CHAPTER V.—COMPARATIVE REVIEW.—PERONOSPOREAE. 133 
are being formed by comparatively few transverse walls disposed at irregular 
intervals. Many of the parasitic species put out from the flanks of the intercellular 
thallus-tubes numerous small branches, which vary in form according to the species 
and penetrate into the interior of the adjacent cells in the form of haustoria (Fig. 8). 
The development of the thallus ceases with the formation of sexual organs: female 

FIG. 61. Formation of oospores and Processes of fertilisation in the Peronosporeae. J—V/. Pythium gracile. Suc- 
cessive states of an ] mature ; to the right of it is an antheridial branch formed but not yet 
delimited // antheridi imited by a transverse wall. /// the oosphere has rounded itself off in the oogonium, 
and a thin zone of periplasm lies between the oosphere and the wall of the oogonium. // the antheridium has put 
out the fertilisation-tube, and a clear receptive spot is visible on the oosphere. Y passage of the gonoplasm from the 
antheridi h pore sur! ded by a thick membrane and almost entirely filling the cavity 


into the oosp Vi ripe 
of the oogonium. VII. Peronospora arborescens; an oogonium with antheridium attached which has put outa fertilisa- 
tion-tube. The oosp! is already i d with a thick ; outside it is a comparatively broad zone of peri- 
plasm, which is contracting to form the exosporium round the oospore. /—Y/ magn. about 800, V/// 600 times. 



organs, oogonia, in each of which one oosphere is formed, and male organs, 
antheridia, by which fertilisation is effected in the oosphere which developes into the 
oospore. The behaviour of these organs in the process of fertilisation varies con- 
siderably in the different genera. It shall be described first in the genus Pythium. 
The oogonia of this genus (Fig. 61) are terminal or intercalary spherical 
a 

FIG. 62. Sexual organs of Peronospora Alsinearum, Casp. a young state. 5 formation of the oosphere and 
fertilisation-tube. c after fertilisation ; i d by the p ion, and the fertilisation-tube 
unusually thick in this speci 2 antheridi o i Magn. nearly 350 times. 


swellings of the thallus-tubes. Their surface remains smooth as it enlarges, or is 
rendered spiky in some species by projecting points. Their wall of cellulose is 
moderately stout. When they have reached their full size they are delimited from 
the thallus by one, or, if their position is intercalary, by two transverse walls, and are 
then filled with dense protoplasm containing numerous drops of oil (Fig. 61, Z). 
