NATURE OF PLANTS 23 



light are produced. In our fires we are dealing with the heat and 

 sunlight that have been slowly stored up by the plant. 



13. The Third Function of the Leaf, Transpiration. — This 

 function refers to the giving off of water by the plant. While 

 other parts of the plant assist in transpiration, the leaf is the 

 principal organ upon which this very considerable work devolves. 

 Water is given off from the plant as a vapor and for this reason 

 transpiration is a more familiar phenomenon than photosynthesis 

 and respiration, where we are dealing with an interchange of 

 invisible gases. We see the vapor from plants growing in a 

 window precipitated on the cool window panes in the form of 

 drops. On hot summer days the leaves of plants droop. This 

 is because they have transpired so much water that their cells 

 are no longer distended by the water, consequently the cells shrink 

 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. This means that an acre of such trees would 

 give off in the course of the season 3,168,000 pounds of water. 

 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 doubt- 

 less be greatly lessened. This water, however, contains other 

 materials that are 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 necessary foods from 

 the soil the plant is obliged to absorb large volumes of water. 



