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



are more nearly closed in winter than in summer, and those of nearly 

 all evergreen shrubs and trees close early in the autumn when the 

 available water supply commences to be diminished, according to 

 Biisgen and Munch (33, p. 218). If cut twigs of Ilex or Taxus, how- 

 ever, are placed in water in a warm room, the stomata reopen in Ilex 

 within a few hours and in Taxus within a week. 



Weber (231) studied the condition of the stomata on two trees of 

 Aesculus hippo casianum in Austria over an entire growing period, 

 using the infiltration method. His results do not disagree with those 

 of other workers, but it would seem impossible to draw any very 

 valuable conclusions from two trees even though many observations 

 were made on each tree. 



This entire relation of the stomata to water and light has been very 

 well summed up in the work of Stalfelt (205), who carried out many 

 observations on Picea excelsa in the field, measuring the effect of light 

 and water on the stomata at different hours of the day and under 

 different external conditions. He concluded that the behavior of the 

 stomata is conditioned by three separate systems, which he calls the 

 "passive", the "photo-active", and the "hydro-active", all three being 

 related to the water content of the plant. When there is plenty of 

 water the guard cells of the stomata open "passively", due to their 

 internal osmotic properties. The "photo-active" system results from 

 the effect of light either on the osmotic substances within the guard 

 cells or from a more direct stimulating effect. When water is at an 

 optimum, the stomata are very sensitive to light, and under these 

 conditions there is a delicate balance between the passive and the 

 photo-active effects. When the water content becomes suboptimal 

 the stomatal movement is dominated largely by the "hydro-active" 

 system and the stomatal cells then play an active part. As the water 

 content continues to fall below the optimum there comes a time when 

 the photo-active and the hydro-active systems are balanced as to their 

 importance in the regulation of the stomatal opening. In general, 

 this means that when there is plenty of water, light is the dominant 

 controlling factor, but when water is lacking, light becomes secondary. 

 Most of the relations between light and water discussed above can 

 be fitted into this picture. We thus see why it takes some little time 

 for the stomata to close even after wilting has commenced; light and 

 water are acting in opposing directions. Likewise, the temporary 

 widening of the stomatal aperture, which frequently precedes the 

 decrease of transpiration during wilting, may possibly be explained 

 by opposing hydro-active and passive effects. The opening of the 

 stomata at the close of wilting, commonly found in dried-up leaves, 

 is also probably a passive posthumous effect. 



Before closing this subject of adaptations to drought, in view of 

 the economic importance in silviculture of our coniferous species it 

 may be well to cite some of the special adaptations to the diminished 

 water supply exhibited by these forms. Attention has been called 

 already to the waxy coatings commonly present, as well as to the 

 thickenings of the epidermal cells, which in some cases reduce the 

 lumina to almost nothing. These epidermal cells are generally stiff 

 and hard, and towards the interior of the leaf they are frequently 

 adjoined by similar thick-walled hypodermal cells, which also protect 

 the interior of the leaf from water losses. This may explain the vari- 

 ability of their occurrence with the moisture of the habitat, as pointed 



