PHYCOMYCETES— (a) OOMYCETES 411 



arising as a rule below it on the same hypha, forms the anther idium. 

 Each is shut off by a septum, and contains dense protoplasm with 

 numerous nuclei. A single egg, or ovum, is differentiated in the 

 middle of the oogonium, by passage of all the nuclei but one to a 

 peripheral position. The uninucleate ovum is then delimited from 

 the multinucleate periplasm. The anthcridium penetrates the 

 oogonium by means of a fertilising tube, the apex of which opens 

 into the ovum, and transmits a single male nucleus (Fig. 310, a). 

 The resulting zygote soon surrounds itself with a membrane, while 

 the periplasm contributes to the thickening of its wall. A period 

 of rest may ensue. On germination the contents of the zygote 

 divide, giving rise to a number of zoospores, which may cause a fresh 

 infection in the same way as those produced from the sporangia. 



The parallel between this structure and that of Vaucheria is close 

 as regards the origin of the uninucleate ovum. The method of 

 fertilisation by a fertilising tube in place of the liberated spermatozoids 

 moving freely in water, offers an interesting parallel with the pollen- 

 tube in Seed-Plants. In both cases the male gamete is conveyed 

 to the female by a method suitable for land-living plants. Comparison 

 shows that both are modifications of organs originally developed 

 to secure fertilisation through the medium of external water. The 

 question will naturally arise whether any fungal type still shows a 

 motile male gamete. This is found in Monoblepharis, a fungus that 

 lives saprophytically in water (Fig. 311). Here a terminal oogonium 

 contains a single ovum, which is fertilised by spermatozoids, each 

 motile by a single cilium. ' This case is unique among Fungi. It 

 holds a place comparable with the motile spermatozoids of the Cycads 

 and Ginkgo, in that it gives evidence of the transition from an aquatic 

 to a terrestrial type of fertilisation. (Compare Chapter XXXIV.) 



The origin of such Phycomycetous Fungi as those described may 

 have been from some Siphonaceous source. In the case of parasites 

 the first step would be the adoption of an endophytic life, as in 

 Phyllo siphon. This would be naturally followed by parasitic nutrition 

 and loss of chlorophyll and chloroplasts. As regards the propagative 

 organs, the sporangium of Pythium or of Phytophthora are such as to 

 make for easy detachment and transport through the air. But it 

 shows its real nature on germination by producing zoospores which 

 are liberated in water. In this connection it may be noted that in the 

 Peronosporales as a whole there is a marked tendency for the sporan- 

 gium to become, or to behave as, an air-borne conidium, as in Peron- 

 ospora in which zoospores are unknown. Sexual reproduction in the 



