232 SUMMAKY OF CUBEENT RESEARCHES RELATING TO 



saprophyte on the dead tissues, or on dead animals. The oogonia 

 appear to be produced only under water, and only on some of its 

 vegetable hosts. Experiments completely failed to infect the potato 

 or tomato with this species, which de Bary identifies with Hartig's 

 P. Fagi, which produces the destructive disease on seedling beeches,* 

 with Schenk's Peronosjjora Sempervivi, and probably with Cohn's 

 P. Cactorum. This species agrees in all essential points of structure 

 with the well-known P, infestans. The conidia or zoosporangia are 

 considerably larger than in that species, but vary in size, the 

 average length being about 50-60 /x, and the average breadth about 

 35 fx. ; their granular protoplasm is of a darker colour. The ripe 

 oogonia are spherical, with a thick, smooth wall, and smaller than 

 in most species of Peronospora, about 24-30 /a in diameter. They 

 are usually terminal, and are produced on lateral swellings or 

 branches. In the process of fecundation there is not, as in Pythium, 

 the formation of any distinct gonoplasm-layer. This species agrees 

 with P. infestans in the peculiarity which distinguishes the latter 

 from all others of Peronospora, viz. the successive formation of several 

 conidia on one conidiophore. The two species are, however, un- 

 doubtedly distinct, and in all probability the unknown oospores of 

 P. infestans resemble those of P. omnivora in their smooth surface, 

 and in other particulars. Although the name P. Cactorum has the 

 claim of priority for this species, de Bary prefers the more descriptive 

 P. omnivora. 



In Peronospora the history of development of the sexual organs is 

 very similar to that in Phytophthora. There is no evident passage of 

 any considerable quantity of protoplasm from the fertilizing tube to 

 the oosphere. 



In the species of Saprolegnia belonging to the ferax group (in- 

 cluding S. monoica, Thureti, and torulosa), the oogonia are as a rule 

 terminal on primary or lateral branches. Here also no passage of 

 protoplasm from the fertilizing tube into the oosphere was ever 

 observed. The mutual function of the two organs appears to consist 

 simply in their close contact, and movements of the protoplasm in 

 each of the organs. The tube puts out an appendix which creeps in a 

 sinuous course over the surface of the oosphere ; it at length loses its 

 protoplasm, and finally disappears altogether. When an oogonium 

 contains several oospheres, the tube grows from one to another of 

 them (except in the case ofS. torulosa), and the same process is repeated 

 in each case. In S. Thureti and torulosa instances frequently occurred 

 of oospores ripening without any contact with the antheridia, 



S. asteropliora, distinguished by its spiny oogonium, differs in no 

 important point from S. monoica. Here again no opening could be 

 detected at any time in the fertilizing tube. Normal oospores are 

 occasionally formed without the co-operation of antheridia. 



Aclilya prolifera and polyandra resemble one another in all 



essential points. The development of the oogonia presents no 



essential difference from that in Saprolegnia monoica. The most 



important distinction is that in Aclilya the protoplasm of the 



* See this Journal, ii. (1879) p. 923. 



