22 THE TREE BOOK 



conditions, the object being to prevent the sun 

 from drying up the leaf juices. The India-rub- 

 ber tree, of the East Indies, has a leaf skin com- 

 posed of three layers of cells. Always the leaf 

 skin must be thin enough for the water which 

 constantly streams up from below to evaporate 

 readily, otherwise the starch factory would soon 

 be submerged 1 



Fifth: To aid in this work of evaporation, 

 the lower leaf -skin is full of little pores, for the 

 easy passage of air and vapor. Some leaves 

 have such pores on the upper surface too, and 

 they are found on green stems and young 

 branches less than a year old, and also on green 

 fruit. These little pores or mouths are called 

 stomata. It is through the leaf stomata that 

 the great quantity of vapor, which we have al- 

 ready spoken of, rises from the trees. The lit- 

 tle mouths swell up whistle-shape in wet weather 

 and collapse in times of drought. So smooth 

 and polished is the leaf-skin that, in a rain 

 storm, most leaves shed water like tiny um- 

 brellas. 



On a bright summer day, the leaves of a good 

 sized tree will make over a pound of starch. 

 To do this several hundred pounds of soil water 

 must be taken in, sorted, and passed off in 

 vapor. Starch is the vital food for all plant 



