14 



BULLETIN 1263, U. S. DEPARTMENT OF AGRICULTURE 



1. Under laboratory conditions approaching natural field condi- 

 tions, the wilting of seedlings of all species was mainly in the nature 

 of a collapse of the stem at the ground line. In this particular, the 

 small, frail, shallow-rooted seedlings of Engelmann spruce and lodge- 

 pole pine were much more susceptible to injury than the larger, 

 deeper-rooted and firmer-stemmed seedlings of western yellow pine 

 and Douglas fir. 



2. Seedlings removed from the soil at the age of about 30 days, 

 and subjected to heating in air practically saturated with water 

 vapor, showed considerable injury after a 6-minute exposure at 

 111° F., and increasing injury with higher temperatures, so that it- 

 may be estimated that an exposure of one minute at approximately 

 141° F. would cause critical injury to all seedlings of the four species. 

 The injury at the higher temperatures took the form of almost im- 

 mediate drooping or wilting of the entire aerial portion, and little 



Number "Tests 5 Number- ree-fs 10 Number -rests 6 



Average rempervrure I22.S6F. f/ll.4-l3S°) Average Temperature I43.4SF (l-!2Z-i$4.6) Average le-nperature I7222°F Osi.l-IQI.Q) 

 Average period of exposure Ze'(/4-35) Average period of exposure I5'(6S0) Average period of 'exposure //'(S-30J 



Average age all ' seed Zings SEdays (-K-9/) Average age a/I send/Inge 7S. 7 daya feS-dl) Average age a// ' sead/ings 5*days (45-dl) 



7" 



135 MO 145 ISO 155 

 DEGREES FAHRENHEIT 



Fig. 1. — Average extent of injury at various temperatures under direct radiation. 



or no difference between the species was brought out. It thus appears 

 that conditions designed to heat the entire plant to a temperature 

 of about 140° F., without time or opportunity for appreciable water 

 loss from the leaves, are immediately fatal. 



3. Direct radiation from an electric heating coil with reflector sup- 

 plying greater heating power and less luminous intensity than sun- 

 light, applied to the seedlings at the ages of 46, 64. 90. and 92 days, 

 served to create a condition of internal heating of the leaves similar 

 to that produced by sunlight, but in greater degree and acting more 

 promptly, so that the factors of water loss and water supply would 

 not materially affect the results. The greater sensitiveness of 

 Douglas fir under these conditions, followed closely by spruce 

 seedlings, at leant at the higher temperatures, was very apparent. 

 The seedlings of lodgepole pine showed the least injury at all 

 temperatures. 



