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 fertilised 

 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 moncecious 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 Peronosporeae. 



3. The Peronosporeee ^ 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 ans and nivea^ 

 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, jD) ; in others, as P.pygmcea, 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 



* De Bary, Ann. des Sci. Nat., 4^ sen, i860, vol. XIII ; 1863, vol. XX. [On Phytophthora {Pe- 

 ronospora) infestans, see De Bary, Joum. of Bot. 1876, pp. 105-126, and 149-154: see also Bot. Ztg. 



1 881.] 



T 2 



