Leafy Mosses 



result of an effort on the part of the plant to increase to the high- 

 est degree its absorbing surface without widening the leaf-blade 

 itself. By directing the growth of the delicate cells upward in 

 thin blades, this end is accomplished. 



a. 



Polytrichum. Cross section of a portion of a leaf to show: (a) lamellae, (s) sclerenchyma. 

 Thickened cells of vein. The leaf -blade one cell thick shows on the left. 



Polytrichum slnctum. Cross section of leaf through the midvein to show bead-like lamellae 

 on the upper surface and thick-walled cells on the under-surface. 



THE STEM 



The stems of most mosses are simple in structure, they have 

 no vascular bundles for strengthening the stem and for the pur- 

 pose of carrying liquids from one part to another. The cells of 

 one part differ but little from the cells of another part ; those on 

 the exterior may have thicker walls so as to form a firmer rind- 

 layer, and those of the interior may be elongated and serve for 

 the storage and transmission of albumen and hydrocarbons. 



The stem of the Hairy-cap is perhaps the most highly devel- 

 oped of all moss stems. 



A cross section shows a central portion of thick-walled cells 



45 



