DEVELOPMENT OF PLANTS 



233 



regarding them as constituting an independent division, and by 

 others they are looked upon as fungi. The fact that the fungus 

 forms the bulk of the lichen and lives upon the alga somewhat 

 after the manner of a parasite and is usually alone capable of 

 forming reproductive spore bodies would lead to the latter 



FIG. 142. Common species of Lichens: A, an erect branching form, 

 Cladonia as, ascocarps. B, a foliaceous lichen, Sticta. C, Parmelia spread- 

 ing over the bark of tree. The centrally-placed ascocarps are surrounded 

 by smaller pycnidia. 



position. However, all degrees of relationship appear between 

 the alga and fungus from stages where their association is of 

 little or no consequence to stages where their existence is abso- 

 lutely dependent upon their association. The members of this 

 curious co-partnership are largely ascomycetes and blue-green 

 algae. Numerous species in each of these two groups of plants 

 have become accustomed to living together. The formation of 

 a lichen comes about through the attachment of a hypha of some 

 fungus to one or more algal cells, as shown in Fig. 143, B. In 

 this way, the food manufactured by the algae is absorbed os- 

 motically by the hyphae. In some cases, the fungus obtains 

 its food by means of haustoria which penetrate the algal cells. 

 By the continued growth of the fungus and algae there finally 

 results an interwoven mass of hyphae about the algae. The 

 walls of the fungus are sufficiently transparent to permit the 

 entrance of light and the color of the green algae can readily 

 be seen when the lichen is moistened. 



(a) Structure of the Lichen. The bulk of the lichen is more 

 usually composed of hyphae which show considerable regularity 

 16 



