KXAMINATION OK A LICUEN. 



2C1 



Fig. 249. 



351. Lichens. These plants may be found growing 

 on 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 

 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 Fi ^ 250 - 

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

 showing it to be largely made up 

 of cells, or Jiyphce, similar to those 

 of the Mushroom. But in the. 

 Fig. 251. 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. 219. A frnticose Lichen (Cladonia dinitata) of the natural size ; b, the 

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

 (Thome\) 



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



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



