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



Shreve (192) studied the effect of soil moisture or the transpiration 

 and the size of the stomatal openings in potted plants of Parkinsonia 

 microphylla Torr. The water content of the soil was decreased from 

 15.1 percent to 3.5 percent, or 76.8 percent of the original amount, 

 which led to a lowering of the relative transpiration (in the sense of 

 Livingston) of 98.4 percent. In another plant, lowering of the soil 

 moisture from 14.5 percent to 3.4 percent, i. e., 76.5 percent of the 

 original water content, lowered the relative transpiration 97.2 percent. 

 In this latter case the average diameter of the stomatal openings in 

 the drier soil was only one-seventh of that in the moister soil. 



Bates (11) studied the transpiration of two sets of potted trees at 

 a water content of 9.05 percent and then raised the water content of 

 one set to 12 percent, which brought about a remarkable increase in 

 the transpiration, as follows: 



Percent 



Ponderosa pine 34. 4 



Douglas fir 30. 5 



Lodgepole pine 33. 9 



Engelmann spruce 8. 8 



Bristlecone pine 14. 5 



This increase in transpiration is especially noteworthy since the 

 wilting coefficient of 4.88 percent was never closely approached. 

 This study of Bates is frequently referred to in the literature, because 

 he included in his paper a very comprehensive group of silvicultural 

 factors in order to correlate the natural distribution of the common 

 conifers in Colorado with the climatic factors and to show that the 

 natural distribution in that region was distinctly based upon specific 

 adaptations, especially to the water relations. He correlated the 

 transpiration with the water requirement, sap density, wind, light, 

 temperature, etc., in an attempt to show that the water relations of 

 the various species in regard to these factors were specific, permitting 

 the species to grow in the particular locations where found. The 

 chief criticisms of his work are that it is too comprehensive in that it 

 tries to consider too many changing factors at the same time, and that 

 too few examples were used in each experiment. In many cases only 

 two plants were used; and it is very unfortunate that work which has 

 been cited so frequently should be based upon so few data. It should 

 be added, however, that most of his conclusions have been supported 

 by other workers who devoted their attention to special phases of the 

 problem and examined each of these more exhaustively. 



Pearson (167) carried on a somewhat similar experiment with 

 potted conifers of Arizona and New Mexico. The wilting coefficient 

 of the soil was 10.5 percent; and after the seedlings were planted 

 enough water was added to bring the soil moisture to 20 percent. 

 After this soil-moisture content had been maintained for several 

 months, watering was discontinued in one series of pots; the tran- 

 spiration rate began to fall immediately, and at the end of a month in 

 most cases, it was less than 3 percent of normal. The soil moisture, 

 during this time, had fallen to 0.2 to 0.5 percent above the wilting 

 coefficient. During the next 2 months the transpiration was almost 

 imperceptible, although the plants did not seem to be wilting. When 

 changed from the outdoors (32° F.) to the greenhouse (54°) no appre- 

 ciable increase in transpiration occurred. To learn whether the 

 plants were really living, two of the pots containing yellow pine were 

 watered at the end of 3 months, which restored the moisture content 



