FUNCTIONS OF LEAVES. 101 



Color seems to exercise an important influence in the direc- 

 tion of organs. If they are of deep color, they ascend ; if color- 

 less, or of a pale color, they take a descending- direction. Roots, 

 if they become green, will then ascend, or turn toward the 

 light, if placed in circumstances to have the light come to them 

 in only one direction. 



Section 2. — Functions of Leaves. 



176. From the structure of leaves, we should be led to sup- 

 pose that they perform an important part in vegetation. They 

 have been compared to the lungs of animals, but they perform 

 much more for the plant than this comparison would indicate. 

 They are not only the organs of respiration, but also of diges- 

 tion and nutrition. They perform in every respect for veget- 

 ables what is performed by the lungs and stomach and the 

 whole digestive apparatus in animals. They receive the crude 

 sap from the roots through the stem, and elaborate it by expos- 

 ing it to the action of the atmosphere, throwing off the super- 

 fluous moisture, decomposing water and carbonic acid. They 

 send immediately downward the materials of the alburnum and 

 liber, and nourish with this elaborated food the contiguous 

 parts. 



177. That the nutrition of a plant depends upon its leaves is 

 abundantly proved by depriving a plant of these organs through 

 a season, and it -withers and dies. It does not die immediately, 

 since it possesses the power of putting forth new leaves, which 

 soon come into action, and supply imperfectly the places of 

 those removed; but if it is deprived of its leaves through the 

 season, its power of putting them forth becomes exhausted, and 

 all functions cease. 



The presence of cotyledons also shows the necessity of leaves 

 to prepare food for the embryo. If the cotyledons be removed, 

 the seed seldom germinates, and if it does germinate, it is in a 

 sickly state. The structure of the leaf shows its adaptation to 

 the purposes of respiration. 



178. By what we have called crude sap, we do not mean 

 that it is not changed at all in its ascent through the root and 

 stem, but that it is unfit for assimilation until it has passed 

 through the leaves. 



It would be an important point to determine the real state of 



What exerts an important influence on the direction? — 178. To what 

 have leaves been compared? Do they do more ? and what ?— 177. What 



£ roves that nutrition depends on leaves \ How with the cotyledons ? — 178* 

 i the sap changed before it arrives at the leaves? 



