260 



LICHENS 



scattered throughout the thallus (e.g. Collema) or occupying 

 definite zones (e.g. Xanthoria parietina, Fig. 142, C). 



The association between Alga and Fungus is paralleled by 

 the so-called " green cells " found in several aquatic animals 

 (e.g. freshwater Sponges, Hydra viridis) ; these are due to uni- 



cellular Algae 

 (species of Chlor- 

 ella, etc.), and in 

 all such cases the 

 partnership be- 

 tween plant and 

 animal appears to 

 be of mutual 

 benefit. In this 

 connection it may 

 be noted that 

 various Algae are 

 found commonly 

 inhabiting the 

 intercellular 

 spaces of certain 

 higher plants (e.g. 

 Duckweed, the 

 Liverwort Antho- 

 ceros), but in these 

 cases the Algae 

 appear to be no- 

 thing more than 



FIG. 142. A, soredium, and C, section through 

 the thallus, of the Lichen Xanthoria parietina. 

 al., algal cells ; hy., fungal hyphae. B, an 

 algal cell from the Lichen Cladonia, illustrating 

 the intimate relation between it and the 

 fungal hyphae. (B after Bornet.) 



"space-parasites " 

 seeking protec- 

 tion. 



The ordinary 

 cup-like fructifi- 

 cations of Lichens originate from the Fungus, and the resulting 

 ascospores reproduce the latter only, so that germination of 

 the spores must take place in the presence of the appropriate 

 Alga, if a fresh Lichen is to be formed. But multiplication in- 

 volving the dual organism also occurs ; in this case algal cells 

 surrounded by interlacing hyphae become detached, and when, 



