114 



POPULAR HISTORY OF LICHENS. 



deep brown. The contents of the filaments of the medullary 

 layer of the thallus are coagulated, as in the paraphyses; 

 and in the other cell-elements of the thallus little change is 

 produced, save a greater distinctness of their walls and con- 

 tents. There are various exceptions to, and anomalies in^ 

 the reactions above given ; but they will be found to hold 

 good in the majority of common Lichens. The action of 

 iodine on the reproductive cells of Lichens, in the exami- 

 nation of which it is chiefly useful, ought to be studied in 

 species having comparatively large or distinct thecse and 

 spores, such as Pertusaria communis and P/iyscia ciliaris. 

 The mineral acids are chiefly useful for dissociating the 

 thecse and paraphyses, and enabling the student to examine 

 their true structure and attachments. He will also find it 

 convenient sometimes to boil portions of a Lichen, to fa- 

 cilitate the making of a section or to produce a disaggre- 

 gation of its tissues. 



