26 



DICOTYLEDONS 



act of transpiration and by intense sunlight. The vertical 

 placement of these leaves reduces the effect of heat and light. 



When the leaves are mature, 

 the tissues are strained, and 

 the leaves assume the ordi- 

 nary horizontal position. 



(/) The young leaves of 

 the Mango tree are often red 

 or copper-coloiired. This is 

 caused by a very active 

 process of respiration or 

 breathing, which, as we all 

 know, is most active always 

 in young organisms. For 

 a plant, like an animal, 

 breathes, that is to say, in- 

 hales oxygen in order to 

 burn off some of its carbon 

 and thus produce the heat 

 necessary for the various 

 chemical processes carried 

 on in its body to maintain 

 its life and to grow. The 

 gas produced by the combi- 

 nation of the inhaled oxygen and the carbon in the plant is 

 carbonic acid gas, which is exhaled (see II. Part, Respiration). 

 When the leaves become older, the process of breathing be- 

 comes slower and the copper-hue disappears from the leaves, 

 changing into the ordinary green colour of leaves. The colour 

 is due to little green granules, called chlorophyir'' granules 

 in the inner cells of the leaf. These granules have the power 

 of feeding on the carbonic acid gas circulating through the 

 air-spaces between the cells and to form starch under the influ- 

 ence of the sun's rays. This process is known as "assimilation" 

 (see II. Tart, Assimilation). 



FjV. 22. 



Young shoot of Mango with 

 pendent leaves. 



•From Greek rhloroH, green, and phyllon, a leaf. 



