34 



MISC. PUBLICATION 257, U. S. DEPT. OF AGRICULTURE 



year, as shown in table 13, leading to the conclusion that seedling 

 size determines the capacity of each species to prolong its transpiration. 



Table 13. — Effect of age on transpiration of conifer seedlings at room temperatures 

 (69° to 77° F.) for about 16 hours 



Ponderosa pine 



Lodgepole pine 



Age of seed- 

 lings i 



Average 

 weight- 



Water 

 transpired 

 per gram 

 of dry- 

 weight 



Average 

 weight 



Water 

 transpired 

 per gram 



of dry 



weight 



Douglas fir 



Eneelmann spruce 



Average 

 weight 



Water 

 transpired 

 per gram 



of dry 

 weight 



Average 



weight 



Water 



transpired 



per gram 



of dry 



weight 



68 days.. 

 122 days. 

 1 year... 



Milligrams 

 38 



61 

 SO 



Milligrams Milligrams 

 8. 56 5. 5 



4.00 

 2.76 



Milligrams 

 41.43 

 18.10 



Milligrams 

 8 

 24 

 34 



Milligravii 



26.17 

 6.30 



1.71 



Milligrams 

 4 



Milligrams 



47.60 



24.30 



3.26 



1 Douglas fir approximately 25 days younger. 



This dependency of transpiration upon temperature results natu- 

 rally in daily and seasonal cycles. In the daily cycle especially, how- 

 ever, it is very difficult to separate the heat effect from that of light, 

 since the two parallel each other very closely. Dole (52) found fhat 

 Pinus strobus seedlings 6 to 8 years old showed a regular diurnal curve 

 of transpiration. Beginning with about 15 to 20 g of water loss per 

 plant per hour at 8 a. m., the transpiration rose to a maximum of over 

 30 g at about 3 p.m. and then decreased gradually until 8 p. m. to 

 about 4 g. From then until 11 p. m. the transpiration fell off more 

 slowly to about 2 g per plant per hour, remaining at this level until 

 4 a. m., when it again rose gradually through 8 a. m. to the 3 p. m. 

 level. At the time of greatest transpiration the humidity was the 

 lowest. He furthermore showed that with 3-year-old plants the 

 absolute losses could not be correlated either with relative humidity, 

 with temperature, or with then combined effect, as expressed in 

 terms of vapor pressure. He interpreted his results to indicate that 

 the temperature, in addition to its effect upon humidity, influenced 

 the diffusion. This view is in accordance, as has been pointed out 

 above, with the known physical effects of increased temperature. 



Cribbs (40), in his series of studies on Til in % found that when the 

 soil moisture was plentiful the daily transpiration curve was very 

 much like that reported by Dole, but that the time of maximum 

 transpiration varied with the other factors which influence transpira- 

 tion, namely, light intensity, relative humidity, soil moisture, and soil 

 temperature. In exposed stations on sand the maximum rate was at 

 10 a. m., owing to a saturation deficit. A second (lower) rise then 

 occurred in the afternoon. The depression of the transpiration rate. 

 however, was due not to a lower soil-moisture content but to the 

 inability of the tissues to conduct the water fast enough. In these 

 studies, which will be referred to later, the rate of transpiration 

 (transpiring power) was measured by the cobalt chloride paper 

 method. 



Blaydes (17), however, found that the time of maximum transpira- 

 tion came much earlier in the day than the time of maximum evapora- 

 tive power of the air. Evaporation studies with 148 species of plants 

 in the deciduous forests of Indiana and Ohio, using the cobalt chloride 

 paper method, showed that 93 percent of the plants reached their 



