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 jiileus, the lower part 

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

 plates or lamellae are developed, which radiate from the centre 

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

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

 of the stipes proceeds, the part of the pileus below the air cavity 



Fig. 853. 



Fig, 863. Development of tfce Mushroom, c. Mycelium. 6. Early stage of 

 the development of the goaidiophore. d. Later stage, showing origina- 

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

 formation of the gills. After Sachs. 



becomes very much 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 partiale. The gills are composed of 

 hyphie, which run parallel along their length, ultimately curving 

 slightly outwards. These constitute the trama (fig. 854,/), 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 ceUs are of two kinds, sterile ones, or 



