WORK OF THE LEAVES: TRANSPIRATION 95 



158. How drops of water exude from leaves. If small, 

 actively growing plants, such as the pea, corn, wheat, bean, etc., are 

 put under a bell jar and placed in the sunlight where the tempera- 

 ture is suitable for growth, in a few hours, if conditions are favor- 

 able, there will be drops of water standing out on the margins of 

 the leaves. These drops of water have eluded through the ordinary 

 stomates, or in other cases through what are called water stomates, 

 by the influence of root pressure. The plant being covered by 

 the bell jar, the air soon becomes saturated with moisture and 

 transpiration is checked. Root pressure still goes on, however, 

 and the result is shown in the exuding drops. Root pressure is 

 here in excess of transpiration.* This phenomenon is often to be 

 observed during the summer season in the case of low-growing 

 plants. During the bright- warm day transpiration equals, or may 

 be in excess of, root pressure, and the leaves are consequently 

 limp or flaccid. As nightfall comes on the air becomes more moist, 

 and the conditions of light are such also that transpiration is 

 lessened. Root pressure, however, is still active because the soil 

 is still warm. In these cases, drops of water may be seen exuding 

 from the margins of the leaves, due to the excess of root pressure 

 over transpiration. Were it not for this provision for the escape 

 of the excess of water raised by root pressure, serious injury by 

 lesions, as a result of the great pressure, might happen. The plant 

 is thus to some extent a self -regulatory piece of apparatus so far as 

 root pressure and transpiration are concerned. 



159. Number of stomates. It has been estimated by in- 

 vestigation that in general there are 40-300 stomates to the 

 square millimeter of surface. In some plants this number 

 is exceeded, as in the olive, where there are 625. In an entire 

 leaf of Brassica rapa there are about 11,000,000 stomates, 

 a-nd in an entire leaf of the sunflower there are about 13,000,000 

 stomates. 



160. Amount of water transpired by plants. The amount 

 of water transpired by plants is very great. According to careful 



* These drops should be distinguished from those formed merely as a 

 result of condensation of moisture on the leaves. 



