MORPHOLOGY OF HIGHER PLANTS. 



367 



rounded by thin-walled PAREXCHYMA-SHEATHS, or as in several 

 grasses and sedges by thick-walled mestome-sheaths. In some 

 plants of various families, the midrib is not only stronger devel- 



FIG. 212. Study of the stomata on leaves of Beta vulgaris: A and B, surface sections 

 of the leaf, and C and D, transverse sections of the stomata. In A and C, the stomata 

 are shown with the guard cells (s) distended and the pore (sp) open to allow the passage of 

 vapors and gases. B and D, showing the pore or opening closed due to the plasmolysis 

 of the contents of the guard cells, the internal pressure or tension having been relieved, 

 e, epidermal cells; a, large cavity or intercellular space beneath stomata; and m, loose 

 mesophyll cells with chloroplasts. After Frank. 



oped than the lateral veins, but it may be composed of several, 

 instead of only one, mestome-strand, sometimes representing a 

 true stele. 



