Waterivavs in Plants 



6i 



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 stomata (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 stomata close so that less water 

 escapes. 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.' 



The Water Path from Root to Leaves. — Water does 

 not pass up to the leaves through all parts of the stem. If a 

 Begonia stem is placed in water coloured with red ink, in a few 

 hours the ink will mark the path it has taken. Cut the stem 



1 An area on the under surface which could be covered by the capital 

 O on this page contains o\'er 3000 stomata. 



Fig. 68. — I. HoriEontal section 

 through the epidermis oi the 

 under side of the leaf of Eu- 

 oiiymus japonicus looked at 

 from below ; sp, stomata. 11. 

 Course of development of the 

 stoma of Arth} opcdium cir- 

 fhatum: spm, mother - cell 

 ready for division ; sp\ sp'\ 

 sp'" , successive stages of divi- 

 sion. III. Mature stoma. 

 (From Edmonds and Mar- 

 loth's " Elementary Botany 

 for South Africa".) 



