CHAPTER V.—COMPARATIVE REVIEW.—PERONOSPOREAE. 135 
genera, but the passage of the protoplasm into the oosphere cannot be directly seen, 
and the periplasm in the oogonium is much denser and more copious (Figs. 61 
VII and 62). Lastly the species of Cystopus behave like Peronospora in every 
point that can be distinctly seen, but the details in this genus require further 
investigation. 
In Pythium, Phytophthora, and Peronospora the constituents of the protoplasm 
of the oosphere are rearranged as it begins to mature, and the structure of the ripe 
oospore is of the following kind (Fig. 61 V7; see also section XL, Fig. 69 ¢). The 
membrane becomes thicker and is composed of two main layers, an episporium 
and an endosporium ; the latter usually swells up into a jelly in water in Peronospora, 
and both show the reaction of cellulose. The original oil-drops are dissolved, but 
their substance has collected into a comparatively large sphere pale on the outside 
and of weak refringent powers occupying the centre of the spore-cavity. The 
space between it and the membrane is either filled with protoplasm containing fine 
uniformly disseminated granules, or a layer of protoplasm of this kind lines the wall, 
but is separated from the central sphere of fatty matter by a limpid zone. When the 
spore is ripe a round or elongated and perfectly pellucid spot makes its appearance 
in the parietal layer close to the membrane. The nature of this spot has not been 
clearly ascertained ; it is perhaps a nucleus, which is visible as a transparent central 
body when the oosphere is first formed and afterwards disappears from that position. 
In Pythium and Phytophthora the periplasm cannot be seen to take part in the 
maturing of the oospore; it surrounds it in the form of an inconspicuous sparingly 
granular mass. In Peronospora (Figs. 61 V//, 62) on the contrary it developes 
into a thick membrane usually of a deep brown colour, the exosporzum, which forms 
a closely fitting envelope round the ripe oospore, and has a structure and surface- 
sculpturing characteristic of each species. In this point Cystopus closely resembles 
Peronospora, but the structure of the ripe oospores still wants a more thorough 
examination. The wall of the oogonium undergoes no special changes as the 
oospore matures; in most cases it becomes decomposed; in some of the Perono- 
sporeae it becomes strongly thickened at the time of the formation of the oosphere, 
and may persist after the latter is ripe. 
The oospores remain dormant for some time and then germinate under water; 
in some species they put out a germ-tube, which on a favourable substratum grows 
at once into a thallus which produces oospores in the same way as the mother plant; this 
is the only form of germination in Pythium de Baryanum, Artotrogus, and Peronospora 
Valerianellae. In other species the whole germinating oospore always becomes 
a sporangium ; its protoplasm divides simultaneously into from several to many 
spores which swarm out of a previously formed tubular process, and after a short 
period of movement each developes like the spores of the first-mentioned species into 
a thallus that produces oospores. Cystopus candidus (Fig. 63) is an instance of this 
kind. In a third group of species some oospores show the one, some the other mode 
of germination, sometimes apparently under the influence of external causes; this is 
the case with Pythium vexans and P. gracile. A fourth group containing Pythium 
proliferum and Phytophthora omnivora exhibits a mode of germination which 
is to some extent intermediate between the first and second kind and which will 
be described further on. 
