THALLOPHYTA— FUNGI 



91 



extends centrifugally. An annular cavity is formed in this 

 near its lower part, which increases in size as the head extends. 

 The head can now be recognised as distinct from the lower portion 

 of the hyphal mass, and is known as the p He us, the lower part 

 or stalk being called the stijjes. In the air cavity a number of 

 plates or lamellge are developed, which radiate from the centre 

 to the circumference. These form the giUs of the nmshroom, 

 and on them the spores or gonidia are developed. As the growth 

 of the stipes proceeds, the part of the pileu& below the air cavity 



Fm. 853. 



Fig. 853. Development of the Mushroom, c. Mycelium, b. Early stage of 

 the development of the gonidiophore. d. Later stage, sho^viug origina- 

 tion of the gill-chamber, a. Later stage, showing commencement of 

 formation of the gills. After Sachs. 



becomes very nnich stretched and forms a membrane, extending 

 from the stipes to the lower margin of the pileus. It ultimately 

 ruptures and leaves the gills exposed. This membrane is 

 known as the velum jpartiale. The gills are composed of 

 hyphse, which rmi parallel along their length, ultimately curving 

 shghtly outwards. These constitute the trama {fig. 854, f), and 

 are covered by the hymenial layer, which is composed of short 

 cells derived from the hyphse of the trama, set at right angles 

 to its long axis. These cells are of two kinds, sterile ones, or 



