WATER UTILIZATION BY TREES 59 



Trees show specific differences in their ability to close their stomata 

 under unfavorable conditions. In some species the stomata are much 

 more adjustable than in others. Thus, in the aspen, which generally 

 occurs on wet soil but is able to live on comparatively dry soil, the 

 stomata close completely when water shortage occurs. The willows, 

 on the other hand, are generally forced to live in more moist habitats 

 because they lack the power to close their stomata rapidly. Similarly 

 the stomata of trees of tropical rain forests show little mobility, accord- 

 ing to observations of Shreve in Jamaica (193). 



Betula alba and Alnus glutinosa, according to Biisgen and Munch 

 (33, p. 217), also are unable to regulate their transpiration appreciably 

 through stomatal movements, and it is noteworthy that these plants 

 are confined to fairly moist habitats, although Burgerstein (27) states 

 that Alnus and Betula close their stomata very quickly when the water 

 supply begins to fail. 



The water supply and the humidity are, along with the light, by 

 far the most important factors governing stomatal behavior. Neger 

 and Lakon (161 ) found in 1-year-old needles of Abies, Picea, and Tsuga 

 that the stomata were open when the leaves were in a fresh condition 

 and closed when wilted. Similar results were obtained by Dengler. 

 In fact, most of the workers who have studied this question have 

 come to the conclusion that excessive losses of water are associated 

 with closure of the stomata. 



While most of the work on this subject has been done with herba- 

 ceous plants, many papers have appeared dealing with trees, especially 

 horticultural species. Hendrickson (93), using the fixation method, 

 found the maximum stomatal opening in Prunus at 9 a. m. until 12 

 noon and the minimum at 8 to 11 p. m. In the shade the maximum 

 opening occurred later in the day. The stomatal behavior was cor- 

 related with the temperature and humidity and also with the soil 

 moisture. The stomata, however, did not vary in the width of their 

 openings with the soil moisture until the soil-water content approached 

 the wilting coefficient. In some species, due to insufficient conduc- 

 tivity of the tissues, the leaves began to decrease in water content 

 early in the morning; this is the probable cause of the closure of the 

 stomata in the early afternoon, which permits partial replenishment 

 of the water lost. Similar results were reported by Veihmeyer and 

 Hendrickson (220). 



Magness and Furr (1J+5), in their work on apple leaves, reported 

 very similar results. On dry, warm days the stomata close in the 

 middle of the day regardless of the soil-moisture supply. They like- 

 wise report that the stomata were not affected by soil moisture until 

 the wilting coefficient was reached, and consider the condition of the 

 stomata to be an excellent way of determining whether the tree is 

 getting sufficient moisture. Sitton (196) observed similar behavior 

 in pecans. 



Burgerstein (27), who studied the relation between wilting and 

 the closing of the stomata on 140 species of trees, found that wilting 

 resulted in complete closure in 56 percent, partial closure in 34 percent, 

 and no marked change in 10 percent of the species. He found also 

 that wilting affected the closure of stomata in a higher percentage of 

 trees than of herbs. 



Since light and moisture are the important factors determining 

 stomatal behavior, it is natural to expect a seasonal effect: the stomata 



