74 EFFECT OF PHYSICAL FACTORS AND PLANT CONDITIONS. 



appeareance at noon of the shaved leaves is evidence that hairs, 

 especially when as dense as on the leaves of this plant, are of con- 

 siderable value in reducing water-loss. 



Plants adjust and adapt themselves to their environment in 

 a number of ways, one of which is stomatal behavior. The hy- 

 drophytes studied had permanently open stomata, since they usually 

 have no need of the mobile kind. In fact, night closure would prob- 

 ably be disadvantageous to a plant such as Scirpus validus, since dif- 

 fusion of gases through the air-passages to the submerged parts must 

 necessarily be slow and is therefore continued throughout the night. 



Plants which grow normally in a very humid region, where the 

 weather rarely becomes hot and dry and the water-content low, do 

 not show the rapid response to evaporation that a plant developed 

 in a more arid habitat does. Thus, alfalfa shows a greater response 

 to low humidity than potato or sugar-beet, and a greater response 

 than even the white-flowered sweet clover (Melilotus alba), which 

 has very similar leaves and stomata. The plants of the arid regions 

 have generally adapted themselves in one of two ways, some 

 functioning at all times, and others only during the rainy season. 

 Opuntia versicolor belongs to the former class (E. B. Shreve, 1916), 

 its stomata functioning normally in the manner that alfalfa stomata 

 do under extreme conditions. Fouquiera splendens, on the other 

 hand, produces a crop of leaves when opportunity permits, which 

 function until the water-supply runs low, in the same manner as 

 a mesophyte under optimum conditions. When the water-content 

 runs low the stomata begin to show less and less opening, until 

 toward the last, when the leaves begin to change color, they open 

 very little, and for only a short time each day. This behavior 

 duplicates that of an aging leaf of alfalfa. Encelia farinosa, which 

 tends to retain some leaves at all times, shows behavior much like 

 that of Fouquiera when conditions are favorable, and somewhat 

 the same behavior as Opuntia versicolor when these are unfavorable. 

 Hence, it is intermediate between the two, especially since it drops 

 most of its leaves upon approach of unfavorable conditions, and 

 those that remain probably show opening only at night, if at all. 



SUMMARY. 



1. Light induces opening of the stomata after daybreak by 

 initiating conversion of the starch within the guard-cells into sugar. 

 This increases the osmotic pressure of the guard-cells, which in 

 turn causes the increase of turgor necessary to produce opening. 

 The starch-content of the guard-cells never wholly disappears, but 

 usually is at its lowest about 10 a. m. When no other factor influ- 

 ences the movement, the starch-content rises from this time until 

 just before daylight the next morning. During the middle part of 

 the day and until shortly before the stomata start to close, the rise in 

 the starch-content is very slow, but is rapid during closure. After 



