CHAPTER V.—COMPARATIVE REVIEW.—SAPROLEGNIEAE. 143 
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 ate in the motile state as they nn ie nee sores Bone saictiues 
issue from the sporangium, and that the branch after ejection of the gonidia, a few only remaining 
- = in the sporangium. The larger number are grouped 
of the thallus which bears the sporangium grows at its mouth in a hollow sphere 2 and have become 
through it when it has discharged its spores. away navdeg dir elle 0 betind tear an 
Achlya and Aphanomyces are known by the about 300 times. 
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! 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 

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