THE FUNGI 



243 



Underneath the dense upper cortex comes a broad 

 zone of loosely packed hyphae, leaving large air spaces 

 between them. In the upper part of this medullary 

 zone, numerous large green cells are embedded, lying in 

 the interstices between the hyphae (Fig. 99, a). Some 

 of these chlorophyll-containing cells are in course of 

 active division. The green cells of Lichens bear the 

 name of gonidia, and the part of the thallus in which 

 they are contained is distin- 

 guished as the gonidial layer. 

 Underneath the medullary zone 

 is a lower cortical layer resem- 

 bling that on the upper surface. 

 From the lower cortex the 

 strands of hyphae arise, which 

 constitute the rhizoids and are 

 analogous to roots. 



Now the most important 



question as to the thallus of a FIG. 99. Physcia parietina -, 

 T i , i vertical section of thallus 



Lichen concerns the nature of ^ npper cortial Iayer . a> a> 

 the green cells, or gonidia. gonidia ( = Cystococcus) em- 



rr,, -, * , . T . bedded among medullary 



They play an essential part m hyphae . w> lo ver cortica j 



the economy of the plant, for, layer. Magnified 500. 

 ,., ., ,, i 11 (After Sen wendener.) 



like other chlorophyll -contain- 

 ing cells, they are able to assimilate carbon from the 

 carbon-dioxide of the air, a fact which has been experi- 

 mentally proved, and thus render the Lichen completely 

 independent of organic food. Hence arises the profound 

 physiological difference between Lichens and all other 

 Fungi. 



For a long time, in fact down to about the year 1868, 

 the gonidia were regarded as forming, like the hyphae, a 

 constituent part of the thallus. So long as that view 





