226 STKUCTUKAL BOTANY 



period of reproduction approaches. Previous to this the 

 bulk of the protoplasm has travelled into the more 

 terminal portions of the mycelium, where the repro- 

 ductive cells are to be produced ; the transverse walls 

 may serve the purpose of keeping it where it is wanted. 



The protoplasmic contents of the antheridium, like 

 those of the oogonium, undergo a severance into a 

 central fertile portion and an external layer of periplasm 

 and here also it is the former alone which plays an active 

 part in the reproductive process. There is no division 

 into spermatozoids, and in fact these bodies are almost 

 unknown among Fungi, another point in which the 

 adaptation to a terrestrial habit of life has involved 

 the disappearance of active reproductive cells. The 

 antheridium sends out a short branch, the fertilising 

 tube, which penetrates the wall of the oogonium, and 

 reaches the ovum (Fig. 95). The fertilising tube opens 

 at its end, and now the whole contents of the antheridium 

 (with the exception of the periplasm) pass through the 

 tube, and unite with the protoplasm of the ovum (Fig. 

 95, B). The whole process can be directly followed with 

 ease, under a high power of the microscope, and, indeed, 

 Pythium is one of the most favourable plants for the 

 immediate observation of the fertilising act. It is 

 probable, from analogy with allied Fungi, that only a 

 single nucleus passes over with the male protoplasm, 

 and unites with that of the ovum. 



As the result of fertilisation, the ovum surrounds 

 itself with a thick cell-wall, the outer layer of which is 

 derived from the periplasm by which it is surrounded. 

 The ovum has now become an oospore ; its contents 

 form a quantity of oil, as a reserve of carbonaceous food 

 and it now passes into a period of rest. 



