110 



ELEMENTS OP BOTANY. 



later, but it will make matters simpler to state at once that 

 the epidermis serves as a mechanical protection to the parts 

 beneath and prevents excessive evaporation, that the palisade 

 cells (which it may not be easy to make out very clearly in a 

 roughly prepared section) help to prevent too rapid evaporar 

 tion of sap from the leaf when exposed to excessive dryness, 

 heat, and direct sunlight, and that they hold large quantities 

 of the green coloring-matter of the leaf in a position where it 



I n 



Fig. 97. — Epidermis of Leaf of Althaea. (Much magnified.) 



I, from upper surface ; II, from lower surface ; h, star-shaped compound hairs ; 



st, stomata ; p, upper ends of palisade cells, seen through the epidermis. 



can receive enough, but not too much sunlight. The stomata 

 admit air to the interior of the leaf (where the air-spaces 

 serve to store and to distribute it) and, above all, they regulate 

 the evaporation of water from the plant. 



137. Leaf of ^'- India-Eubber Plant." ^ — Study with the microscope, 

 as the lily leaf was studied, make the same set of sketches, note the 

 differences in structure between the two leaves, and try to make out their 



meaning. 



' Ficu.'i elastica. 



