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 Vaiicheria. The sexual organs have 

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

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

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

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

 a septum, and contains dense protoplasm with numerous nuclei. 



Fig. 355. 

 stages of germination of one of ttie conidia of Phytop!itkoi-a. a, the ripe conidium 

 in water, b, protoplasmic contents brealiing up into blocics, which separate and escape 

 (r, d) as minute itidney-shaped zoospores {e), each mth two cilia. /, g, the zoospore 

 coming to rest, and losing its cilia. /;, i, ;', 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 multimicleale 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 zvgote 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 di\'ide, gix'ing rise to a number of zoospores, 

 which may cause a fresli infeclion in the same wa\- as tliose produced 

 from the conidia. 



The parallel between this structure and !liat of ]'auchciia is close 

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

 fertilisation liy a fertilising tube in place of the liberated spermatozoids 



