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A TEXTBOOK OF THEORETICAL BOTANY 



Under favourable conditions for germination the spore wall bursts and a 

 germ tube is put out, which grows over the surface of the leaf until it comes 

 to the pore of a stoma, when it immediately passes down into the leaf and 

 forms the mycelium which, as we have already said, ramifies in the spaces 



Conidiospore 



Haustorium 



Host cell 



Fig. 222. — Peronospora sp. A, Branched conidiophore emerging from stoma. 

 B, Germination of conidiospore. C, Mycelium in host tissue. 



between the cells. Considerable investigation has been directed to the 

 problem of what induces the growth of the germ tube into the stomatal 

 opening and the growth of the young conidiophore out of it at a later stage. 

 Chemotropism and hydrotropism have both been suggested, but the true 

 explanation is still a matter of some uncertainty. 



Sexual Reproduction 



Sexual reproduction is of an oogamous type, resembling in general that of 

 Pythium. It takes place under conditions adverse to the Fungus and in the 

 deeper layers of the host tissue, generally in the stem, and especially in the 

 pith. The sex organs consist of antheridia and oogonia, and are formed 

 terminally at the apices of either the same or of separate hyphae (Fig. 223). 

 The oogonium is a swollen structure containing, at an early stage, a large 

 quantity of cytoplasm and many nuclei. The antheridium is more tubular 

 in shape, and it, too, is multinucleate. 



