FIRST GROUP. THALLOPHYTES. 



the antheridium. The sexual process was first fully described by De Bary in this case, 

 as in that of the Peronosporeae. Saprolegnia ferax may serve as an example. In this 

 species the smaller oogonia contain only one oosphere, the larger from ten to twenty. The 

 wall of the oogonium is marked with roundish unthickened areas (pits), but the anthe- 

 ridia send their tubes through the thickened as well as the unthickened portions of the 

 wall; there is therefore no morphologically definite spot for the attachment of the 

 antheridium to the oogonium. The oosphere has in its centre a clearer spot, the nuclear 

 spot, which is probably only a nucleus ; its outer surface is formed of a thin pellicle 

 free from granules. The volume of the oosphere shrinks in the process of formation ; 

 there is therefore a loss of water. The oospheres when formed lie as spherical bodies in 

 the centre of the oogonium, in which there is then nothing but the oospheres and some water. 



FIG. 60. Two zoosporangia of Achlya. FIG. 6r. Oogonia and antheridia of Achlya lignicola, growing on 



A still closed. B open to discharge the wood in water ; course of development according to the letters from A to 



spores; a spores ejected but still resting, E- a the antheridium, b its tube forcing its way into the oogonium. Magn. 



c swarm-spores, which have left their mem- 550 times. Compare the text. 

 branes at b behind them. Magn. about 

 300 times. 



Meanwhile the contents of the antheridium have been differentiated into a layer of pro- 

 toplasm lining the walls of the cell and a central watery portion. Then the antheridia 

 begin to put out tubular processes at the point of contact with the oogonium, each 

 antheridium producing two or three such tubes ; these are the fertilisation-tubes, which 

 penetrate throug"h the wall of the oogonium. If there is only one oosphere in the oogonium, 

 the tube grows up to it and attaches its extremity firmly to it ; after a few minutes a pro- 

 tuberance appears at the margin of the point of contact of the fertilisation-tube and the 

 oosphere, and rapidly developes into another tube, which at first travels over the surface 

 of the oosphere but eventually takes another direction. If there are more oospheres than 

 one, and only one tube enters the oogonium, it grows towards the nearest oosphere ; the 

 second tube, the protuberance just mentioned, then grows over the first oosphere and 



