WATER UTILIZATION BY TREES 59 



Need for further study has been indicated at various places in the 

 body of the discussion, but several lines of work seem especially 

 promising: 



(1) Is the size of the vessels of the wood a good index to the water 

 demands of a tree? Can the size of the vessels of the same species in 

 various sites be taken as an index to the site possibilities, and are the 

 sizes of the vessels in different species on the same site indicative of 

 the relative water demands on that site? If so, a study of wood 

 and root anatomy in this connection would give valuable results as 

 to site adaptations. 



(2) It should also be advantageous to know more about the water 

 relations in various forest soils of the country. Not only their water 

 content at various seasons of the year and at various depths should 

 be more carefully studied, but also their wilting coefficients. Since 

 plants do not wilt much until the wilting coefficient of the soil is 

 reached, it would be advantageous to know over a period of years how 

 many months of the year and in what seasons the soil-moisture 

 content approaches the wilting coefficient. 



Such information in connection with the water requirements of 

 individual species should prove valuable not only in planning long- 

 time planting projects over extensive regions, but also in local problems 

 of seeding, rejuvenation, and management. Why are some trees 

 able to withstand apparent droughts and seemingly unfavorable 

 conditions so much better than others? If more were known about 

 the internal anatomy of the tree in relation to its immediate external 

 environment (air and soil), we should be able to give more satisfactory 

 answers to these problems than at present. 



(3) There is need also for more extensive work on the role of tran- 

 spiration. It should be settled once and for all whether transpiration 

 in trees aids in the absorption of minerals from the soil and whether 

 plants that have high rates of transpiration utilize more soil salts than 

 others. It is possible that, owing to the method of water translocation 

 and the physical problems connected therewith, trees behave in this 

 respect differently from shrubs and herbs. If it can be established 

 that there is a connection between water requirements and mineral 

 intake, with a resultant effect upon the soil mineral content, an im- 

 portant advance will be made. Not only would this provide silvi- 

 culture with an easily measurable index to the relative water require- 

 ments of different species on the same site, but it would also shed some 

 light on the general mineral needs of forest trees. 



"ADAPTATIONS" OF TREES TO DRY CONDITIONS 



For many years it was thought that plants living in dry regions 

 transpired less than those in moister localities; and in xerophytes 

 transpiration was thought to be at a minimum. Maximo v, however, 

 has done much to modify this opinion. His views on tins subject have 

 been summarized in book form (148); but the essence of Maximov's 

 contributions has been stated also in more concise articles (149). 

 According to him, plants commonly called xerophytes may transpire 

 more intensely than many mesophytes, and the real essence of xerophy t- 

 ism is the ability to exist in arid regions regardless of the amount of 

 water lost. 



