1879-] Leaves and their Functions . 409 
a blue material : their union forms chlorophyll, or leaf-green. 
It is thought that the yellow colour of leaves at maturity is 
caused by the predominance of xanthophyll at that time. 
Besides chlorophyll the leaf-cells contain the proximate 
principles of the plant ; and here the real work of building 
plant structure is performed. But this brings us to the 
consideration of the second part of our subject, viz., the 
functions of leaves. 
In treating this branch of the subject it will be necessary 
to consider the leaf under several different characters. 
Leaves should be considered as real living beings, capable 
of performing vital functions— as workers performing a large 
amount of important work. We may first consider the leaf 
as a pump. One of its most important offices is to pump up 
water from the soil through the roots and stems of plants. 
This it exhales through its stomata in the form of invisible 
vapour. By this means a large quantity of water is carried 
up from the soil to the atmosphere. Thus a large portion 
of water that would quickly settle down through the deeper 
soil, and find its way into underground passages, is carried 
up and given off to the atmosphere, where it is condensed 
into clouds and descends in rain, thus watering and making 
fruitful the earth. Without this work many parts of the 
earth that now blossom as the rose would become arid 
wastes. The amount of moisture thus carried up and ex- 
haled by the foliage of trees and plants is immense. A 
sunflower, with a leaf surface of 39 square feet, exhaled 
3 lbs. of water in twenty-four hours. A corn plant, in about 
three and a half months, gave off in vapour thirty-six times 
its own weight of water. A medium-sized forest tree will 
pump up and exhale about 5 barrels of water in twenty-four 
hours : this will give about 800 barrels to the acre. An 
acre of grain or grass will do about the same. From this 
it may be seen why forests exert such a powerful influence 
on the rainfall of a country. 
Again, we may consider the leaf as a lightning conductor . 
It Is one of the most efficient conductors of electricity ever 
made. Most leaves have notched edges ; each of these 
“ points ” is powerful to attract the eleCtric fluid from the 
air, and through the stem convey it silently to the ground. 
A single blade of grass is said to be three times as powerful 
to attract electricity as a fine cambric needle, and a twig 
covered with leaves is more efficient than the best con- 
structed “ patent point.” A tree covered with leaves is the 
most efficient safeguard from lightning that can be found. 
A green tree is constantly conveying electricity from the 
