THALLOPHYTA— FUNGI 



79 



sporangia, after a period of rest. Zygospores are also formed, 

 but the differentiation of the sexual organs is not so complete as 

 in the Mucorinse. The Entomophthoraces are parasitic on 

 insects. They differ from Mucor chiefly in the mycelium being 

 incompletely septate, being thus composed of a number of 

 cosnocytes, instead of a single one. 



Oomycetes. — The chief members of this group are the 

 PeronosporesB and the Saprolegnise. The former are chiefly 

 parasitic, causing many of the diseases of Phanerogamic plants, 

 though saprophytic forms occur. A typical instance of them is 

 the fungus Phyiophthora infestans, which causes the potato 

 disease. The hyphiE ramify usually 

 between the cells of the plants, 

 sending haustoria into their interior 

 and so absorbing their nutritive 

 juices {fig. 831). The mycelium 

 sends out hyphse through the sto- 

 inata of the leaves, which bear 

 gonidangia at their apices. In 

 Phytophthora these hyphs are 

 branched and bear several goni- 

 dangia; in Peronospora each bears 

 a single one. In some cases the 

 gonidaugium gives rise to zoogo- 

 nidia, in others it behaves like a 

 gonidium and puts out a hypha 

 directly. 



The sexual reproduction of this 

 group is seen best in such forms as 

 Pythium and Cystopus. A hypha 

 cuts off a cell at its apex, which 

 becomes swollen and rounded and 



forms an oogonium. Its contents give rise to a single oosphere, 

 part of the protoplasm only being concerned in its formation 

 whUe the rest forms a peripheral layer, the periplasm. The 

 male organ or poUinodium is produced in a similar manner 

 from a hypha, which sometimes springs off as a lateral branch 

 from that which bears the oogonium and sometimes from an 

 adjacent one. The poUinodium becomes closely applied to the 

 oogonium and penetrates its wall by means of a delicate tube 

 which serves to conduct the male cell into the oogonium, where 

 it fuses with the oosphere, forming the oospore, which secretes 

 a cell-wall round it. After a period of rest the latter germinates. 



Fig. 831. Foiir cells of potato 

 plant, infested witti Phyto- 

 phthora. b. Hypha running be- 

 tween the cells and sending 

 haustoria, a, into their interior. 



