THE MANGO FAMILY 25^ 



have a certain rigidity in order to spread out a wide, green 

 surface to the sun, and this is obtained by the system of veins 

 or ribs. 



(c) The leaves are also leathery^ due to a thick epidermis 

 (coat), which reduces the rate oj evaporation of the water in the 

 tree and is, therefore, of great importance to it. Every one 

 knows that the ^lango tree is evergreen and able to keep its 

 leaves on all the year round. It thus preserves its roots much 

 cooler than a tree which has at times no leaves, like the Teak. 

 It is also able to go on storing up food all through the year, 

 and as its fruit forms at a time when many other trees are leafless, 

 this is another very great advantage to the Mango tree. But it 

 does not follow from this that the Mango tree (or any other 

 evergreen tree) has no resting period. "There is a delinite peri- 

 odicity — that is, a regular alternation of resting and working 

 periods — noticeable in all tropical trees, although the seasons at 

 which episodes like the fall of leaves or the shooting of buds 

 take place, vary greatly in different species, in different indivi- 

 duals of the same species, nay in different branches of one 

 individual. In the Mango tree, for example, one or two branch 

 systems may alone be putting forth the reddish-brown young 

 leaves at a time when the rest of the crown retains the dark- 

 green adult foliage" (J. M. F. Drummond). 



(d) Further, the leaves are smooth and shining somewhat like 

 a looking glass. Now, we all know that light-rays are reflected 

 from the surface of a mirror and not absorbed. The same is the 

 case with the heat-rays which accompany the former, and by thus 

 reflecting the heat-rays the temperature of the leaf is kept down 

 and a further source of evaporation taken aivay. Evaporation 

 increases in proportion as the temperature rises. 



{e) The young leaves of the Mango tree are specially protected 

 against any injury from excessive heat and light. When the tree 

 is budding you can see all the young leaves hang loosely as if they 

 were fading (pendent leaves, fig. 22). They have grown at an enor- 

 mous rate and attained the size of full-grown leaves in a very short 

 time. This being so, the tender cells of which they are compos- 

 ed and their contents could be easily destroyed by a vigorous 



