THE LEAF 213 



matically the anatomical features of the leaf of Alnus incana. 

 Above is a layer of cells containing only protoplasm and a nucleus. 

 A similar situation is presented by the lower surface of the leaf, 

 with the exception that the continuity of the epidermis is locally 

 perforated by stomatic openings. The elements related to these, 

 the guard cells, are distinguished from the rest of the epidermal 

 layer by the presence of chloroplastids. The upper epidermis is 

 contrasted with the lower, not only by the absence of stomata, but 

 also by the presence of a rough impervious covering, the cuticle. 

 Another feature of interest in the superior epidermal cells is the 

 mucilaginous modification of their inner walls, and this is expressed 

 in the illustration by a thick layering. The central portion of the 

 leaf is occupied by the mesophyll, composed of the palisade (upper) 

 region and the spongy (lower) region. The development of palisade 

 parenchyma in foliar organs is definitely related to the amount of 

 insolation or exposure to light, while the spongy layer is less well 

 developed when the leaf is strongly illuminated, and becomes much 

 more accentuated in foliar organs exposed to shade and a damp 

 atmosphere. The leaf of the monocotyledons supplies little in a 

 general way which is of interest from the standpoint of evolutionary 

 anatomy, except the occasional persistence of cambial activity. 



