94 



FIRST GROUP. THALLOPHYTES. 



Alsinearum and others. In the genus Cystopus either all the gonidia are alike, that 

 is, they all produce swarm-spores when they have fallen into a drop of water (C. 

 candidus], or the uppermost gonidium in a chain puts out a germ-tube, if it is capable 

 of germination, while the other cells of the chain produce swarm-spores (C. Portulacae). 

 When the swarm-spores come to rest they settle on the cuticle of the host and become 

 invested with a delicate cell-wall ; in Phytophthora i?ifestans they insert a slender 

 germ-tube directly into a cell of the epidermis by piercing the cell-wall ; the tube thus 

 introduced (Fig. 56, H] receives the whole of the protoplasm of the swarm-spore, and 

 having pierced through the inner wall of fhe epidermal cell reaches an intercellular 

 space, where the development of the mycelium then begins. On the other hand, the 

 swarm-spores of Cvstopus place themselves near the stomata of the host and send their 



FIG. 57. Cystopus candidus. A mycelium with young oogonia. B oogonium ? withoosphere os andantheridium an. 

 C ripe oogonium. D ripe oospore. E, F, G formation of swarm-spores from oospores ; i endosporium. After 

 De Bary, magn. 400 times. 



germ-tubes through the opening (Fig. 56, G) directly into the intercellular spaces. If 

 the mycelium is once established in the parenchyma of the host, it grows vigorously 

 and often spreads through the whole plant, and protrudes the branches which produce 

 the gonidia at various points in stem, leaf or inflorescence into the open air. The 

 mycelium may also live through the winter inside the host, as Phytophthora infestans 

 inside the potato-tuber, and spread again in the next spring in the young shoots. The 

 sexual organs of Peronospora, Phytophthora, Cystopus, and others, are formed inside 

 the host, outside it also in the saprophytic species of Pythium. A portion of a branch 

 of the mycelium, either the extremity or not unfrequently a part between the two 

 extremities, swells into a globular form, and the protoplasm moves into the swollen 

 part. The oogonium thus formed is then cut off by one transverse wall, if it is at the 



