24 AMOUNT OF TRANSPIRATION 



and recombines them into foods which represent a certain amount 

 of stored-up energy. Respiration breaks down the products 

 formed from these foods and liberates the energy necessary for 

 the growth and activities of 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. It is estimated 

 that from 200 to 500 lbs. of water is transpired in the construction 

 of one pound of dry substance and that a square meter of leaf 

 surface evaporates about 50 gms. per hour. 



The question naturally arises why are such large volumes of 

 water transpired? This depends in part upon the size of the leaf 

 and external conditions, the size of the leaf being, determined by 

 the relation of transpiration to photosynthesis. Conditions mak- 



