4 2 



ones are formed from the cortical layer. The change from cortical 

 to tegumentary cells consists in a retardation of growth in a vertical 

 direction and a stretching combined with growth in a horizontal 

 direction. As a result of these combined influences the cells become 

 much flattened. 



The primary function of the dermis is, doubtless, to prevent the 

 sudden and excessive evaporation of moisture. It also provides a 

 mechanical protection to the underlying tissues. 



(/>>) The Upper Cortical Layer. 



This layer, in different degrees of perfection, is present in all 

 warty, squamose, foliose and fruticose thalli. It is usually desig- 

 nated as a semicortical or semiparenchymatous tissue. Structurally 

 it certainly resembles very closely the parenchymatous tissue of 

 higher plants. The cells are somewhat elongated in a vertical di- 

 rection and lie in actual contact ; in outline they are quite irregular ; 

 the walls are usually considerably thickened, especially at the 

 angles ; the lumen is in part filled with air. The cells of this 

 layer are directly continuous below with the hyphal cells of the algal 

 layer and above with those of the dermis. There is a continual 

 regeneration or development of new cortical cells proceeding upward 

 from the algal layer, while the upper cortical are converted into 

 dermal. 



The cortical layer varies greatly in thickness in different species 

 as well as in different portions of the same thallus. From the fact that 

 the thallus performs the function of assimilation as well as that of me- 

 chanical support, it is evident that the structures destined to perform 

 these functions must be suitably adapted and arranged with reference 

 to each other. The upper cortical layer, whose function is primarily 

 mechanical, is variously modified so as to adapt it to the requirements 

 of the underlying assimilating tissue. This is very beautifully shown in 

 Solorina saccata : in this species the lower outline of the upper cortical 

 layer is serrate, that is, the cortical layer is alternately thicker and thin- 

 ner ; this gives great mechanical support and at the same time favors 

 the grouping of the algae near the surface for exposure to the 

 influence of sunlight, and to permit the exchange of gases. We 

 find this layer thickest in those lichens having no lower cortical 

 layer, very likely because the exchange of gas can readily take 

 place downward. There are, however, other provisions to permit 



