Whether water passes upward from cell to cell or through especially provided tubes, it reaches at last the leaves, where evaporation takes place.  It is necessary to consider in greater detail what takes place in leaves in order that we may more clearly understand the loss due to transpiration.  One half or more of every plant is made up of the element carbon.  The remainder of the plant consists of the mineral substances taken from the soil (not more than two to 10 per cent of the dry plant) and water which has been combined with the carbon and these mineral substances to form the characteristic products of plant life.  The carbon which forms over half of the plant substance is gathered from the air by the leaves and it is evident that the leaves are very active agents of plant growth.  The atmosphere consists chiefly of the gases oxygen and nitrogen in the proportion of one to four, but associated with them are small quantities of various other substances.  Chief among the secondary constituents of the atmosphere is the gas carbon dioxid, which is formed when carbon burns, that is, when carbon unites with the oxygen of the air.  Whenever coal or wood or any carbonaceous substance burns, carbon dioxid is formed.  Leaves have the power of absorbing the gas carbon dioxid from the air and separating the carbon from the oxygen.  The oxygen is returned to the atmosphere while the carbon is retained to be used as the fundamental substance in the construction by the plant of oils, fats, starches, sugars, protein, and all the other products of plant growth.

This important process known as carbon assimilation is made possible by the aid of countless small openings which exist chicfly on the surfaces of leaves and known as “stomata.”  The stomata are delicately balanced valves, exceedingly sensitive to external influences.  They are more numerous on the lower side than on the upper side of plants.  In fact, there is often five times more on the under side than on the upper side of a leaf.  It has been estimated that 150,000 stomata or more are often found per square inch on the under side of the leaves of ordinary cultivated plants.  The stomata or breathing-pores are so constructed that they may open and close very readily.  In wilted leaves they are practically closed; often they also close immediately after a rain; but in strong sunlight they are usually wide open.  It is through the stomata that the gases of the air enter the plant through which the discarded oxygen returns to the atmosphere.

It is also through the stomata that the water which is drawn from the soil by the roots through the stems is evaporated into the air.  There is some evaporation of water from the stems and branches of plants, but it is seldom more than a thirtieth or a fortieth of the total transpiration.  The evaporation of water from the leaves through the breathing-pores is the so-called transpiration, which is the greatest cause of the loss of soil-water under dry-farm conditions.  It is to the prevention of this transpiration that much investigation must be given by future students of dry-farming.