10* GROWTH AND, WORK OF PLANTS 



tubes. In this way the leaves lose less water than if they re- 

 mained flat. It will be remembered that freezing of plant 

 tissue has a drying effect. 



162. 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 tem- 

 perature is suitable for growth, in a few hours, if conditions are 

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

 of the leaves. These drops of water have exuded 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 phenom- 

 enon is often to be observed during the summer season in the 

 case of low-growing plants. During the bright warm day trans- 

 piration 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. 



163. 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, 



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

 result of condensation of moisture on the leaves. 



