452 



BOTANY OF THE LIVING PLANT 



called a hymenium, exposed to the air, from which the spores are shed, 

 as in the Mushrooms, Toadstools, and Shelf-Fungi (Figs. 386, 387). 



The mycelium may obtain nourishment in various ways. It is 

 sometimes parasitic as in the Honey Agaric (Armillaria mellea), which 

 penetrates the trunks of forest trees, ravaging the cambium, and killing 

 them (see Fig. 340). Many of the Shelf- Fungi (Polyporus) grow parasi- 

 tically at the expense of the heart-wood of trees, making them hollow. 

 The infection comes through injury by wind, which exposes the 

 internal tissues to the invading spores. The mycelium may live for 

 years, digesting the lignified walls, till it is sufficiently nourished to 



FIG. 388. 



Radial longitudinal section through wood infested with Polyporus igniarius. 

 Highly magnified. (After Hartig.) 



form a fruit-body (Fig. 388). On the other hand the Dry Rot Fungus 

 (Merulius lacrymans) lives saprophytically, its mycelium digesting the 

 substance from dead wood-work in houses and ships, where confined 

 in a close damp space. Later it forms the cake-like fruit bodies. The 

 Common Mushroom is an example of a very common habitat of sapro- 

 phytic mycelium, viz. in the sod of grass-land. It is found especially 

 where horses have been grazing. But the mycelium can be bought in 

 bricks of " mushroom-spawn," made up of a compost of dung, clay, 

 and loam, in which it can be seen as fine white threads, either spread- 

 ing as single hyphae, or as numerous hyphae running parallel to form 

 thicker strands. If the brick be broken into pieces and spread 

 through a similar compost, and kept warm and moist in the dark, 

 the mycelium spreads ; in a few weeks it forms mushrooms of various 



