MORPHOLOGY AND PHYSIOLOGY. 43 



tective layer for the tissues beneath and also supplies 

 mechanical support to prevent the breaking or tear- 

 ing of the thallus due to winds and other external 

 forces. The upper portion of this layer is sometimes 

 colored, due to a deposit of acid crystals. The func- 

 tion of this colored substance is, perhaps, twofold : it 

 probably -modifies the influence of sunlight, and keeps 

 away animals which feed upon lichens, such as snails. 

 The thickness of this layer varies greatly in different 

 lichens. The layer is often not uniform in thickness, 

 even in the same plant; for example, in Peltigera it is 

 alternately thicker and thinner, and shows a remark- 

 able double adaptation — to supply the required mechan- 

 ical support, and at the same time allow the algae to 

 approach nearer the surface for the purposes of greater 

 chlorophyllian activity (carbon assimilation). 



2. Algal Layer. — This lies beneath the layer just 

 described, and consists of a loose network of hypha? in 

 which the algae are suspended. It is the layer in 

 which carbon assimilation is carried on. Certain 

 hyphal branches, the haustoria, enclose and even pen- 

 etrate the algae, and take from them the assimilated 

 food-substances required by the fungal symbiont, while 

 the alga in return receives water and certain soluble 

 food-substances from the fungal symbiont. 



3. Medullary Layer. — This consists of a very loose 

 network of hyphae. It contains air, and therefore the 

 carbon dioxide required to carry on the function of as- 

 similation. The thickness of this layer varies con- 

 siderably. 



^. Lower Cortical Layer. — When present it re- 



