47 



many of the higher foliaceous lichens, as, for example, in Nephroma, 

 Psora and Peltigera. 



From the lower surface are developed the rhizoids. In it also oc- 

 cur the cyphellae, whose structure and function will be considered 

 later. 



The principal function of this layer is doubtless mechanical. 

 Since it is continuous below with the rhizoids and above with the 

 medullary hyphae, it is evident that the cortical cells must transmit 

 the soluble food substance taken up by the rhizoids. This layer is 

 more highly hygroscopic than the upper cortical. 



The student of comparative morphology will be impressed with 

 the close analogy between a typical foliose thallus and a foliage leaf. 

 This analogy is close both as to structure and function. Morpho- 

 logically we find the following similarities : The epidermal layers 

 (upper and lower) of the leaf may be compared to the upper and 

 lower cortical layers (inclusive of the dermis) of the thallus ; the 

 palisade layer to the algal layer ; the spongy tissue to the medullary 

 layer ; the vascular bundles (veins and midrib) to the hyphal bundles 

 (mechanical tissue). The special function of the tissues in the leaf 

 is also closely similar to the special function of the corresponding 

 tissues in the thallus ; that is, epidermis (of leaf) and cortical 

 layers (of thallus) have a mechanical and protective function ; pali- 

 sade cells and algal layer are assimilative ; spongy tissue and medul- 

 lary layer form the aerating and conducting tissue ; the vascular 

 bundles and hyphal bundles are essentially conducting and mechani- 

 cal. 



2. TYPES OF THALLI. 



There are three types or forms of lichen-thalli recognized by 

 nearly all lichenologists : (a) The crustaceous ; (5) The foliose 

 (foliaceous, frondose) ; (c) The fruticose (fruticulose) . It must not, 

 however, be supposed that these types are absolutely distinct ; there 

 are all intermediate forms between the simplest network of hyphae 

 intermingled with a few algae and the most highly developed fruticose 

 •thallus. Since the great advance made in the morphological investi- 

 gations of lichens it becomes doubly necessary to draw some lines of 

 distinction between these different forms of thalli. A recognition of 

 these characteristic forms is important from the standpoint of 

 morphology and physiology as well as in classification. As taxo- 



