138 CYSTOPUS, OR WHITE RUST. 



The mycelium of Cysfopus, besides producing the chains of 

 spores already described, produces also other bodies, which may 

 be roughly compared to the pistils and anthers of flowering plants. 

 The terminal portion of a tube of mycelium swells out into a 

 large spheroidal cell, with thickened walls and granular protoplas- 

 mic contents, which are shut off from the mycelium-tube by a 

 partition. This is the female organ or oogonium, and the granu- 

 lar protoplasmic contents accumulating in the centre form the 

 oosphere. A neighbouring portion of mycelium-tube also becomes 

 i:)artitioned off, assumes an obovate form, and encloses within its 

 thin membrane a mass of fine granular protoplasm. This is the 

 male organ or antheridium, which presently comes in contact with 

 the oogonium, projects a fine beak through its wall, and pierces 

 the oosphere within. This is the act of fertiUsation, answering to 

 the discharge of pollen on to the stigma in flowering plants. In 

 the same way as an ovule becomes a seed after fertiUsation, so the 

 oosphere becomes an oospore after the contact of the antheri- 

 dium. A membrane is now formed around the oospore, which is 

 at first very thin, but gradually becomes thickened by deposits 

 from the surrounding fluid, and is at length yellowish-brown in 

 colour, and has its surface studded with large obtuse warts, one of 

 which, larger than the rest, surrounds the fecundating tube (PI. 

 XV., Figs. 8—12). Zalewski, who has made some detailed obser- 

 vations of these fungij finds that the oospore is enclosed in a 

 double coat, composed of endospore and exospore. The endo- 

 spore consists entirely of pure cellulose, and the exospore is 

 usually clearly differentiated into three layers, the innermost of 

 which is thin, homogeneous, and cuticularised ; the middle layer 

 suberised, finely granular, and composed of very thin round or 

 angular columns, placed very closely at right angles to the surface; 

 and the outermost composed of cellulose, and dark brown, cuticu- 

 larised or suberised externally. The mature oospore contains a 

 ball of protoplasm, occupying from one-half to nearly two-thirds 

 its diameter, which itself contains a large amount of oil. The 

 protoplasm between this and the wall of the oospore is dense and 

 granular ; in its outer portion are a number of bright, round spots, 

 probably vacuoles. The oospore grows and matures itself whilst 

 still within the supporting leaf or stem for many months, and does 



