4'2 GUIDE TO THE STUDY OF LICHENS. 



is difficult to determine whether a given thallus is 

 crustose or foliose. A typical crustose thallus is 

 always closely adnate to the substratum and never has 

 a lower cortical layer ; the margin sometimes becomes 

 lobed, as in some representatives of Placodium. 



It is questioned whether many of the lower crustose 

 lichens should not be classified as fungi. If a thallus 

 is present, no matter how rudimentary, it is unmistak- 

 ably a lichen ; it must be remembered that careful 

 search is often necessary to detect such rudimentary 

 thalli, as in Pyrenula and other genera. (Plates II 

 and IV.) 



2. The Foliose Type. 



Most of the lichens with which the amateur collector 

 will come in contact belong to the foliose type. As 

 a rule, the thallus is large, consisting of branching 

 lobes which are only loosely attached to the substra- 

 tum. In some instances the thallus is entire, varying 

 from very small, as in Dermatocarpon and Psora, to 

 very large, as in JJtr^iUcaria and Gyrophora. One 

 thing must be kept in mind and that is the fact that 

 the foliose type merges into the fruticose type in the 

 ascending series, and into the crustose type in the de- 

 scending series. The characters here given refer to 

 the more typical forms as represented by the Parmelias, 

 Physcias, Stictas, Stictinas. 



If we make a vertical section of the thallus and 

 examine it under the microscope we find the following 

 structures. 



1. Upper Cortical Layer. — This consists of a 

 compact tissue of short-celled hyphae. It forms a pro- 



