CHAPTER V.—COMPARATIVE REVIEW.—PERONOSPOREAE, 137 
and especially of genera, may be taken chiefly from the gonidial formations, while the 
few species which have no gonidia are not easily classified. 
The main features in the formation of gonidia in the genera and subgenera of the 
Peronosporeae are as follows :— 
Pythium. A persistent cell, usually the terminal cell of a branch, is delimited by a 
transverse wall and becomes a spore-mother-cell (sporangium). The gelatinously 
thickened wall at its apex suddenly expands into a thin-walled spherical vesicle, and 
into this at the same moment the whole of the protoplasm of the cell, which is hitherto 
undivided or has only shown transitory beginnings of division, streams rapidly, within a 
few minutes’ time at most; there it breaks up at once-into a number of swarm-spores, 
which issue from the delicate swelling vesicle and finally germinate. The sporangia in 
some species are of the same form as the gonidia of Phytophthora (Fig. 64),—round or 
ovoid vesicles prolonged at the extremity into a neck or beak, in the apex of which the 
swarm-spores are formed ; in others any portion, and often a very long portion, of the 
cylindric filamentous thallus-tube is delimited to form the sporangium, and its apex in 
which the swarm-cells are formed is then a small knob-like enlargement at the 
extremity of a branch, but is 
not otherwise distinguished by 
any particular form. There is 
usually no strict regularity dis- 
cernible in the arrangement of 
the sporangia. In species with 
vesicular sporangia the fila- 
ment which bears them often 
grows on into the empty spor- 
angium from the point of its 
insertion, or through its entire 
length; and then forms a new 
terminal sporangium as in 4 
Saprolegnia (see p. 46). A 
certain kind of regular ar- 
rangement and succession oc- FIG. 64. Phytophthora infestans. Extremities’of two simple sporophores. a for- 
] E i f the fir idi: i a «a: 
curs in one species only, mation of the first gonidia on the tip of each branch. 4 two ripe gonidia on each 
branch, a third beginning to form. Magn. about 200 times, 
Pythium intermedium, and in 
two ways, sporangia being either formed sympodially and successively on a branch 
of the thallus and separated by elongated portions of the thallus, as is described on 
pages 47 and 65, or by serial successive abjunction (see p. 66). In the same species 
the sporangia with their wall of delimitation are easily and abundantly shed from their 
sporangiophore, so that they may equally 
well be called spores, and the name is 
still further justified by the fact that 
under certain circumstances they may 
put out a germ-tube immediately without 
forming swarm-spores. P. de Baryanum 
also often forms spores (gonidia), which 
have the same form as the sporangia, 
in place of sporangia, but germinate 
directly by the emission of a tube. : ore 
Phytoplithora (Figures 64, 65). emails teure « Sormetm I wove 
Branches of the thallus either solitary or forest and bank (eres in the motile ime one 
growing side by side in small tufts con- 
stitute peculiar gonidiophores and form gonidia. They generally send out a few branches 
monopodially disposed, and each of these secondary branches or the unbranched 
gonidiophore forms a number of gonidia sympodially and successively at elongated 

