314 A TEXTBOOK OF THEORETICAL BOTANY 



or less spherical knob of tissue which bursts through the bark. It may 

 appear either on the main trunk or on the branches, but only boughs of 

 considerable size normally bear fruiting bodies. They appear either high 



Fig. 306. — Polyporus betitUmis. Birch bark covered with a papery 

 tissue consisting of the Fungus mycelium which forms a layer 

 between the bark and the wood. 



up or low down on the trunk and are not formed in acropetal succession but 

 purely haphazardly. 



This knob of tissue now enlarges and it is possible to recognize a very 

 short stalk by which it is attached to the tree and a terminal rounded mass 

 which is convex above and flat or slightly tilted to the horizontal below. 

 The margin of the lower surface is much infolded (Fig. 307). As develop- 

 ment continues the upper surface becomes flatter until finally it assumes a 

 slightly undulating or completely smooth form. Meanwhile the margin 

 has expanded and the lower surface has become flat and reniform or rounded 

 in outline. 



The lower surface remains smooth and flat, and is at first spongy, being 

 covered by a fine downy layer of the mycelium. As development continues 

 this layer becomes harder and minute holes appear in it, which are the 

 beginnings of the pores of the hymenial tubes. These pores gradually 

 increase in size by the drying up and dissolution of the covering membrane, 

 until at maturity the pores appear as minute circular holes distributed over 

 the entire lower surface of the fruiting body (Fig. 308). 



A section through the sporophore shows it to be divisible into 

 several distinct layers. The upper surface consists of a narrow zone of 

 thick-walled hyphae which run parallel to the surface. These give off short 

 branches, which terminate in oval cells and form a very fine roughening of 

 the general surface. Below this the greater part of the body of the fructi- 

 fication is composed of very fine, anastomosing hyphae with large irregular 

 spaces between them. This zone comprises about four-fifths of the thick- 

 ness of the fructification. Towards the lower surface these anastomosing 



