PLANTS AS WATER-CARRIERS. 495 



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 im- 

 pervious to gases, it is seen that the stomates are the organs that regu- 

 late the evaporation stream. 



That the amount of water carried is very great scarcely needs 

 to be emphasized. Kote 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 after- 

 wards 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 



