NATURE OF PLANTS 23 



and the leaves contract or wilt. When plants are covered by a 

 bell jar or placed in a tight glass jar the water given off soon 

 saturates the air and collects in drops on the sides of the jar. 

 The amount of water transpired by a plant is surprisingly large 

 and it is probably safe to state that usually it amounts daily 

 during the hot summer months to more than the plant's weight. 

 An oak with seven hundred thousand leaves was estimated by 

 Ward to transpire from June to October 244,695 pounds of water. 

 A birch with 200,000 leaves transpired 700 to 900 gallons on hot 

 summer days. From careful measurements of the amount of 

 water given off by grass plants it has been calculated that six 

 and one-half tons per acre may be transpired daily during the 

 summer. The question naturally arises, why is the transpiration 

 of such large volumes necessary? We have noticed that the 

 veins carry water to the green cells of the leaf. If water were 

 the only food that the chlorenchyma required, the amount of 

 transpiration would doubtless be greatly lessened. This water, 

 however, contains other foods that are very necessary for the plant 

 and which exist in very dilute solutions, probably not more than 

 one part in ten thousand. In order therefore to secure the neces- 

 sary foods from the soil the plant is obliged to absorb large 

 volumes of water. Only a small portion of the water is required 

 and the surplus is forced out of the cells and finds its way in the 

 form of vapor along the intercellular spaces in the spongy meso- 

 phyll through the stomata to the air. The transpiration of these 

 large volumes of water must be of importance also in keeping 

 down the temperature of the plant during the burning summer 

 heat. Transpiration is often compared to the evaporation of 

 water from a dish. While it is controlled to a limited extent in 

 the same manner as evaporation, it should be borne in mind that 

 the giving off of water is intimately associated with the vital 

 activities of the cells and that the loss of water is to a degree under 

 the control of the plant. The cuticle, which is practically im- 

 pervious to water and gases, extends as a thin coat over all parts 

 of the plant body, and materially assists in controlling the amount 

 of transpiration. As a result of this water-proof coat the vapor 

 can only escape from the leaf through the stomata. These minute 



