OOSPORES. 
^15 
Two other remarkable facts have still to be mentioned : — in the first place that 
determined by Pringsheim, that parthenogenesis is not an uncommon phenomenon in 
Saprolegnia and Achlya. Some or all of the oogonia on a plant may not be fertiUsed 
at all, the formation of antheridia on them being altogether suppressed; and yet the 
oospores are perfectly developed and are capable of germinating. We shall refer 
more in detail in Book III. Sect. 30 to this and other instances of parthenogenesis in the 
vegetable kingdom ; here it need only be mentioned that the forms hitherto considered 
as dioecious — no antheridial branches being found on the oogonia — are not distinct 
species, but only parthenogenetic forms of monoecious species. In contrast to the 
development of oospores without fertilisation is the occurrence of antheridia the tubes 
of which do not penetrate into oogonia, but open and expel the fertilising particles into 
the surrounding water. In these, as well as in those tubes which pierce the oogonia, 
Pringsheim observed that the expulsion does not take place all at once, but in repeated 
jerks and after considerable intervals. 
The oospores or fertilised oospheres clothe themselves with a thick firm cell-wall 
and remain dormant in the oogonium for months. Their germination takes place 
subsequently in two different ways : — either the oospore puts out a germinating filament 
which at once developes into a branched plant in which zoogonidia are subsequently 
formed ; or it puts out a short filament which opens at its apex and allows the whole 
of its contents to escape in the form of zoogonidia. Sometimes after the com- 
mencement of the formation of a filament the contents break up into gonidia, or 
the entire contents, surrounded by an inner cell-wall, escape from the outer envelope, 
and then germinate. In their mode of germination the Saprolegnieae approach on one 
hand the Mucorini, on the other hand the Peronosporese. 
3. The Peronosporese^ are all parasitic on the succulent parenchymatous tissue 
of living Phanerogams (Dicotyledons), the irregularly-branched mycelium spreading over 
large areas in the intercellular spaces, and putting out at a number of places appendages 
(Haustoria) which penetrate into the interior of the cells of the host, and enable it 
to absorb the cell-contents as food. As in the preceding families, the entire vegetative 
body or mycelium consists of a single tubular cell not divided by septa. Multiplica- 
tion again takes place at the commencement of the period of growth exclusively in the 
non-sexual mode by the production of gonidia. These are abstricted and fall off from 
the end of branches, and are therefore stylogonidia, or, as they are usually termed, 
conidia. In the genus Peronospora, which comprises a large number of species, long 
slender branches of the mycelium emerge into the air through the stomata of the host 
for the purpose of the production of conidia, then branch in a somewhat arborescent 
manner, and form at the end of each branch a comparatively large ellipsoidal conidium. 
In Cystopus a large number of short club-shaped branches (Fig. 180, B) are formed 
densely crowded together on the mycelium beneath the epidermis of the host, and 
each of these produces at its extremity a series of conidia, until finally they burst 
the epidermis and escape in the form of a fine dust. The conidia germinate in 
different ways. In Peronospora there are some species, as P. Infest cms and ni^vea, 
in which the contents of the conidia, after falling off and reaching a drop of 
rain or dew, break up into a large number of swarming zoogonidia and escape 
(Fig. 180, C, i)) ; in others, as P. pygmcBüy the whole of the protoplasm escapes out of 
the conidium and forms a roundish cell which at once puts out a germinating filament. 
In a third and fourth section of the genus the conidium puts out at once a germinating 
filament which emerges either at a definite spot {P. gangliformis) or indifferently at any 
spot {P. parasitica^ calotheca, Alsinearum, &c.). In Cystopus either all the conidia are 
alike, i.e. when they reach a drop of water they produce swarming zoogonidia 
1 De Bary, Ann. des Sei. Nat., 4^ ser., i860, vol. XIII ; 1863, vol. XX. [On Phytophthora {Pe- 
ronospora) infestans, see De Bary, Journ. of Bot. 1876, pp. 105-126, and 149-154: see also Bot. Ztg. 
1881.] 
