4 1 



ciple, which contains algae, and is, therefore, assimilative. The 

 term is, however, usually limited to the alga-bearing portion, ex- 

 clusive of apothecia. On account of the perfect mutualistic adapta- 

 tion of the lichen symbionts we find some very marked structural 

 characters to meet the requirements of different cases. In the lower 

 forms of lichens this structural adaptation is not so readily recogniz- 

 able, but in the higher forms the physiological and mechanical adapta- 

 tions are, in fact, more marked than they are in the higher plants. 

 We will now take up the discussion of a typical foliose thallus 

 because in it the morphological differentiations are most highly 

 developed. After having a thorough understanding of the highest 

 type of thallus there will be little difficulty in understanding the lower 

 forms. The following are the tissues met with in a vertical section 

 of the thallus of Sticta. 



(a) Tegitincntary Layer. (Dennis.) 



(b) Upper Cortical Layer. 



(c) Algal Layer. (Gonidial Layer.} 



(d) Medullary Layer. 



(e) Lower Cortical Layer. 



These layers will be described in this order ; the rhizoids, cy- 

 phellae, breathing pores, cephalodia and other accessories of the 

 thallus will be treated independently. 



(a) The Tegnii/entary Layer. (Dennis.} 



This is the uppermost layer of the thallus and is usually rudimen- 

 tary or wanting, except in the higher lichens, especially Sticta 

 and Stictina. It consists of several layers of flattened hyphal cells 

 extending at right angles to the underlying cortical cells. The cells 

 are rather irregular in form ; their walls are usually more gelatinous 

 than those of the cortical layer. The walls of the more internal 

 cells are frequently slightly colored, due to a deposit of lichenic acid. 

 The outermost cell-walls are, however, perfectly colorless and 

 gelatinized. Sometimes the outermost cells develop hair-like pro- 

 longations whose function is not definitely known. In PeUigera 

 apkthosa, for example, they doubtless serve to retain the soredia, 

 which subsequently develop into cephalodia. They, perhaps, also 

 serve to retain moisture. The dermis contains intercellular canals 

 which are continuous with the canals of the cortical layer below. 

 The outermost cells are continually removed by abrasion, while new 



