Watej'ways hi Plants 



59 



cobalt chloride paper. The leaf should be dried from all surface 

 water ; then place between two plates of glass. On all place a 

 light weight. In half an hour, or less, examine. The paper 

 next the under side of the leaf will be red ; that on the upper 

 surface will be only slightly changed. 



The greater amount of water passes off from the 

 lower surface of the leaf. 

 This is true of many leaves^ but 

 not of all. Is the result the 

 same for the lower broad leaves 

 and the upper narrow leaves of 

 the blue gum ? Where does the 

 water escape from the water-lily 

 leaf? From silver leaves? From 

 " April Fool " leaves ? 



Why do plants usually 

 lose more water from the 

 under side ? How the leaf 

 controls the escape of vapour can 

 be seen with a microscope. A 

 picture will help to make it clear 

 for the present. Scattered over 

 the under surface of the apricot 

 leaf are very small holes called 

 stomates (singular stoma) or sto- Fig. 

 mata (mouths). Two crescent- 

 shaped cells surround the opening. 

 These lip cells open and close. 

 During the day they are open, 

 and evaporation keeps the leaves 

 cool. If leaves were as hot as 

 the stones around them they would die. In very dry weather 

 the cells lose some of their water and close up so that less can 

 escape. These stomata open into spaces within the leaf into 

 which water passes from surrounding cells. On a summer 

 day leaves lift up and lose tons of water. Leaves borne on 

 the surface of water or close to the soil have their stomata 

 on the upper surface. 



— I. Horizontal section through 

 e epidermis of the under side of 

 the leaf of Eiionyiiius japonicus 

 looked at from below : sp, stomata. 

 II. Course of development of the 

 stoma of Arthropodimn cirrhatum : 

 spin, mother-cell ready for division ; 

 sp', sp", sp'", successive stages of 

 division. III. Mature stoma. (From 

 Edmonds and Marloth's " Elementary 

 Botany for South Africa.") 



