38 BOTANY AND PHARMACOGNOSY. 



alga. In its development the fungus forms a mycelium which 

 encloses the alga, the growth of which latter is not hindered. The 

 two organisms then continue to grow simultaneously forming 

 lichen patches. A section of a lichen shows a differentiation into 

 several parts, namely, a more or less compact row of cells on both 

 surfaces forming two epidermal layers ; and an inner portion made 

 up of the hyphal tissue of the fungus in which the alga is embed- 

 ded either in a single layer or throughout the mycelium. The 

 mode of growth and branching is influenced largely by the fungus, 

 although in some cases the alga may exert the most influence. In 

 some cases the lichen consists of a thallus which is irregular in 

 outline, growth taking place at no definite point, and in other 

 cases branches which are more or less regular are formed, growth 

 taking place at the apex. 



Groups of Lichens. According to the manner of grow^tli 

 and the manner of attachment to the substratum three principal 

 groups (Fig. 26) of lichens are distinguished: namely, (i) Crus- 

 taceous Lichens, where the thallus adheres closely to the stones 

 and barks of trees and practically can not be removed without 

 injury; (2) Foliose Lichens, or those which are more or less 

 flattened, somewhat leaf-like and attached at different points; (3) 

 Fruticose Lichens, or those which are attached at a particular 

 part of the thallus, and form diffusely branching clumps. To this 

 latter group belongs Cetraria islmidica or Iceland moss (Fig. 26), 

 which is used in medicine (p. 690), Usnea barbata and the 

 red-fruiting Cladonias which are so common. 



Reproduction in the Lichens takes place in several ways. In 

 all of them there is a vegetative mode by means of what are 

 known as soredia. These are small spherical bodies consisting of 

 a group of algal cells, which are surrounded by a mass of hyphse, 

 and which when cut off" from the main body are able to grow. 

 Lichens also produce spores of a number of kinds. In the largest 

 group, the one to which Cetraria islandica (Fig. 26) belongs, the 

 spores are found in special spherical receptacles, known as pyc- 

 NiDiA, which are formed on the teeth of the margin of the thallus. 

 The spores arise from the ends of hyphse at the base of the pyc- 

 nidia and are in the nature of conidiospores. To these spores 

 the name pycnoconidia has been applied. Cetraria also pro- 



