Page:Popular Science Monthly Volume 59.djvu/505

Rh passing buckets at a fire, or tossing melons in the loading of a schooner for a northern market.

The whole story of water-carrying is not ended with the above. One of the most delicate of all plant mechanisms is that which is associated with the transportation of its liquids. The leaves and green surfaces generally are closely studded with minute structures, 100,000 or more to a square inch, that open or close as the emergencies of the case demand. They are vitalized and exceedingly sensitive valves, usually constructed of two crescent-shaped cells set in the skin and highly charged with protoplasm. These organs are influenced by sunlight and darkness, by heat and cold; in fact, their functioning calls forth the admiration of any careful student of the subject. The two guard cells are so hung that they become turgid when the leaf is well filled with water, and thus enlarge the opening to its full capacity for the passage of vapor-laden gases. As soon as these guard cells lose much water, they become less plump, and this brings about the closing of the pore. They are, therefore, valves of safety, and, as the other portions of the leaf are covered with a cuticle more or less impervious to gases, it is seen that the stomates are the organs that regulate the evaporation stream.

That the amount of water carried is very great scarcely needs to be emphasized. Note the rapidity with which grass wilts when cut for hay or the leaves upon a branch that has received any injury. If a melon vine with twelve leaves will carry a liter of water in a single day, as it has been known to do, what must be the vastness of the lift in a forest of a thousand acres upon a dry day when the leaves are fresh and most active!

That it needs to be great is seen from the requirements of the plant. The soil water is weak in all salts that a plant must acquire, and to take them in concentrated form would be as poison. The whole plan, therefore, is to carry large quantities of a dilute solution, and afterwards bring it to the required strength. In the evaporation there is a cooling obtained that may possibly save the plant from destruction.

We thus far have seen that an ordinary plant has its slender, delicate, insinuating root-hairs closely applied to the soil particles from which they imbibe the adhering moisture. It has further been shown that the opposite terminal of the waterways has also a vast number of delicate living cells exposed, not dangerously, to the drying action of the atmosphere. Between these two extremities is the body of the tree, the main roots and branches, and it is for us to determine through what parts the upward flow takes place. This admits of demonstration by the removal of certain portions and observing the effects. That it does not take place through the central or heart wood is to be expected, for the cells here are often all filled up with lignin and coloring