EXAMINATION OF A LICHEN. 



2<h 



3^1. Lichens. These plants may be found growing 

 m the bark of -trees, on old fences, on rocks, or on the 

 ground. They differ widely in external 

 appearance, sometimes growing erect and 

 imitating a stem and branches, as in Fig. 

 249 ; sometimes forming flat expansions 

 which adhere to the surface upon which 

 they grow, as in Fig. 250. Some species 

 are yellow, others red, others grey. A 

 very common one is that represented in 

 Fig. 250. It may be found upon many 

 Fig. 249. tree-trunks, and will be easily recognized 

 by the yellow disks which dot its 

 surface. 



352. The flat part of the Lichen is 

 the thallus, or vegetative portion, 

 while the yellow, cup-shaped disks 

 (the apothecia) contain the fructifica- 

 tion. Fig. 251 shows a section of the 

 apothecium, and also the lobing of 

 the margin of the thallus. Fig. 252 Fig - 25 - 

 is a very highly magnified view of a section of a thallus, 

 showing it to be largely made up 

 of cells, or hyphce, similar to those 

 of the Mushroom. But in the 

 ?. 25i. Lichen there are visible, in addi- 



tion, large numbers of spherical green cells (g g in the 

 Fig.) known as gonidia, which either occupy well-marked 



Fig. 249. A fruticose Lichen (Cladonia digitata) of the natural size ; b, the 

 cup ; c, the thallus ; the rounded bodies at the summit are the apothecia. 

 (Thorn 4. ) 



Fig. 250. A foliaceous Lichen growing on a stone ; natural size. (Gray.) 



Fig. 251. Section of an apotheoium. (Gray.) 



