BOTANICAL CHARACTERS 



83 



Take the width at 6 inches from the tip of the leaf. CHAP 



Take the average of these two measurements and multiply by the length of IV. 



leaf between the two points of measurement. 

 Add the area of the isosceles triangle formed by the 6 inches left at the tip. 

 Multiply by 2 for the two surfaces. 

 Multiply by the number of leaves on the plant. 



The leaves absorb air into their tissues through microscopic 

 openings called stomata (Figs. 14 and 27). At the suggestion 

 of the writer, Mr. H. A. Wager (1), of the Transvaal University 

 College, Pretoria, kindly undertook to determine the number 

 of stomata on the maize leaf. He found that on the under 

 surface they varied from 75 to 126 per square millimetre, and 

 on the upper surface from 60 to 97. Carbon-dioxide (CO.,) 

 is one of the gases of which the air is composed, and consists 

 of the two chemical elements 

 carbon and oxygen. When the 

 air comes in contact with the 

 chloroplasts (11 61) in the leaf- 

 cells, in the presence of light 

 and moderate warmth, the 

 carbon-dioxide is decomposed, 

 and some of the oxygen is 

 given off into the air. The 

 carbon is retained, and, com- 

 bining with the water and 

 chemical substances obtained 

 from the soil through the roots, 

 various complex organic com- 

 pounds are formed. This chemical action takes place chiefly 

 during the day-time, and only in the presence of light, and 

 is therefore called photosynthesis. The new compounds are 

 used in the building up of tissues required for the increasing 

 growth of the plant. 



All of that enormous quantity of starch required to fill out 

 the endosperm of the maize grain must first be chemically 

 formed in the leaf before it is carried to the grain on the ear, 

 where it is finally deposited. The importance of the leaf in 

 the life-history of the plant is thus evident ; it is a chemical 

 laboratory in which the various elements of plant-food are separ- 

 ated out from the compounds in which they originally occur, and 

 are re-united into such forms as can be made use of by the plant. 



6* 



Fig. 27. — Three stomata with sur- 

 rounding epidermic cells (E). G, G, 

 guard cells of a stoma. From Sir F. 

 Darwin's Elements of Botany (Cam- 

 bridge University Press). 



