Leaves. 



to the earth, it being estimated, for instance, that when the 

 sun is in the zenith the energy communicated by it to the 

 earth's surface amounts to one horse power for each area 

 five feet square. It is impossible to conceive of the amount 

 of energy coming in this way to the leaves of plants over 

 the entire surface of the earth ; but the student will find it 

 instructive to estimate the amount for a single definite field 

 of wheat or corn. 



64. The Leaf a Manufac- 

 tory. The leaf intercepts 

 the sun's energy and is en- 

 abled by it to manufacture 

 the starch and other food 

 materials necessary to the 

 life and growth of the plant. 

 We must therefore look upon 

 a leaf as a manufactory which 

 uses energy directly from the 

 sun to do its work. To use 

 economically the energy of 

 the sun directly is, as yet, for 



FIG. 34. 



Diagram showing the distance apart of 

 the branches of vascular bundles in 

 the sunflower leaf, as seen when look- 

 ing through a bleached leaf with a 

 medium power. The horizontal line 

 shows A mm. magnified to the same 

 extent as the figure. 



us an unsolved problem, but 



plants have solved it for themselves. We must examine the 

 construction of a leaf to see how this is accomplished. We 

 note that a typical leaf is spread out in the form of a flat 

 lamina, which insures the interception of a maximum 

 amount of light, and the ready ingress and egress of gases 

 to and from all parts of the leaf. We have seen in Obser- 

 vation 87 that the vascular bundles ramify throughout the 

 leaf so completely as to bring all parts of it in close com- 

 munication with the water brought up from the roots, and 

 to quickly carry out of the leaf its manufactured products 

 (Fig. 34). And we have further seen by Observation 92 



