106 



OOMYCETES. 



FIG. 91. PhytopMliora infestans : a-c conidia de- 

 tached ; in c the swarm-cells are leaving the mother- 

 cell; d two free-swimming swarm-cells. 



(Fig. 90 c c), and immediately underneath it another is formed, 

 which pushes the first to one side, and so on. These conidia some- 

 times germinate directly, and form a mycelium, but most frequently 

 their protoplasm divides into many small masses, each of which 

 becomes a pear-shaped zoospore provided with two cilia (Fig. 91). 

 Water is required for their germination, and when the ripe conidia 



are placed in a drop of 

 water the swarm-cells are 

 formed in the course of 

 about five hours. They 

 swarm about in rain and 

 dewdrops in the Potato- 

 fields, and are carried with 

 the water to the Potato- 

 plants and to the tubers 

 in the soil. The wind also 

 very easily conveys the conidia to healthy Potato fields and infects 

 them. The enormous quantity of conidia and swarm-cells that 

 may be formed in the course of a summer explains the rapid 

 spreading of the disease ; and the preceding makes it clear why 

 wet summers are favourable to its existence. When the swarm- 

 cells germinate, they round off, and then surround themselves 

 with a cell-wall which grows out into the germ-tube, and pian's 

 through the epidermis of the host-plant (Fig. 92). Having entered 



the host, a new mycelium 

 is formed. The potato 

 disease, since 1845, has 

 been rampant in Europe ; 

 it has, no doubt, been in- 

 troduced from America, 

 which, it must be remem- 

 bered, is the home of the 

 Potato-plant. 



The conidia exhibit various 

 characters which are employed 

 for the separation of the genera. 

 Pythium is the most simple 

 form. The contents of the ter- 

 minally-formed conidia emerge as a spherical mass and divide into swarmspores. 

 P. de Baryanum lives in the seedlings of many different Flowering-plants, 

 which it completely destroys. Phytophthora is distinguished by the circum- 

 stance that the sparsely-branched conidiophores bear, sympodially, chains of 



FIG. 92. PUytopUliora infestans. Cross section 

 through a portion of a Potato-stalk. Two germinat- 

 ing conidia (a, b) piercing the epidermis, and the 

 mycelium penetrating the cells. 



