420 



BOTANY OF THE LIVING PLANT 



some other plants it seems to be able to propagate itself indefinitely 

 without the recurrence of sexual reproduction. But in other members 

 of the Peronosporeae the details which have been observed show a 

 striking parallelism with those of Vaucheria. The sexual organs have 

 been found in Peronospora and Albugo to be formed within the host- 

 plant (Fig. 357). The oogonium appears as a spherical swelling on 

 the end of a hypha, while a thinner branch, arising as a rule below 

 it on the same hypha, forms the antheridium. Each is shut off by 

 a septum, and contains dense protoplasm with numerous nuclei. 



FIG- 355- 



Stages of germination oi one of the conidia of Phytopiithora. a, the ripe conidium 

 in water, b, protoplasmic contents breaking up into blocks, which separate and escape 

 (f, d) as minute kidney-shaped zoospores (), each with two cilia. /, g, the zoospore 

 coming to rest, and losing its cilia, h, i, j, k, stages of germination of the zoospore. 

 Highly magnified. (After Marshall Ward.) 



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 antheridium then penetrates the oogonium by means of a. fertilising 

 tube, the apex of which opens into the ovum, and transmits a single 

 male nucleus (Fig. 357, 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 conidia. 



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 



