WHITE RUSTS. 131 



and is called an ^^ oospore/^ The membrane which 

 at first invests this organ is very thin^ but by- 

 deposits from the surrounding fluid it attains to 

 a greater thickness_, and is at length of a yellowish- 

 brown colour_, having its surface studded with large 

 obtuse warts (plate X. fig. 206). One of these 

 warts, larger than the rest, forms a kind of thick 

 sheath around the fecundating tube. 



The oospores do not give evidence of any ap- 

 preciable change for some months. For instance, 

 those collected by De Bary in June did not attain 

 their ulterior development until the commencement 

 of December. The method adopted was as fol- 

 lows : — Parts of the plants containing ripe oospores 

 were preserved in the dried state. When examina- 

 tion was considered desirable, the portion to be 

 employed was immersed in water for a day or two ; 

 it was then placed on a humid soil, or mould 

 covered with blotting-paper. The tissues enclosing 

 the oospores were decomposed, and at the end of 

 from four to eight days their germination might 

 be observed when placed in a drop of water. This 

 method again corresponds with the ordinary pro- 

 cesses by which the plant naturally decays on 

 exposure to the influences of the atmosphere, and 

 the oospores germinate under the favour of a 

 shower of rain. 



If the oospore, after the decay of the tissues, 

 is isolated and placed in a drop of water, the 

 brown investing membrane will be seen to rupture 



k2 



