WATER UTILIZATION BY TKEES 17 



"drip points", as in Ficus religiosa L., aid in this process. Other 

 reasons why the leaf should be freed from rainwater as quickly as 

 possible are that its presence favors the establishment of parasites 

 and also the film of water checks normal transpiration and metabolism. 

 Although drip points are associated chiefly with climates of high rain- 

 fall, some trees in temperate climates, e. g., species of Betula, Populus, 

 and Alnus, are provided with these structures. 



The draining of water from the crowns of trees in the Temperate 

 Zone generally proceeds either centrifugally, as in the linden, where 

 the water runs off the edge of the tree first, or cen tripe tally, as in the 

 beech, where the water from the leaves passes first to the branches and 

 thence to the main stem, from which it passes to the ground. 



WATER LOST THROUGH TRANSPIRATION 



By far the greatest losses of water occur through transpiration, by 

 which is ordinarily meant the losses through evaporation from the 

 leaves. Van Tieghem (218) originally used the term for the loss of 

 water from the tissues that did not contain chlorophyll and reserved 

 the term "chlorovaporisation" for the loss of water from the cells 

 with chlorophyll. But neither these distinctions nor those of Henslow 

 (94), who distinguished between transpiration from the protoplasm 

 and that from the chlorophyll, have been maintained in recent work; 

 and the term "transpiration" is now used to include all water losses 

 from the leaf tissues. 



The distinction between cuticular and stomatal transpiration of 

 Von Hohnel (97) is still held, however, the former being the water 

 lost from the epidermal cells of the leaf while the latter is that from 

 the internal cells through the stomata. In addition, Buscalioni and 

 Pollacci (31) have designated as "stomato-cuticular transpiration" 

 that water lost from the surface of the guard cells, but this seems to be 

 a rather fine point and has not been considered very seriously by later 

 workers. 



The term "specific transpiration" is also in use and refers to the 

 percentage of water in a leafy twig that is lost in a unit of time. Thus, 

 if the water content of a fresh shoot is 63 percent and if the water loss 

 during a given period were 9 percent, the specific transpiration as 



used by Neger and Lakon (161) would be — ^— or 14.3. 



By "absolute transpiration" is meant the loss of water in mass 

 units from a given tissue, while the term "relative transpiration" was 

 introduced by Livingston (188) to mean the ratio of the transpiration 

 rate per unit of leaf surface to the evaporation rate per unit of water 

 surface, as measured with the "atmometers" devised by him. These 

 are porous clay structures connected with a water supply permitting 

 free evaporation from their surfaces. Inasmuch as the leaf is a living 

 structure and, therefore, exercises some control over the water losses, 

 evaporation from the leaf is generally less than that from the atmom- 

 eter. A comparison of water losses from equal areas of leaf surface 

 and atmometer surface gives one, therefore, the relative transpiration. 

 This term, "relative transpiration", occurs very frequently in the 

 literature and should be well understood by a student of tins subject. 



121727°— 37 2 



