1857.] The Leaf— Its Functions, etc. 197 



formation of woody fibers, and all the solid naatters of similar com- 

 positions. 



The organs of assimilation, of the mature foliage, receive accord- 

 ing to LlEBiG and others, more nourishment from the atmosphere 

 than they employ in their own substance ; and when the formation 

 of the wood substance has advanced to a certain extent, the expend- 

 iture of the nutriment, the supply of which still remains the same, 

 takes a new direction, and blossoms are produced. The functions of 

 the leaves of most plants cease upon the ripening of their fruit, be- 

 cause the products of their action are no longer needed. They now 

 yield to the chemical influence of the oxygen of the air, generally 

 suffer a change in color, and fall off. A peculiar transformation of 

 the matter contained in all plants takes place in the period between 

 the blossoming and ripening of the fruit; new compounds are pro- 

 duced, which furnish constituents to the blossom, fruit and seed. — 

 The precise nature of these transformations, and their modus oper- 

 andi, furnish a wide field of inquiry, as well as of speculation. Mod- 

 ern chemistry has afforded us many proofs of the functions of the 

 leaves of plants. The experiments of Priestly, Ellis, Decandole, 

 DuHAMEL, Marriotte, and others, have established both the ab- 

 sorption and exhalation of gases through the leaves. From some 

 recent experiments also, it appears, that vegetables, by decomposition 

 of moisture, can supply an atmosphere to themselves; and thus, that 

 plants will grow and thrive when inclosed in glass eases, perfectly 

 impervious to external changes of atmosphere, simply by decompos- 

 ing the water of the moist soil, with which they are furnished. 



This decomposition of the carbonic acid absorbed from the air is 

 effected in the parenchyma of the leaves, as well as in all the other 

 green and herbacious parts of the vegetable. 



When vegetables are exposed to the action of the sun, they absorb 

 carbonic acid, retaining the carbon and disengaging the oxygen. — 

 The reverse takes place when they are withdrawn from the influence 

 of light, in which case they extract from the air a portion of its ox- 

 ygen, which they replace by disengaging an equal quantity of car- 

 bonic acid gas. Thus, vegetables become blanched when removed 

 from the influence of the sun, lose their green color, become soft 

 and watery, and contain a larger proportion of sacharine matter. — 

 Hence this parenchyma is evidently the seat of absorption and res- 

 piration ; for it is the part that changes color in exercising these 

 functions. 



