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



enter, the stomata are completely closed. Other workers use other 

 injection materials but the same principles apply. 



FACTORS INFLUENCING STOMATAL MOVEMENTS 



Factors other than soil moisture and humidity influence the move- 

 ments of the stomata, but since the movements affect transpiration, 

 no matter what may be their origin, it will not be out of place to 

 mention some of these factors. The stomata are completely mov- 

 able only in mature leaves. When young, the stomata tend to close 

 or open only under especially favorable conditions. The leaves 

 nearest to the base are generally the first to have movable stomata. 

 As the leaves become advanced in age the stomata lose their mobility, 

 and in evergreen leaves (or needles) that remain on the trees for 

 several years the mobility may completely cease, as shown by the 

 studies of Dengler (45) and others. In older leaves the walls of the 

 guard cells become somewhat lignified, and it is this lignincation 

 doubtless which checks their mobility. This hardening of the guard 

 cells has been reported in conifers by Noack (162) and others as well 

 as in Laurus nobilis L. and Quercus ilex by Von Guttenberg (77). 



Likewise, the stomata in autumnal leaves, especially after the 

 photosynthetic function has ceased, have been shown by Stahl (203) 

 to be closed. Even some of the willows, which ordinarily do not 

 close their stomata when wilting, showed a decrease in the size of 

 the openings in late summer. Similar results are reported by Molisch 

 (156) and Burgerstein (27), who found in plants with green leaves 

 on one side and yellow leaves on the other that in the former the 

 stomata were open, while in the latter they were closed, exceptions 

 being Populus alba and some Salix species, where the yellow leaves 

 also showed open stomata. 



About half an hour after twigs are cut and placed in water the 

 stomata tend to close, as pointed out by Huber (104). This effect 

 of cutting on the closing of the stomata is, of course, a source of 

 error in work based upon cut twigs and should be taken into account 

 in any absolute measurements attempted by this method. 



As is to be expected when the real function of stomata is considered, 

 light influences their behavior. The stomatal opening should be wide 

 enough to admit sufficient C0 2 and yet not so wide as to permit 

 excessive water losses. These two requirements are often contradic- 

 tory and result in a compromise. Since the supply of C0 2 is of use 

 to the plant only when in the light, the width of the stomata must 

 depend in the first place on illumination. Light, therefore, causes 

 the stomatal opening to be wider and darkness causes it to be narrower. 

 Thus Sitton (196) found that the stomata of pecan leaves opened 

 very soon after exposure to direct sunlight and closed soon after being 

 shaded. Also the stomata on completely shaded leaves in the interior 

 of the trees were not open. 



The stomata are generally most widely open between 11 a. m. and 

 3 p.m., while at night or on dull days they may be completely closed. 

 On the other hand, if the darkness is unduly prolonged, the stomata 

 may reopen to permit the entrance of oxygeD. This had been shown 

 by Dengler (45) to be the case in conifers, by Weber (231) in Aesculus, 

 and by Magness and Furr (145) in apple trees, to cite only a few of 

 the papers on this subject. 



