I 



OOSPORES, 



^n 



arming about (Fig. 178, B). It also sometimes occurs that the gonidia clothe them- 

 Ives with a delicate cell-wall while still within the mother-cell, forming a kind of 

 parenchyma, and then escape from the mother-cell through numerous holes in its wall. 

 These various modes of formation of the gonidia, by means of which species and even 

 genera have hitherto been distinguished, can, as Pringsheim has shown, take place 

 simultaneously on the same plant, as in Saprolegnia and Achlya. In Saprolegnia, 

 when the gonidia have escaped from the terminal cell of a branch, the septum 

 bulges out and developes into a new gonidial receptacle which takes the place of the 

 one that is now empty; in Achlya a lateral branch beneath the septum becomes the 

 new gonidial receptacle. In the very small and simple genus Pythkim, which is parasitic, 



~IG. 178.— Two gonidial receptacles of 

 chlya; A one still closed; B with the 



fonidia escaping; a gonidium just es- 

 ?ed and still in a state of rest, c after'com- 



icing to swarm, having abandoned their 



•walls b. 



Fig. 179. — Oogonia and antheridia oi Achlya lignicola, growing on 

 wood in water ; the course of development is indicated by the letters A —F. 

 a the antheridium, b its tube penetrating into the oogonium (X 550). 



laments emerge which open at their apex and allow the protoplasm to escape in a ball 

 which then breaks up into a number of zoogonidia. After coming to rest the zoo- 

 gonidia of the Saprolegnieae give rise to new plants, which, when they have access to 

 a fresh substratum, such as a dead fly, produce in succession several generations of 

 non-sexual gonidia-forming individuals. 



It is only towards the close of the period of growth that sexual individuals make 

 their appearance. The extremities of the filaments then swell up into a globular 

 form (Fig. 179), a septum is formed beneath the swelling, which constitutes the oogo- 

 nium, and its protoplasmic contents either simply contract to form the oosphere, as 

 in Pythium monospermum and Aphanomyces, or the contents divide into two or more 



T 



