2 BULLETIN 1263, U. S. DEPARTMENT OF AGRICULTURE 



to the inability of the seedling to supply moisture equal to the 

 transpiration. Of course, this loss of water may be accompanied 

 by direct heat injury as well: but, in general, if the soil drying has 

 advanced very far, the result should be thought of as due primarily 

 to lack of water. 



In the present study the effort has been made to devise tests which 

 would show the ability of seedlings to withstand high temperatures, 

 as distinguished from a lack of water in the stems or leaves. The 

 general assumption has been that if the protoplasm is heated to a 

 temperature of 140° F. or 60° C., direct injury, from which re- 

 covery is not probable, will ensue almost immediately. It seems im- 

 probable that the qualities of protoplasm differ appreciably in this 

 respect as between the several species. On the other hand, the ex- 

 tent to which the cells of the plant remain below the air tempera- 

 ture, or fail to be heated by radiation penetrating to their interior, 

 must vary considerably according to the absorptive nature of the 

 cell walls" and plastids, and the rate at which the heat is being dis- 

 sipated by vaporization and transpiration. Information has been 

 particularly sought to answer the questions: Are the seedlings of 

 different species protected in different degrees by their varying 

 transpiration rates? Is this the basis for their apparently dif- 

 ferent heat requirements, heat tolerances, and light tolerances? 



LITERATURE AVAILABLE. 



There is little information in the scientific literature on direct- 

 heat injury to plants growing under natural conditions. That ex- 

 cessive temperatures from direct insolation may be a deciding factor 

 between one species and another was claimed by Bates in a recent 

 article (&) 1 , and is brought out again in his more elaborate dis- 

 cussion (3) of the physiological requirements of the Rocky Moun- 

 tain trees and in his extensive data on climatic and soil conditions 

 just published (3A). In the second citation it is clearly shown that 

 the common belief that spruce requires more water than pine is not 

 well founded, and is, in fact, definitely controverted by transpira- 

 tion measurements. Engelmann spruce (Picea engelmannii) , Doug- 

 las fir (Pseudotsuga taxijolia), lodgepole pine (Pinus contorta), 

 and western yellow pine (P. ponderosa), require increasing amounts 

 of water in the order named, either on a basis of the increments or 

 the leaf-exposure area. 2 



The concept is therefore reached that since spruce transpires less 

 water than pine it may be more sensitive to heat, primarily because 

 the incident energy is not so largely utilized in the evaporation proc- 

 ess. It is, however, admitted by Bates (3) that since spruce has 

 long been accustomed to seek cool places, where alone its seedlings 

 may survive, it has acquired the habit of slow, shallow rooting, be- 

 cause these cool places are rarely subjected to rapid drying or to 

 exl reme drying of the surface soil. Conversely, western yellow pine, 

 since it needs much heat for its proper functioning and develop- 

 ment, has become accustomed to soils which are directly exposed to 



• The numbers in italics in parentheses refer to " Literature Cited" at the end of this 

 bulletin. 



* Reference to citation (2) will show tbal (his arrangement depends on several condi- 

 tion . The arrangemenl given is the Qrsi deduction from transpiration tests in 1917. 



