INTRODUCTION TO BOTANY. xlvii 



The leaves and the roots are the principal organs of absorp- 

 tion and nutrition in vegetables : they absorb from the atmos- 

 phere nutritive substances which are made subservient to their 

 growth. They also serve for the purpose of expiring and of 

 exhaling fluids, which are become useless to the vegetable, and 

 it is by their means, that the sap is divested of its watery con- 

 stituent, and acquires all its nutritious qualities. 



The leaves of herbaceous plants, which are immersed in an 

 atmosphere constantly moist, absorb equally by their upper 

 and under surfaces. Place the leaves of trees upon water, on 

 their under surfaces, they will continue green for several 

 months ; place them on their upper surfaces, and in a few 

 days, they will completely wither. 



When leaves are exposed to the action of the sun, they de- 

 compose carbonic acid gas, (fixed air,) retain the carbon, and 

 set free the oxygen, (pure part of the air.) The contrary 

 takes place when they are withdrawn from the action of the 

 light ; they then take a portion of oxygen from the air, and 

 replace it with an equal quantity of carbonic acid. 



Leaves are susceptible of certain motions, which depend on 

 the irritability with which they are endued. 



Fall of the Leaf. 



Trees whose leaves are earliest expanded, are generally the 

 first to lose them. In the elder, (sambucus,) however, the 

 leaves appear very early, and are late in falling : in the com- 

 mon ash, the leaves appear very late, and fall towards the end 

 of summer. 



The fall of the leaf may be ascribed to the suspension of 

 vegetation, to the want of nourishment, which leaves expe- 

 rience at that period of the year. The vessels of the leaf 

 become contracted and dry, and the leaves fall off. 



Medical leaves are chiefly those possesed of emollient, tonic, 

 stimulant, narcotic, and purgative qualities. 



Stipulas 



Never exist in monocotyledonous vegetables. They furnish ex- 

 cellent characters for the construction of natural orders. The 



