6 4 



sparingly or frequently branched ; their upper ends, besides being 

 variously colored, are also more or less enlarged. Sometimes they 

 are partially or almost entirely gelatinized, as in the Verrucariaceae. 

 Again, they may be granular, so as to make it difficult to determine 

 their structure. As a rule, they are few-celled, scarcely ever single- 

 celled. They are usually looked upon as as sterile spore-sacs. 



The mature paraphyses likewise vary greatly in length, less in 

 thickness ; in some cases only the tips are colored, and again the entire 

 thecium is more or less tinged with the coloring substance deposited 

 in the cell-walls. In some lichens the paraphyses unite to form 

 semicortical vertical plates extending through the thecium. Their 

 function is, no doubt, mechanical, and has to do with the ejection of 

 spores. 



(c) The Hypothecium. 



The hypothecium is the hyphal structure found immediately 

 below the thecium. In the higher foliose lichens it usually consists 

 of two layers : an upper one, in which the cells extend in a hori- 

 zontal direction, and a lower one, in which the cells, extend in a verti- 

 cal direction ; frequently the two layers can not be distinguished. 

 The cellular structure of the hypothecium varies greatly ; in some 

 lichens (especially the higher foliose forms) it is cortical ; again 

 there is no trace of a cortical structure, particularly in the lower 

 forms. The cortical tissue of the hypothecium differs, however, 

 from the cortical tissue of the thallus ; the cells are smaller and 

 usually more gelatinized. In the non-cortical hypothecium the hyphal 

 cells are rather short and also considerably gelatinized ; they are 

 always smaller in diameter than the medullary hyphae. 



The margin of the hypothecium forms a part of the exciple, as 

 will be explained more fully later. Here the hyphal threads de- 

 scribe trajectory curves corresponding to the direction of growth ; 

 those above the median plane of growth curve upward and enter 

 into the formation of the upper hypothecial layer as well as into that 

 of the paraphyses and spore-sacs ; those below the median plane ex- 

 tend downward and enter into the formation of the lower hypothecial 

 layer and the exciple. 



In the lower lichens the hypothecium is often colored, usually 

 black, and varies greatly in consistency ; sometimes it is quite soft, 

 again it is hard and brittle ; it also varies considerably in thickness. 



