4 LEAF-MINING INSECTS 



tions are sometimes added upon its surface. Plant hairs 

 are commonly developed from single cells of the epidermis, 

 more abundantly, as a rule, upon the lower surface of the 

 leaf. 



Midrib and veins have a twofold function in the leaf. 

 They contain hard parts that make them supporting struc- 

 tures, and they contain the vessels that are the channels of 

 circulation within the plant, bringing up water with its 

 content of mineral salts from the ground, and taking to 

 other parts of the plant the sugars and other products of 

 assimilation that are manufactured in the leaves. In so 

 far as the veins contain cells having hard walls, they offer 

 an impediment to the work of the leaf-miners. 



The part of the leaf for which all other parts exist and 

 on whose products all other parts of the plant subsist, 

 is the soft living, protoplasmic, chlorophyl-containing 

 parenchyma. This is variously disposed in different types 

 of leaves, but in general it tends to be differentiated into 

 two layers: a palisade layer of one or more rows of closely 

 placed, usually columnar cells next the epidermis, and a 

 spongy layer of irregularly placed and openly spaced cells 

 beneath or within. The stomates of the epidermis open 

 into the interspaces between the cells of the spongy layer. 



It is a part of the fitness of things that the close packed 

 palisade cells, with their rich content of chlorophyl should 

 be placed directly in contact with the transparent upper 

 epidermis of the leaf where they are reached by effective 

 light; and that stomates should be relegated mostly to the 

 lower epidermis where they control the admission of air to 

 the moist chamber of the spongy parenchyma, maintaining 

 proper conditions there for the exchange of gases in me- 

 tabolism. The palisade layer is the chief seat of food pro- 

 duction. It is the part of the leaf that is sought out by the 

 more specialized leaf -miners: the lower, more easily pene- 

 trable, spongy layer is the first sought by the unspecialized. 



