WATER UTILIZATION BY TREES 7 



sence of such fungi the trees do not thrive so well, owing to some 

 sort of inherited "adaptation" in the course of evolution. But this 

 point has not yet been fully cleared up. At any rate, the mycorhiza, 

 when present, apparently help the tree to absorb available moisture. 



SUMMARY AND SUGGESTIONS FOR RESEARCH 



Water is absorbed to a small extent by leaves and twigs but chiefly 

 by the roots. The amount which they absorb depends not only upon 

 the phytic o chemical nature of the soil and its moisture content but 

 also upon inherent specific differences between plants and upon the 

 biotic environment. This latter includes not only roots from other 

 members of the same species but also those of the other species of 

 plants in the same soil. In addition the role of micro-organisms and 

 of mycorhiza must be considered. Although much yet needs to be 

 done in determining the manner in which trees check transpiration, 

 the role of roots in supplying water should not be overlooked. Under 

 what conditions do the deep-lying roots function in supplying water 

 and how effective are they? 



While the broad outlines of these problems have been well under- 

 stood for some time, we have little data of an intensive nature. In 

 just what regions of the soil are the absorbing roots of the various 

 species? Which layers of the soil are the most important for the 

 water supply of individual species, and what adaptations are possible 

 in the various tree species which will permit them to accommodate 

 themselves to a changing soil-moisture content or to a falling ground- 

 water level? To just what extent can environment change the genetic 

 pattern of root systems, and to what degree is the inherited tendency 

 of a species to form a taproot influenced by root mutilations, such as 

 occur in transplanting nursery and other planting stock? Also what 

 is the optimum spacing (density of stand) for the various soil-moisture 

 conditions? And to what extent are range grasses and other minor 

 vegetation such as chaparral and brush species compatible with trees 

 and forests? In answering satisfactorily all these questions, much 

 work still remains to be done. 



STORAGE OF WATER IN TREES AND WATER 



CONTENT OF TISSUES 



WATER CONTENT OF WOOD 



Since water is passing from the roots to the leaves through the stems 

 and branches, it is to be supposed that the stem tissue, although 

 largely composed of dead cells, will, nevertheless, be rich in water. 



The living cells of the wood parenchyma, cambium, rays, and phloem 

 contain a certain amount of water in their vacuoles; but, in addition, 

 one must consider the water content of the wood proper (especially 

 the sap wood), as well as that of the pith and outer bark, since all of 

 these tissues differ in their water content. The amount present also 

 varies with the season, with the time of day, and with the species. 

 Exact figures, taken mostly from European work, show that the aver- 

 age maximum is about 55 percent and the average minimum about 40 

 percent. The water content of a stem is determined by cutting it 

 into pieces of equal size, weighing, drying at about 110° C, and 



