ZOOLOGY AND BOTANY, MICROSCOPY^ ETC. 231 



prothallium of Equiseta and on potatoes. The oogonia and oospores 

 are smaller than those of P. de Baryamim, the former measuring 

 15-18^, the latter 12-15 /x. It also differs in the mode of germi- 

 nation, and in the abundant formation of zoospores from the freshly 

 formed oospores. It also shows no indication of its thallus penetra- 

 ting the living tissue of the host ; it is a saprophyte, not a parasite. 



P. megalacanthum n, sp. is found, along with the first species, on 

 cress ; but only on tissue which is already dead, and is hence not a 

 true parasite. The zoospores are of comparatively large size, having 

 an average diameter of 18-20 jx after coming to rest; and 12-15 or 

 more are formed in a zoosporangium. The oogonia are characterized 

 by a large number of vertical conical protuberances, averaging about 

 one-half the length of the radius of the oogonium. The formation of 

 oogonia and oospores takes place chiefly within the tissue of the host. 

 There is a less sharp distinction between gonoplasm and periplasm. 



P. intermedium n. sp. is also saprophytic on Lejndium and 

 Amaranihus. The conidia are formed in rows of from 2-5 by 

 successive abstriction, in a manner different from that kno\ni in any 

 other species of Pythium. The author has at present failed to detect 

 oogonia and antheridia. 



P. proliferum appears on dead insects floating in water that 

 contains algae ; it does not attack living plants. This species closely 

 resembles P. de Baryanum in its general morphology, and in the size 

 of the oogonia and oospores. It is characterized by the successive 

 formation of new zoosporangia by a process of prolification. A 

 slightly different form, possibly permanently distinct, is named by 

 the author P. ferax. 



All the species of Pythium hitherto described have more or less 

 globular zoospores ; in P. monospermum, reptans, and gracile, the 

 zoosporangium is filiform, the zoospores being formed in the terminal 

 cell of an ordinary branch of the thallus. 



P. gracile occurs in dead flies, in water that contains algee, and 

 can be cultivated on dead plants of Lepidium or Camelina. The 

 oogonia are very minute, and are formed only in and between the 

 cells of the dead plant. On warm days in summer the oospores are 

 mature in from 24-48 hours after fecundation, and remain then for 

 months in a resting condition. The other forms with filiform zoo- 

 sporangium are exceedingly similar, and perhaps identical, but appear 

 to be truly parasitic. 



Associated with the species already described, and especially with 

 P. de Baryanum, there is commonly found one with spiny oogonia, 

 described by Montague and Berkeley under the name Artotrogus 

 hydnosporus. This is the foundation of Montague's genus Artotrogus, 

 formed, as de Bary thinks, on insufficient grounds, chiefly from the 

 negative character of the absence of zoospores, and he proposes for it 

 the name Pythium Artotrogus. The antheridia are never, the oogonia 

 rarely, formed from terminal cells of the branch. 



Phytophthora omnivora is a parasite on a large number of healthy 

 plants, rapidly killing them, especially in wet seasons, or when 

 otherwise well supplied with moisture, and then living as a 



