4 BULLETIN 1263, U. S. DEPARTMENT OF AGRICULTURE 



when transplanting them the ground was sufficiently shaded by the 

 foliage to practically eliminate the injury which they termed " stem- 

 girdle." 3 



That the optimum temperature for assimiliation and other chemi- 

 cal activities within the plant may be considerably below the maxi- 

 mum temperatures experienced, has of course long been recognized. 

 Cieslar (4) , for example, discussing the experiments of Lubimenko 

 to determine the effect of light and temperature upon assimilation of 

 Abies, Picea, Pinus, Lariat, Taxus, Tilia, Robinia, and Betula, states 

 that " with medium light intensity, the assimilation rises to a tem- 

 perature optimum, varying with" species, then sinks (with higher 

 temperatures), excepting with the larch, which continues to increase 

 its assimiliation to the highest temperature used. Similar results 

 were secured under the strongest light." Cieslar concluded that 

 " a close relation between light and temperature influence exists and 

 that the (shade) tolerant species behave differently from the intoler- 

 ant. The former decrease in (assimilative) energy very rapidly after 

 the maximum ; the latter very little, or not at all. " So it may be 

 said that the foliage of intolerant species is less sensitive to heat 

 than that of tolerant." The results of the experiments reported 

 here substantiate this last statement. 



MacDougal and Working (£), experimenting with Opuntia at 

 the Desert Laboratory, found that " joints of this plant maintained 

 a fair rate of enlargement at a temperature of 56.5° C, the air sur- 

 rounding them being 58° C. (136° F.), and the growth of young 

 joints of the same plant stopped and shrinkage ensued when the 

 temperature rose to 144° F. in an air temperature of 146° F., but 

 growth was resumed when their temperature fell below 122° F." Of 

 special significance was the finding that " data from observations in 

 which temperatures were taken from the air have but little value in 

 any estimation of the working temperature of active protoplasm,. 

 by reason of the abnormal hydration and transpiration conditions 

 introduced. These conditions as well as the proportions and state 

 of the main colloidal components must determine the temperature 

 effects." It is very likely that the transpiration conditions involved 

 in the above study were considerably more severe than those involved 

 in the present consideration of the central Kocky Mountain conifers. 

 In a comparative study of species, however, we are not more con- 

 cerned with absolute plant temperatures than with the recording 

 of air temperatures and the different reactions to specific external 

 conditions. 



The usually close connection between heat injury and excessive 

 transpiration is likewise illustrated in a discussion by Munns (7) 

 of the effect of the Santa Ana wind on various eucalypts. This wind 

 is a hot current which occurs in southern California in the vicinity 

 of the mountains. While the effect of the wind on the foliage of 

 eucalypts is described as a "burning," the conclusion is that this 



3 Ono of the factors that has no doubt retarded the study of possible injuries from 

 excessive heat has been the lack of suitable thermometers for recording the plant tem- 

 peratures closely enough to establisb the connection between them and tbe injury. While 

 the apparatus lias not been used in the present study, some of the difficulties of tem- 

 perature measurements have been solved by K. Shreve \u). The apparatus described con- 

 sists of a Comparatively simple and small thermocouple outfit, tbe sensitive junction of 

 which may be pressed against the surface of a leaf, obtaining a quick response to its 

 temperature and therefore not seriously interfering with the conditions which most di- 

 rectly influence that temperature. 



