96 FIRST GROUP. — THALLOPHYTES. 



moves in the form of a band of irregular thickness into the central space. Then 

 begins a movement of the gonoplasm through the tube into the oosphere, till it has all 

 made its way into it ; the movement is slow, and the transit lasts from one to two 

 hours. When the transit begins, the more granular portion of the protoplasm of the 

 oosphere draws back from round the point of contact with the fertihsation-tube, leaving 

 a narrow hyaline spot free, the receptive spot. The particles of the gonoplasm enter 

 one after another into the substance of this spot, then move towards the dark granular 

 substance and disappear in it. Here therefore fertilisation is effected by the union 

 (conjugation) of two masses of protoplasm. After fertilisation has taken place the 

 oospore is seen to be invested with a coat of cellulose, the periplasm of the oogonium 

 shrinks to a loose sac, and the wall of the oogonium remains intact till the germination 

 of the oospore. Sometimes more than one antheridium pours its gonoplasm into one 

 oosphere, as in the case of P. proliferwn. 



In PhytophtJioj'Li omnivora, a parasitic plant which attacks many Phanerogams, no 

 separation of the contents of the antheridium into gonoplasm (the only portion employed 

 in Pythium for fertilisation) and periplasm is to be seen ; a very small portion of the 

 contents of the antheridium passes over as gonoplasm into the oosphere, and the 

 appearance of this portion is not such as to make it evident that it had been previously 

 separated from the rest; but the transit is plainly seen. The same is the case with 

 Peroftospora ; but here only very minute quantities of the contents of the antheridium 

 pass over into the oosphere, nor can the transit be actually seen. In the nearly allied 

 group of the Saprolegnieae fertilisation appears no longer to take place, as will be seen by 

 the account to be given below, although the sexual organs are present in many cases in 

 the same form as in the Peronosporeae. The cellulose-wall of the ripe oospore is differen- 

 tiated into a thick outer layer, the episporium,and athin inner layer, the endosporium,and 

 the contents are composed of a central portion rich in oil, and an outer granular proto- 

 plasm surrounding it, which shows in one place a small hyaline spot. The oospore 

 olPeronospora and Cystopus has an additional envelope formed from the periplasm, which 

 hardens into a firm deep yellowish-brown coat fitting close to the oospore, and having 

 its surface coarsely and irregularly tubercled. The oospores enter when mature on a 

 period of rest, the duration of which varies in different species ; usually they remain 

 dormant during the winter and germinate in the next spring. The mode of germination 

 varies. The oospore enlarges and bursts the episporium and puts out a germ-tube, 

 which may remain shorter than the diameter of the oospore or exceed it several times 

 in length. This germ-tube either becomes a zoosporangium {Cystopus, Fig. 57, F), or 

 it ramifies and forms several sporangia, or, if it falls upon a suitable substratum, it 

 grows at once into a mycelium. These different modes of germination may occur in 

 one and the same species, according to the supply of nutriment (see above under Miicor) ; 

 but some species are limited to one mode. 



3. The SAPROLEGNIEAE' include the genera Saprolegnia, Achy la, Dictyiichus 

 and Aphanojiiyccs ; Pythium was till quite recently placed in this group but now ranks 

 with the Peronosporeae, from which the Saprolegnieae differ in growth and in the mode 

 of propagation. The Peronosporeae are chiefly endophytic parasites ; in the Sapro- 

 legnieae the spore settles on the outside of the substratum and sends out one germ-tube 



the protoplasm not expended in the formation of the spermatozoids is ejected ; compare also the 

 development of the spermatozoids in Chara and the Vascular Cryptogams, where it is really only 

 the nuclear substance of the mother-cells that is used to form them. Perhaps the gonoplasm of the 

 antheridia of the Peronosporeae consists chiefly of nuclear substance. 



' De Bary, Beitiage &c., IV. Reihe, and Unters. ii. d. Peronosporeen u. Saprolegnieen (Abh. d. 

 Senckenb. nat. Ges. Bd. XIII. pp. 225-370, Frankfurt 1881) ; [also Vergl. Morphol. u. Biologic der 

 Pilze, Myceotozoen u. Bacterien, Leipzig, 1884]. Pringsheim, Jahrb. Bd. I, II, IX. Other works in 

 De Bary, loc. cit. Schmitz, Ueber die Zellkerne der Thallophytcn (Niedenh. Ges. 18S0). 



