurs by means of isogametes that are formed like the zoospores but 

 escape and fuse in pairs. The zygote sends a short tube into a host cell, 

 after which its protoplast enters, enlarges, and secretes a thick wall. 

 After resting over the winter, it gives rise to a number of uniciliate 

 zoospores. 



Synchytrium. This form attacks the epidermal cells of various seed 

 plants, such as cranberry, primrose, hog peanut, filaree, and many others. 

 A disease called cranberry gall is caused by Synchytrium vaccinii, while 



