46 



ally we find groups of tissue elements distributed through this layer 

 whose primary function is mechanical. This is, however, rare ; 

 mechanical tissues as a rule occurring outside of the medullary 

 layer. 



In some lichens, especially the so-called Lichenes gelatinosi 

 (Collemaceae;, there is no distinct medullary layer ; that is the algal 

 and medullary layers are, so to speak, intermingled. There is no 

 medullary tissue proper in the crustaceous lichens, since none is nec- 

 essary at least as far as the requirements of aeration are concerned. 



Structurally this layer is adapted to perform two functions. The 

 hyphae conduct food substances such as water, mineral salts, carbon 

 compounds, nitrogen, hydrogen, to and from the algae. They 

 form the paths of exchange between the products of assimilation 

 formed by the algae and the soluble food substances taken up by the 

 fungal portion. Much work is, however, yet to be done in the 

 physiological study of these functions. 



According to Zukal (no) the medullary hyphae also serve as 

 storage tissues, accumulating for future use various carbo-hydrates 

 (lichenin, isolichenin), gelatine, fats and fatty oils. Lichenin is 

 stored in the cell-walls, causing them to become considerably thick- 

 ened. The excess may or may not become reabsorbed in the future 

 life processes of "the plant. Fat is stored within the cells and varies 

 considerably in amount but never occurs in large quantities ; it seems 

 probable that the fat is stored at one point to be again utilized at some 

 other point in the same plant. Fat also frequently occurs in the 

 spores, where it doubtless plays an important part in the processes of 

 germination. The sphaeroid-cells occurring in the calcivorous crus- 

 taceous lichens are special fat-storing structures. It is as yet a dis- 

 puted question whether this fat is waste product or whether it is 

 really utilized in further metabolic processes. 



(<?) The Lower Cortical Layer. 



Structurally this layer very closely resembles the upper cortical 

 layer. The cells have the same essential characters. They are 

 somewhat elongated in a direction vertical to the thallus. As a 

 whole this layer is thinner and the cells are more loosely united than 

 the corresponding cells of the upper cortical layer. The dermal 

 layer is always wanting. Quite generally there occurs a dark color- 

 ing substance in the cell-walls. This layer is wholly wanting in 



