FUNGI. PERONOSPOREAE. 



93 



and gonidia are, as comparison with the other species shows, equivalent formations 

 which behave differently according to the external conditions. Pythium, which lives 

 both in water-plants (Algae and others) and in succulent land-plants (seedlings, &c.), 

 discharges the contents of its gonidia (zoosporangia) while they remain on the mycelial 

 tube which produced them ; in Peronospora and Cystopus, on the contrary, the gonidia 

 at the extremities of branches of the mycelium germinate only after having been sepa- 

 rated from them. 



In Peronospora, a genus with many species, long slender branches of the mycelium 

 push through the stomata of the host into the open air, where they ramify and form 

 a comparatively large roundish elongate gonidium at the extremity of each branch. In 

 Cystopus, on the contrary, a number of short club-shaped branches (Fig. 56, B] appear 

 close together on the parasitic mycelium beneath the epidermis of the host, and each 



FIG. 56. A G Cystopus candtdus. H Phytophthora infestans. A branch of mycelium growing- at the 

 apex / with haustoria h between the cells of the pith of Leptdium sativunt. B branch of mycelium bearing gonidia. 

 C E formation of swarm-spores from gonidia. F swarm-spores germinating. G swarm-spores germinating on a stoma 

 and piercing the epidermis of the stem of a potato at H. After De Bary, magn. about 400 times. 



of these forms at its free extremity a row of round gonidia, a gonidia-chain, till at last 

 the multiplication of these gonidia bursts the epidermis and they issue forth as a 

 white dust. The behaviour of the gonidia in germination varies. In certain species of 

 Peronospora, as P. infestans, P. nivea, if the gonidia on separating from the parent plant 

 fall into a drop of water (dew, rain), their contents break up into a number of zoogonidia 

 and disperse (Fig. 56, C, D] ; in P. pygmaea the whole of the protoplasm escapes from 

 the gonidium and forms a roundish cell which at once developes a germ-tube. In a 

 third and fourth section of the genus the gonidium puts out a tube at once, and either 

 at a fixed spot, as in P. gangliformis, or at any point, as in P. parasitica, P. calotheca, 



