WATER UTILIZATION BY TREES 71 



out by Stahl {202). In the needles of many species of fir growing in 

 the sun, these hard cells form an almost continuous layer beneath the 

 upper epidermis; while in the shade they are only slightly developed. 

 Likewise, species of southern origin, e. g., Abies cephalonica Loud., 

 show a stronger development of the hard cells than those in moister, 

 more northerly regions, and the leaves of silver fir seedlings in the 

 shade where there is plenty of water have almost no hard cells; while 

 Groom (74) called attention to the xerophytic structure of the leaves 

 of northern conifers, which live in regions where there is plenty of 

 moisture but where, owing to the extreme cold, it is for the most 

 part unavailable (physiologically dry habitats). 



In his paper on the relation of the leaf structure of western conifers 

 to light and moisture, Larson (132) compared the general shape of 

 the leaf, shape of the stomatal depressions, location of the stomata 

 in relation to grooves, nature of the parenchyma, ratio of the xylem 

 to the entire leaf, amount of xylem per stoma, amount and condition 

 of intercellular spaces, and presence or absence of endodermis and 

 degree of its lignification, in Tsuga heterophylla Sarg., Picea engel- 

 manni, Abies grandis, Pseudotsuga taxifolia, Pinus monticola D. Don., 

 P. contorta murrayana Engelm., P. ponderosa, and P. albicaulis. 

 His results show distinctly the relation of structure to tolerance and 

 moisture requirements. Unfortunately, in this work the . effects of 

 light and moisture have not been separated, but on the other hand, 

 as has so frequently been observed, these two factors are very difficult 

 to separate ecologically. 



METHODS OF ASSURING AN ADEQUATE WATER SUPPLY 

 INCREASING THE ROOT SYSTEM 



Turning now to the methods by which plants might assure an 

 adequate supply of water in their leaves, the most obvious method 

 would be that of expanding the root system. But, on closer observa- 

 tion, it will be seen that to expand the root system in times of drought 

 or when the water supply is endangered is somewhat like lifting oneself 

 by one's bootstraps. The roots cannot grow unless they have food 

 and water, both of which presuppose a supply of water. It is true 

 that roots are positively hydrotropic and will follow up a gradually 

 receding water supply, but this is a somewhat different matter. 



While, as we have seen from the work of Schroder (182), leaves do 

 not begin to die until they have lost 50 to 80 percent of their water, 

 roots show signs of water deficiency when the leaves have lost only 

 40 percent of their water. Evidently roots are more sensitive to 

 drought than are the leaves, possibly because they are nearer the 

 source of supply and consequently are the first to feel the effects of 

 drought. 



Dingier (47) determined the transpiring area of the leaves of a 

 7-year-old potted plant of Acer platanoides to be 906 cm 2 , while the 

 size of the entire root surface was 1,720 cm 2 , that of the absorbing 

 roots being 1,000 cm 2 . Such calculations are, however, of little value 

 since neither the area of the root hairs is considered nor that of the 

 mesophyll cells where the actual transpiration occurs. 



The depth to which roots will go when soil conditions will permit 

 them is, in some cases, astonishing. Renner (176) reported that when 

 the Suez Canal was dug, roots of tamarisks were found 30 m from 



