122 FUNCTIONS OF LEAVES. 



by a process of absorption, and. an outward current of the components 

 of the plant-juices by a process of exhalation. In the cells of the 

 leaves changes take place under the agency of light, by which oxygen 

 is given oflf and carbon fixed. These will be considered under the 

 head of vegetable respiration. The absorption of carbonic acid and 

 of fluids is carried on by the leaves, chiefly through their stomata, 

 and most rapidly by the under surface of ordinary leaves in which 

 the cuticle is thinnest, the cellular tissue least condensed, and stomata 

 most abundant ; the upper surface of the .leaf, which usually pre- 

 sents a polished and dense epidermis, with few stomata, taking little 

 part in such a process. Hoffman has ascertained that leaves absorb 

 fluids in large quantities ; that during a fall of rain the vegetable 

 fluids undergo from such a cause a process of dilution, leading to an 

 immediate and more rapid descent of sap, which under such circum- 

 stances is capable of general diffusion throughout the several vege- 

 table tissues. Some physiologists have expressed doubts as to absorp- 

 tion being carried on by the leaves in ordinary circumstances. Leaves 

 also absorb gaseous matters. Saussure states that oxygen is absorbed 

 by the leaves during night, the quantity varying according to the 

 nature of the plant. . Boussingault found that the leaves of the Vine 

 absorbed carbonic acid from the air. Other experiments prove that 

 ammonia and nitrogen are similarly acted on. 



Leaves also give off" gases and liquids by a process of exhalation 

 or transpiration. A moderate amount of carbonic acid is exhaled 

 during darkness, and a large quantity of liquid is given off by tran- 

 spiration. The number and size of the stomata regulate the transpi- 

 ration of fluids, and it is modified by the nature of the epidermis. 

 The absorbing power of leaves depending on simUar causes, is capable 

 of being increased by any process which removes either natural or 

 imposed obstructions to the free action of their surface. It is thus 

 that rain, while supplying the material for absorption, at the same 

 time renders the leaf more capable of such action. In plants with a 

 thick and hard epidermal covering, exhalation is less vigorous than in 

 those where it is thin and soft. Some succulent plants of warm 

 climates have a very thick covering. The peculiar character of the 

 phyllodia of Australian plants is probably connected with the dry 

 nature of the climate. The process of transpiration is more under the 

 influence of light than of heat. It assists the process of endosmose, 

 by rendering the fluid in the cells thicker, and thus promotes the 

 circulation of sap. 



The quantity of fluid exhaled varies in amount in different plants. 

 A Sunflower three feet high gave off twenty ounces of watery fluid 

 daily. Hales found that a Cabbage, with a surface of 2736 square 

 inches, transpired on an average nineteen ounces per day ; a Vine, 

 of 1820 square inches, from five to six ounces. Deheran found that 



