CHAPTER V. COMPARATIVE REVIEW. SAPROLEGNIEAE. 



141 



SAPROLEGNIEAE. 



SECTION XL. These plants, which live on dead organic bodies in water, closely 

 resemble the Peronosporeae in the course of their development and to some extent 

 also in habit ; they are most of them of large growth, with tubular hyphae 1-2 cm. in 

 length standing out from the substratum and slender rhizoids spreading through it 

 (Fig. 68). They differ from the Peronosporeae chiefly in the development of the 

 oosphere and in the circumstance that in all the better-known species the antheridia 

 though existent do not perform their fertilising function, or are entirely wanting. 

 The oogonia are formed on branches of the thallus-tubes as in the Peronosporeae, 

 and the whole of the fatty protoplasm is transformed into a single oosphere, or divides 

 into several portions which become so many round oospheres, without any separation 

 of periplasm. The number of oospheres varies in both species and individuals. Most 

 species have aa a rule several oospheres in an oogonium, some have as many as 30 or 

 40 or more, feeble specimens often only 

 2-4. The oospheres ripen into oospores, 

 which in most cases have the same struc- 

 ture as those of the Peronosporeae, es- 

 pecially the Pythieae (Fig. 69 C). A few 

 species are unlike the rest in this respect. 



Like the Peronosporeae many Sapro- 

 legnieae have antheridia ; in a few species 

 only (Saprolegnia hypogyna, Pringsh.) the 

 antheridium is the stalk-cell which bears the 

 oogonium, as in Monoblepharis (Fig. 67): 

 usually it is the obliquely club-shaped or 

 cylindrical terminal cell of slender branches 

 which grow one or more in number to- 

 wards the oogonium and apply themselves 

 closely to it. These lateral antheridial 

 branches spring, according to the species, 

 either from the branch of the thallus 

 that bears the oogonium to which it at- 

 taches itself, and then usually close to it 



(androgynous forms; Fig. 69 A,B}; or from special branches of the thallus which 

 do not bear oogonia (diclinous forms) ; cases intermediate between the two extremes 

 are of comparatively rare occurrence. The antheridial branches apply themselves to 

 the oogonia. The first appearance of the lateral branches and the delimitation of 

 the antheridia take place before the formation of the oospheres. When these are 

 formed, the antheridia usually send out i, 2, or 3 delicate tubular processes which, like 

 the fertilisation-tubes of Pythium, grow into the oogonium and in the direction of the 

 nearest oosphere and apply their apex firmly to it; but they do not open, and 

 nothing like a discharge of their protoplasm has ever been observed (Fig. 69 B} ; on 

 the contrary they generally continue to elongate after their first contact with an 

 oosphere and grow over its surface and not unfrequently beyond it. When there 



FIG. 68. Acklya prolifera. A germ-plant twenty-four 

 hours old, about 1.5 mm. in height, growing on the surface of 

 the larva of a gnat indicated by the line aa ; rl indication of 

 branches of the primary rhizoid which have penetrated into 

 the substance of the larva ; r secondary rhizoid-branches 

 growing towards the substratum from the erect and sub- 

 sequently fertile branches. 



