THE FUNGI WHICH CAUSE PLANT DISEASE 



79 



causing the "white rusts." The conidia are borne in white 

 bUster-like sori under the raised and finally ruptured epidermis 

 of the host. The conidiophores are short, club-shaped, arranged 



Fig. 50. — ^Albugo. A, section through a sorus showing epidermis, conidia, 

 conidiophores and mycelium; B, conidiophores and conidia; C, myce- 

 lium and haustoria. After Bergen and Davis. 



in clusters; the spores are borne in basipetal succession and 

 remain attached in rather long chains tmless disturbed. 



The mycelium is very fine, intercellular and penetrates the cells 

 by globular haustoria. The rudimentary oogonium is multi- 

 nucleate and filled with uniform proto- 

 plasm. As the oogonium grows older 

 the protoplasm within differentiates 

 into two parts, the inner part of dense 

 protoplasm, the oosphere, and the 

 outer part less dense, the periplasm.^' 

 Figs. 51, 53, 54. During this process 

 the nuclei enlarge, undergo one or two 

 mitoses. Fig. 54, and in some species 

 all the nuclei except one pass to the 



periplasm. In other species the Fig. 51.— Multiple fertilization 

 .. , . ,i. 1 J. X J. -x inA.bliti. Anthendial tube 



oosphere is multinucleate at maturity, discharging sperms. After 

 The latter type is fertilized by nu- s*«^^°^- 

 merous nuclei from the antheridium, the former by a single nu- 

 cleus.**""' ^^ After fertilization the oosphere matures to an 

 oospore. 



