128 BOTANY 
been estimated that roughly about four hundred 
pounds of water is transpired, or given out by the 
plant, for every pound of carbohydrate formed. The 
transpiration current, as we have seen by standing 
plants with their roots in eosin solution, carries the 
water containing dissolved minerals from the root hairs 
through the cortex to the vessels of the wood. The 
course then follows these vessels through the root and 
stem, finally reaching the leaves, where the branching 
of the vascular bundles into a close network allows the 
solution to supply every part of the blade. We are 
concerned now with the means by which the surplus 
water is got rid of. 
The leaves form the chief organ of transpiration. 
Fill two long-necked flasks with water. In one stand 
a leafy shoot: the geranium or nasturtium admirably 
serves the purpose. In the other, place a similar shoot 
from which all leaves have been removed. Now, to 
prevent evaporation, cover the surface of the water 
with a thin layer of olive oil (Fig. 87). Be sure to put 
the shoots into the water first. Otherwise the cut end 
will become clogged with oil. It will be found that 
under favourable weather conditions the water in the 
fiask bearing the leafy shoot soon falls, while, even 
after several hours, the water in the other flask will 
remain at practically the same level. This goes to 
show that the leaf is the chief avenue of transpiration. 
The weight of water transpired by the leafy shoot may 
be caleulated by measuring in centimetres the amount 
of the fall and the bore of the neck of the flask, and 
reckoning a cubic centimetre to the gram. By 
measuring the total area of the leaves the transpiration 
per sq. centimetre may be found. By thus using a 
leafy shoot under different conditions the following 
facts will be discovered. Transpiration is more 
vigorous on a dry day than when the air is charged 
with moisture. When the wind is blowing and the 
