Vegetable Physiology. 241 



the softer part, and distributing the sap throughout the whole. 

 The parenchyma is composed, as can be seen in many leaves 

 tfitb the naked eye, of separate cells, which when magnified, 

 ^e found to contain each a globule of the chlorophylls which 



• ves them their green color. These cells are arranged differ- 

 ently in different parts of the leaf, being usually more closely 

 packed together near the upper surface than the lower, where 

 there are more interspaces between them. The cuticle or 

 outer skin of the leaf is furnished with stomata, heretofore 

 described, by which watery vapor can pass out, and air can 

 enter. These breathing pores communicate with the cells, so 

 as to allow the passage of air through the whole interior of the 

 leaf. When the plant is too full of sap, they open in a peculiar 

 manner, so as to allow the escape of the superabundant moist- 

 ure, and when it becomes dry, they close so as to retain a suffi- 

 cient amount to sustain life. These pores are principally 

 found on the lower surface of the leaf, since they are too deli- 

 cate to endure the full influence of the sun. 



The object of this arrangement, says Dr. Gray, will appear 

 evident, when we consider that the spaces between the cells, 

 filled with air, communicate freely with each other throughout 

 the leaf, and also with the external air, by means of the 

 stomata, and when we consider the powerful action of the sun 

 to promote evaporation, especially in dry air, and that the thin 

 walls of the cells, like all vegetable membrane, allow of the 

 free escape of the contained moisture by transudation. The 

 light and air necessarily being freely admitted into the texture 

 of the leaf, the sap it contains would be liable to escape by- 

 evaporation faster than it can be supplied from the stem and 

 roots ; and the leaf consequently shrivel and perish just as it 

 does when plucked from the stem. A safeguard against 

 excessive evaporation is to some extent afforded by the more 

 compact arrangement of the upper stratum, which is exposed to 

 the direct action of the sun, as well as by the form and vertical 

 position of these cells, which present the least possible surface 

 to the sun's rays. This provision is the more complete in the 

 case of plants indigenous to hot, arid regions, where the soil is 

 frequently so parched for long periods as to afford only the 

 scantiest supply of moisture to the roots. 



