Mosses and Lichens 



The leaves have no epidermis' and no breathing pores as do 

 the leaves of higher plants. 



LEAVES OF POLYTRICHUM 



The leaves of a Polytrichum represent about the highest stage 

 in the development of mosses. The mid-vein is broad, and only 

 at the extreme margins is the leaf-blade one-layered. 



The central tissue of the mid-ribs of the leaves 

 continue so as to unite with the central axis of 

 the stem in a manner quite analogous to that found 

 in stems of higher plants. A cross section of a 

 leaf shows that the marginal cells and a 

 line of cells running through the central 

 part are comparatively thin- walled and are 



Pogonatum Al- 

 pinum. Upper face 

 of leaf to show deli- 

 cate lamellae. 



Catharinea angustata. Cross 

 section of leaf to show the thin 

 blade and two lamellae rising 

 from the vein. 



Catharinea undulata. Upper 

 surface of the apex of a leaf 

 showing lamellae with thin 

 leaf-blade on either side. 



empty water-conducting cells similar to the wood-ducts 

 (tracheae) of a fibro-vascular bundle in a higher plant. The next 

 layer is composed of similar but smaller cells containing starch. 

 The rest are thick-walled cells (sclerenchyma) . The outer cells 

 contain more or less leaf-green (chlorophyll). When breathing 

 pores occur they are on the spore-case walls. 



The cells of the upper surfaces, have their walls exceedingly 

 delicate, so that they can absorb gases and permit gases or water 

 to leave them. The thin blades (lamella) are undoubtedly the 



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