40 GUIDE TO THE STUDY OF LICHENS. 



From what has already been said of lichens, the 

 reader knows that the thallus is functionally wholly 

 different from the vegetative portion of fungi. The 

 main difference may be stated as follows : In fungi 

 the structural development and arrangement of tissue- 

 elements, of the vegetative portion, is for the special 

 purpose of furthering the function of reproduction, 

 that is, to supply a suitable structural arrangement 

 for the maturation and distribution of the spores. In 

 lichens the structural development and arrangement 

 of the tissue-elements is for the special purpose of 

 furthering the function of assimilation. 



Structurally and functionally, the thallus of lichens 

 is also analogous to a foliage leaf of higher plants. 

 This becomes apparent on comparing vertical sections. 

 The upper and lower cortical layers of the thallus are 

 functionally analogous to the upper and lower epider- 

 mis of the leaf ; the algal layer of the thallus to the 

 palisade tissue of the leaf ; the medullary tissue of the 

 thallus to the spongy tissue of the leaf. The analogy 

 is, in fact, very striking, particularly between foliose 

 thalli and the ordinary flattened foliage leaves. Fru- 

 ticose thalli are analogous to leaves with " isolateral " 

 or "centric" structure. Crustose thalli are in reality 

 rudimentary foliose thalli, and are, therefore, remotely 

 analogous to ordinary foliage leaves. 



For practical purposes, the thalli of all lichens may 

 be divided into three types or kinds, namely, crustose, 

 foliose, and fruticose. Their structural differences are 

 as follows : 



