110 FUNCTION OP LEAVES. , 



sume a spiral form. The cause in these cases is made plain 

 by a curious experiment contrived by Dutrochet. It was 

 found that the Mistletoe liad the same impulse towards the 

 center of the branch of a tree on which it grows, that most 

 other plants have to the center of the earth, and it was also 

 found that the Mistle toe might be made to germinate on a 

 thread so small that jt would exercise no inlluence over the 

 direction of the root. This he did, and then fixed it to a fine 

 needle, and had it accurately balanced so as to turn like a 

 compass needle w^th the slightest force. He then placed 

 near the radicle a piece of wood and covered the whole with 

 a glass, and in process of time the radicle was seen to turn 

 directly towards the wood, and that too without moving the 

 needle ; showing that the quantity of matter controlled the di- 

 rection of the radicle, but by the exercise of no power with 

 which we are acquainted. It could not have been by attrac- 

 tion, for then it would have produced a movement of the nee- 

 dle. It seems to be an exercise of power over the vital en- 

 ergies in producing the turning of the radicle in that direc- 

 tion. The earth no doubt exercises its influence in the same 

 manner. This influence, however, has been counteractedby 

 the application of agents in an unusual manner. Prof. 

 Schultz is said to have succeeded in reversing the growth of 

 plants by planting them in moss, and so arranged that the 

 light which they received was the solar rays reflected from a 

 mirror from beluw upwards. Under such circumstances, it 

 is said, the roots take their directions upwards, and the stems 

 downwards ; similar experiments have not succeeded in the 

 hands of others. 



Notwithstanding such experiments may succeed, we may 

 nevertheless draw the conclusion, that all roots direct them- 

 selves perpendicularly to the surface of the body on which 

 they naturally germinate ; if they are parasites they will be 

 perpendicular to the surface on which they grow. 



Color seems to exercise an important influence in the di- 

 rection of organs; if they are of deep color they ascend, if 

 colorless, or of a pale color, they take a descending direction. 

 Roots if they become green will then ascend, or turn towards 

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

 them in only one direction. 



Section 2. Function of Leaves. 



135. From the structure of leaves, we should be led to suppose 

 that they perform an important part in vegetation. They 



