76 EFFECT OF PHYSICAL FACTORS AND PLANT CONDITIONS. 



14. Water-logging the soil causes closure of the stomata and sub- 

 sequent wilting by inhibiting the functioning of the roots. 



15. The maximum of leaf turgor in most cases is reached about 

 midnight. After stomatal opening at daybreak, or shortly after- 

 ward, it begins to decrease, the rate being dependent upon transpi- 

 ration on the one hand and water-supply on the other. Part of 

 the water present in the leaf at the start is a working margin and 

 the stomata do not close until this is gone. If the roots can keep 

 up a sufficient rate of supply this working margin does not wholly 

 disappear during the day and the stomatal movement is of the nor- 

 mal light-induced type. If the margin is lost, however, the stomata 

 close until it is recovered, at least in part. 



16. The nightly maximum of leaf turgor increases after every 

 rain or irrigation and thereafter decreases until the next rain. Such 

 fluctuation between rains is often very great. In lesser degree, the 

 critical leaf-water or percentage at which the working margin disap- 

 pears also fluctuates between rains or irrigations. 



17. The degree of succulence of a leaf determines to a large extent 

 the amount of water present as a working margin, and therefore 

 the readiness with which the stomata close when transpiration is high. 



18. The growth habit of a plant often affects stomatal movement 

 by reserves of water to increase the rate of supply during periods when 

 loss is greatest and by roots which penetrate to permanently moist soil. 



19. The age or degree of maturity of the plant affects its stomatal 

 movement, and the age of each leaf influences the readiness with 

 which the stomata on it function. 



20. The number and size of the stomata on any given area of 

 leaf are influenced by the conditions under which they were formed. 

 A leaf developed in the shade has fewer and larger stomata per 

 unit area than one produced in sunlight. 



21. The removal of the hairs from the leaf of a plant with very 

 hairy leaves caused the stomata to open somewhat earlier upon the 

 appearance of light and close very much sooner, showing that the 

 water-loss from such a leaf was probably much higher than from 

 the normal leaves. 



22. Plants of desert regions have adapted themselves to their 

 surroundings in two ways, so far as stomatal movement is con- 

 cerned. Some produce a crop of leaves when opportunity per- 

 mits, which wither and disappear when the water-supply becomes 

 critical. Such leaves are mesophytic in type as a rule, and show 

 no specialization for desert conditions. Other plants have per- 

 sistent leaves, or stems adapted for photosynthetic work, which show 

 considerable adaptation to desert conditions, hairs, wax, sunken 

 stomata, and other means being used to reduce water-loss. Such 

 plants often normally show a reversal of the usual behavior of the 

 stomata, opening occurring at night and closure during the day, 

 as in many mesophytes when exposed to similar arid conditions. 



