STUDIES IN CRYPTOGAMS 



195 



nection with which the spores are borne. These aerial parts are 

 the only ones we ordinarily see, and which constitute the " mush- 

 room " part (Fig. 131). 

 Only asexual spores (ba- 

 sidiospores} are produced, 

 and on short stalks (basidia) 

 (Fig. 286). In the puff- 

 balls the spores are inclosed 

 and constitute a large part 

 of the "smoke." In the 

 mushrooms and toadstools 

 they are borne on gills, and 

 in the shelf fungi (Fig. 134) 

 on the walls of minute pores 

 of the underside. The my- 

 celium of these shelf fungi 

 frequently lives and grows 

 for a long time concealed in 

 the substratum before the 

 visible fruit bodies are sent 

 out. Practically all timber 

 decay is caused by such 

 growth, and the damage is 

 largely done before the fruiting bodies appear. 

 counts of mushrooms, see Chap. XIV. 



FIG. 286. PART OF GILL OF THE CUL- 

 TIVATED MUSHROOM. 



tr, trama tissue; s/i, hymenium; b, basidium; 

 st, sterigma; sp, spore. (Atkinson.) 



For other ac- 



FIG. 287. LICHEN ON AN 

 OAK TRUNK. (A species 



LICHENS 



Lichens are so common everywhere 

 that the attention of the student is sure 

 to be drawn to them. They grow on 

 rocks, trunks of trees (Fig. 287), old 

 fences, and on the earth. They are 

 thin, usually gray ragged objects, ap- 

 parently lifeless. Their study is too 

 difficult for beginners, but a few words 

 of explanation may be useful. 



Lichens were formerly supposed to 

 be a distinct or separate division of 

 plants. They are now known to be or- 

 ganisms, each species of which is a con- 

 stant association of a fungus and an alga. 

 The thallus is ordinarily made up of fun- 

 gous mycelium or tissue within which 

 the imprisoned alga is definitely dis- 

 tributed. The result is a growth unlike 

 either component. This association of 



