290 CARNEGIE INSTITUTION OF WASHINGTON. 



Finally, the series of prints for several of the most important species, 

 such as alfalfa, potato, and barley, were mounted in a circle to permit 

 ready measurements and comparison of the openings on both sur- 

 faces for the 24 hours. 



Similar studies were made at the Desert Laboratory on several 

 species of xerophytes and winter annuals. In addition, particular 

 attention was paid to the relation of the stomatal opening to trans- 

 piration and to wilting. The annuals were transplanted into uniform 

 soils in pots and were maintained at different amounts of water-con- 

 tent from the minimum to saturation; 24-hour series of strips and 

 transpiration weighings were made of these as well as of plants as they 

 approached and passed the wilting-point. The results of the investi- 

 gation seem to harmonize the diverging views as to the regulatory 

 action of stomata, since they not only show that species exhibit ex- 

 treme differences in the daily activity of their stomata, but that the 

 upper and lower epiderms of the same species may also vary greatly. 

 Probably the most important result is that all the species studied with 

 reference to wilting indicated that the stomata had a decisive regu- 

 latory action at this time, even in the case of those most passive under 

 less extreme variations in the water relations. 



Transpiration of Trees, by J. E. Weaver and Anne Mogenson. 



This study has dealt with the transpiration of coniferous and de- 

 ciduous seedlings out-of-doors during the winter and summer. The 

 experiments were carried on at the University of Nebraska from Sep- 

 tember to May and at the Alpine Laboratory during June and July. 

 The species employed were Pinus ponderosa, P. hanksiana, Pseudotsuga 

 mucronata, Picea engelmannii, Abies grandis, Acer saccharinum, A. 

 glabrum, Ulmus americana, and Quercus macrocarpa. The material 

 consisted of 2- and 4-year seedling conifers and 1-year deciduous seed- 

 lings, potted in the same soil in sealed containers. Each battery 

 comprised approximately 10 individuals of each species. The containers 

 were placed on the ground and held in place by a frame 12 inches high, 

 the intervals being filled in with soil. A record of the humidity and of 

 both air and soil temperatures was obtained by means of hydrothermo- 

 graphs. The transpiration was measured by weighing from time to 

 time on a balance sensitive to 0.5 gram under a load of 8 to 9 kg. 

 The winter transpiration of the conifers proved to be exceptionally 

 small, ranging from less than 1/100 to less than 1/200 of that for the 

 autumn per unit area. During the autumn, the daily loss from the 

 conifers was nearly twice as great as that from the deciduous species, 

 with the exception of Quercus macrocarpa. The daily loss from the 

 deciduous trees also bore an interesting inverse relation to their water 

 requirements, Quercus macrocarpa transpiring nearly twice as much 

 as Acer saccharinum. The summer transpiration of A. saccharinum 



