CHAPTER V. COMPARATIVE REVIEW. SAPROLEGNIEAE. 



limited to the direct development of a thallus from the germ-tube which proceeds 

 from the oospore, and to the formation of oogonia, oospores, and antheridia on it. 

 There is usually no formation of gonidia in this species. 



In all other cases the fully-grown thallus which forms oospores also produces 

 gonidia; the production is comparatively scanty and uncertain in Achlya spinosa, 

 abundant usually in all other species. The gonidia are formed first, the oospores 

 appearing during a later period of the development, partly on the same main branches 

 of the thallus as the gonidia, partly on special branches. Here, as in the Peronosporeae, 

 and evidently as the result of external causes, 

 the gonidia are produced in much greater 

 quantities than the oospores, and they are them- 

 selves the most effective in the propagation of 

 the species. At the same time no species in the 

 Saprolegnieae is known to be without oospores. 

 The gonidia in all species, except the Aplanes 

 mentioned above, are normally swarm-spores and 

 are formed either in the germinating oospore, or 

 in sporangia, which are usually of some size and 

 borne on branches of the thallus. The species 

 with swarming gonidia have occasionally resting 

 gonidia also, but their appearance is accidental 

 and exceptional. The genera of the Sapro- 

 legnieae, like those of the Peronosporeae, are 

 chiefly distinguished by the sporangia and the 

 formation of the swarm-cells in them. 



The genera Saprolegnia, Achlya and Dictyu- 

 chus, when well developed have club-shaped 

 sporangia, the protoplasm of which divides into 

 numerous spores arranged in many rows (Fig. 

 70 A). Very feeble specimens form only one 

 row of spores, and there is scarcely ever more 

 than one row in the long narrowly cylindrical 

 sporangia of Aphanomyces. (See section XVIII 

 a, p. 74.) 



The distinguishing mark of Saprolegnia is 

 that the spores are in the motile state as they 

 issue from the sporangium, and that the branch 

 of the thallus which bears the sporangium grows 

 through it when it has discharged its spores. 

 Achlya and Aphanomyces are known by the 

 discharged spores collecting into little heads form- 

 ing the hollow spheres described on page 108, which they subsequently leave when they 

 begin to swarm (Figs. 69 A, 70 B}. Another exceptional fact besides the forming of 

 these heads is observed in some species of Achlya and according to Sorokin l in 

 Aphanomyces also ; the spores are invested with a membrane of cellulose in the 

 place in which they are formed inside the sporangium, and afterwards burst through 

 this membrane and through the lateral wall of the sporangium to swarm. A number 



FIG. 70. Sporangia of a species of Achtya. A 

 after formation of the spores (gonidia) but still closed. 

 B after ejection of the gonidia, a few only remaining 

 in the sporangium. The larger number are grouped 

 at its mouth in a hollow sphere a and have become 

 invested with a cell-wall ; at c they begin to swarm 

 away leaving their cell-walls * behind them. Magn. 

 about 300 times. 



1 Ann. d. sc. nat. sr. 6, II (1876). p. 46. 



