68 PLANT PHYSIOLOGY AND ECOLOGY 



spired on the average 75 kg. daily during the summer, while a 

 birch tree possessing about 200,000 leaves lost nearly 400 kg. 

 during one hot day. 



84. Relation between transpiration and absorption. It has 

 been previously shown that transpiration and absorption are neces- 

 sarily reciprocal, or compensatory. The amount of water absorbed 

 determines the amount that can be transpired, and, conversely, 

 the rate of transpiration reacts forcefully upon absorption. Leaves 

 that bear stomata cannot avoid transpiration, but under ordinary 

 conditions this is beneficial instead of harmful. The growth of a 

 plant depends upon the amount of water absorbed, and the amount 

 of nutrient material dissolved in it. Where the water supply is 

 sufficient, plants that transpire the most obtain the greatest 

 amount of water and salts, and grow the best. Under the usua- 

 conditions, the water absorbed by the roots contains about .01% 

 of dissolved salts. In other words, a plant must absorb and 

 transpire 10 kg. in order to obtain 1 gram of nutrient salts for 

 use. In the case of many water plants, the nutrient content is 

 much less, and the amount of water that must be transpired is 

 correspondingly greater. Thus, while transpiration is an in- 

 evitable process for all plants with stomata, it is more significanl 

 for some than for others. Its significance depends upon absorpt 

 tion, i.e., upon water content. For plants in dry soils, transpira- 

 tion is more or less injurious, and it must be prevented in so far 

 as possible. Transpiration is beneficial to plants of moist habitats 

 because it promotes their growth. This benefit is still more pro- 

 nounced in amphibious plants, where the movement of a large 

 amount of water through the plant is necessary to its welfare. 



85. Compensation for increased transpiration. From what 

 has just been said, it is clear that the effect of increased tran- 

 spiration is determined by absorption: the converse is equally true 

 though much less important. Such compensation is naturally 

 possible only when the water supply is adequate to meet the new 

 demand. Consequently, when the water loss is greatly increased^ 

 the final adjustment of the plant is determined by the amount of 

 water available at the root surface. As indicated, the compensation 

 merely results in increased transpiration when the absorption is 

 entirely adequate to the demand. When the water supply is 

 inadequate, the stomata close. If this condition is but temporary, 

 the stomata open as soon as absorption is able to restore the normal 



