PRINCIPAL GROUPS OF PLANTS. 73 



give very striking reactions with solutions of the alkalies and solu- 

 tions containing chlorine. The reaction with iodine solutions is 

 also employed for diagnostic purposes ; some of the Lichens give 

 a blue reaction, while others behave like amylo-dextrin. 



Groups of Lichens. According to the manner of growth 

 and the manner of attachment to the substratum, three principal 

 groups of Lichens may be 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 belong Cetraria islandica or Iceland moss (Fig. 43), 

 which is used in medicine, 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 hyphae, 

 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. 43) 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- 

 duces, like many other Lichens, disk-like or cup-shaped bodies at 

 various places on the surface of the thallus, which are known as 

 APOTHECIA and which may be regarded as exposed or open asco- 

 carps. The inner surface of the apothecia is lined with a number 

 of asci as well as sterile cells, the former giving rise to ascospores. 



Economic Uses of Lichens. A number of the Lichens are 

 used in medicine, as several species of Cetraria, Pertusaria corn- 

 munis, Physica parietina, Sticta pulmonacea, Evernia furfuracea. 

 Some of those used in medicine are also used as foods on account 



