LEAVES AND THEIR USES 241 



the greatest possible amount of light in the early morning 

 and late afternoon when the light is least bright, and the 

 smallest possible amount at midday when it is most intense. 

 This and other plants with similarly vertical leaves are 

 sometimes called " compass plants." 



255. Evaporation of Water. Leaves are the parts of the 

 plant from which water is chiefly lost -by 'evaporation. 

 When we consider the total area of the surfaces of the leaves 

 of an ordinary plant and remember that water is continually 

 evaporating from all these surfaces, we can see that a large 

 amount of water must be lost. It has been calculated that 

 a single oat plant in the course of its life gives off about 14 

 pounds of water ; and that a birch tree with 200,000 leaves 

 loses about two barrels of water per day. The water thus 

 lost by the leaf cells that are in contact with the air is re- 

 placed by water that comes from the cells deeper within the 

 leaf ; these in turn draw upon the water in the wood cells of 

 the veins and leaf -stalk ; these take water from the wood 

 cells (and vessels) of the branch or stem, these from the 

 wood cells of the roots, these from the cells of the cortex, 

 and these from the root hairs, which finally, as we know, 

 absorb water from the soil. Thus there is what amounts to 

 a steady stream of water passing from the soil through vari- 

 ous tissues of the plant, but most of the way through the 

 wood, to the outer cells of the leaves and thence into the air. 

 The comparison of this current of water to a stream must not 

 be pushed too far. The water really passes through a long 

 series of cells and vessels, going from each cell or vessel to 

 the next through the wall which separates them. 



It is because of this continual passage of water upward and 

 outward that the roots of a tree must penetrate to the deeper 

 layers of the soil where a steady supply of water is to be had. 

 It is because of the great need for water that the roots of a 

 tree must branch abundantly, so that there shall be many 

 growing root-tips, each with its zone of absorbing hairs. 



