THE COMMON MUSHROOM 743 



which are arranged in a radiating manner from the centre of the 

 pileus, the oldest of them extending from the edges of the latter 

 to the stem, while younger ones reach only part of the way. 



Attached to the stipe at a point a little more than half-way 

 from its base is an encircling frill or annulus, the remains of the 

 veil {velum partiale, b), which, in the early stages of develop- 

 ment of the sporophore, was stretched in the form of a delicate 

 membrane from the stipe to the outer edge of the pileus and 

 completely enclosed the young lamellae. 



It is upon the laraellas that the basidia and their spores are 

 borne. A thin, tangential slice through the ' mushroom,' as in 

 Fig. 252, cuts the lamellae transversely (see A and B). A portion 

 of one of the latter is shown highly magnified at C: the central 

 substance (i) is known as the trama, and is composed of long, 

 somewhat loosely-woven hyphae, which diverge to the right and 

 left, and become divided into shorter segments in the part (n) 

 termed the sub-hymenial layer. From the ends of the short cells 

 forming this layer the club-shaped basidia arise, each of which 

 bears two to four oval and prominent purple basidiospores, carried 

 singly at the tips of the same number oi sterigmata. 



The basidia are packed closely side by side, and intermixed 

 with smaller sterile cells (x) the paraphyses, the whole spore- 

 bearing surface (ti) of the lamella being spoken of as the 

 hymenium. 



Several wild varieties of the mushroom are met with in pas- 

 tures and woods. 



The cultivated form (var. hortensis) is very variable and 

 somewhat distinct from these. It is at present propagated 

 chiefly from pieces of mycelium or ' spawn,' which are con- 

 tained in the compressed blocks of richly-manured compost 

 sold by seedsmen as ' mushroom spawn.' The conditions neces- 

 sary for the germination of mushroom spores are not yet clearly 

 understood. They can be encouraged to germinate to some 

 extent in solutions containing magnesium phosphite, magnesium 



