IQ MISC. PUBLICATION 257, U. S. DEPT. OF AGRICULTURE 



osmotic pressure of the cell contents, with the result that there is an 

 osmotic gradient between the roots and the leaves, the osmotic pres- 

 sure increasing upward. Whatever makes for water depletion appar- 

 ently tends to increase the osmotic pressure in the cells and vice 

 versa. The extent to which the osmotic pressure may be taken as 

 an index to the site conditions has been studied by several workers, 

 but much here still remains to be done. To what extent can trees 

 use stored- water to tide them over unfavorable conditions? And 

 under what conditions and on what sites will maximum storage 

 result? What use, if any, do trees make of this stored water in repro- 

 duction? And does the water content of trees during heavy seed 

 years differ markedly from that during years when few seeds are 

 produced? What effect do changes in the ground-water level, pre- 

 cipitation, etc., have upon this storage of water, and how is this 

 related to the subsequent drought resistance? And, above all, how 

 can a knowledge of these principles be applied to the practical business 

 of silviculture and watershed management? The answers to all these 

 questions await the developments of future research. 



WATER LOSSES OF TREES 



As has just been indicated, both the amount of water absorbed by 

 plants and the amount stored by them depend to a very large extent 

 upon the amount lost. These water losses occur through (1) bleeding, 

 (2) guttation, and (3) transpiration. 



BLEEDING, GUTTATION, AND DISPOSAL OF EXCESS WATER 



Bleeding is the loss of water (with any included dissolved materials) 

 from cut ends of stems and branches. It is a result of the root pres- 

 sure below, which apparently forces water up the stem whether or 

 not it is being used by the parts above. Some plants bleed much 

 more readily than others. Thus Hartig (87) noted that maples and 

 birches bleed more easily than most other trees, followed by beech 

 and hornbeam. The cut surface becomes distinctly wet in black 

 locust, willows, alders, poplars, firs, and larches; but not even a wetting 

 of the surface can be observed in pine, spruce, oak, horsechestnut, 

 and basswood. Since bleeding occurs only under abnormal or acci- 

 dental conditions, it need be discussed no further at this point. 



Guttation, by which is meant the secretion of water through leaf 

 scars or special glands in the leaves, likewise plays a very minor role 

 in trees of the temperate zones. Although in warm, humid climates, 

 such as that of Louisiana, guttation is very marked in the autumn 

 through leaf scars in some deciduous trees, e. g., the hackberry, 

 it is chiefly of importance in tropical rain forests and, in general, is 

 not especially noticeable in the region under consideration in this 

 publication. 



In the Tropics, where there is a great excess of moisture, leaves 

 very frequently have structures which permit or even favor the rapid 

 run-off of rainwater. Since drops of water remaining on the leaf 

 epidermis frequently act like convex lenses and may result in local 

 burned areas, it is advantageous in bright sunlight for the leaf to rid 

 itself of surface films and drops. Many plants in the Tropics droop 

 and thus permit the rain to run off quickly; and prolonged tips or 



