4^ GUlDE to TrtE STUDY OF LICHENS. 



usually forming two layers ; one imnaediately under- 

 neath the hypothecium, and the other just above the 

 lower cortical layer of the apothecium. The algae may 

 approach quite to the margin of the cup, or they may 

 extend only a part way up. In fact, there are all 

 gradations, from the purely fungal type to the most 

 highly developed thalline type as it occurs in the 

 Parmelias and in Usnea. In general it may be stated 

 that the fungal type occurs in the lower lichens, while 

 the thalline type occurs in the higher lichens. There 

 are, however, important exceptions to this rule. Figs. 

 1 to 5 of Plate IV will serve to illustrate the differ- 

 ences between the types just discussed. 



III. ACCESSORY STRUCTURE. 



The thallus and apothecia constitute the most im- 

 portant lichen-structures ; there are, however, a num- 

 ber of other structures which occur on the thallus, 

 and which are of more or less importance in the lichen- 

 economy, and for that reason it is well that the student 

 should be somewhat familiar with them. They are 

 here discussed in the order of their importance : 



1. The Soredia. 



Examination of the upper surface of the thallus of 

 many lichens reveals patches of a powdery or mealy 

 substance. These patches may be circular or linear, 

 especially along the margin of the thallus. If some 

 of this powder is placed under the microscope and 

 examined, it is found to consist of small irregular 

 bodies made up of a hollow network of hyphae, in the 



