344 CARNEGIE INSTITUTION OF WASHINGTON. 



of this was found, and the results completely correlated with those 

 obtained at Salt Lake City. In addition, some interesting correlations 

 were obtained between the behavior of the stomata of desert plants and 

 those of mesophytic species grown under similar conditions of aridity. 

 In addition to extended results dealing with the relation of stomatal 

 opening to physical factors, the following conclusions have been 

 reached : 



1. The stomatal movement differs in plants of different ages, and 

 often also in different leaves of the same plant. Moreover, the move- 

 ments of the stomata of the upper and lower epiderm of the same leaf 

 are markedly different in many plants. 



2. Stomatal movement in most plants differs from day to day in more 

 or less close response to physical factors. Stomata are to a consider- 

 able degree individual in their response to changing factors, and they 

 act together as a group chiefly because their factor-control is identical. 



3. Stomatal movement is but one of several factors affecting tran- 

 spiration. A plant often shows considerable changes in transpiration 

 because of the factors controlling evaporation, although the stomatal 

 apertures remain the same. Nevertheless, plants of the same species, 

 which have different curves for stomatal opening, also show similar 

 differences in their transpiration curves, 



4. Stomatal closure produces a considerable fall in the rate of tran- 

 spiration, except when evaporation factors counterbalance the effect 

 of closure. 



5. Stomata have a definite regulatory action at the time of wilting, 

 and in many plants perhaps only at this time. When the conditions 

 under which a normal mesophyte is growing become more and more 

 arid, the stomata behave more and more like those of such a xerophyte 

 as Opuntia versicolor, in which the stomata are closed during the day and 

 open at night. 



6. The transpiration of a cut stem can not be compared with that of 

 a potted plant or of one growing naturally, nor can the transpu'ation of 

 a potted plant be compared with that of a field plant, since the stomatal 

 movement of a cut stem or potted plant is only accidentally the same 

 as in a field plant. 



Photosynthetic Efficiency, hy F. E. Clements and Frances Long. 



The results so far obtained in measuring photosynthetic activity 

 show definite correspondence with the light intensity. This is marked 

 in the crowns of such shrubs as Acer glabrum, Prunus demissa, and 

 Ruhus deliciosus. Shade leaves from the center of the crown with a 

 light intensity of 0.04 showed 54 per cent of the photosynthate of leaves 

 in full sun in the case of Acer and 55 per cent in the case of Prunus. 

 Shade leaves of Ruhus in a light intensity of 0.006 showed 27 per cent 

 of the amount found in the sun leaves. Several species of herbs showed 



