18 VEGETABLE PHYSIOLOGY. 



of rains and dews. The leaves which have conciirred in the pro 

 duction of the fruit, and in perfecting the seed, fall as it were ex- 

 hausted ; but they are soon replaced, and their fall is only perceived 

 by their presence on the surface of the ground. 



The perfect plant, therefore, whether it be studied among annuals, 

 or among trees that endure for a century, has analogous organs, 

 destined to fulfil the same functions, to conduce to the same end — 

 the reproduction of the seed. These organs, which we shall study 

 in succession, are, 1st. The roots ; 2d. The stem ; 3d. The leaves ; 

 4th. The appendages of the fructification. 



When we follow the progress of a seed set in a proper soil, we 

 observe that from their very first appearance the roots seek or tend 

 towards the interior of the earth ; the plumule, or young stem, on the 

 contrary, takes a direction diametrically opposite ; it grows verti- 

 cally and seeks the air. 



The lateral shoots in herbaceous plants, and the young branches 

 of shrubs, form various angles with the principal stem or trunk. The 

 first tendency of the branches is to rise vertically ; but as they gain 

 length and weight, they bend more or less downward, yielding to the 

 power of gravitation. Mr. Knight showed, by a series of ingenious 

 experiments, that the direction taken by the roots and branches is 

 mainly due to this force. 



This able observer arranged a wheel of wood in such a way that 

 he could make it turn with different velocities in planes variously 

 inclined to the horizon. The wheel, which was kept in motion by 

 a stream of water, could be made to revolve vertically or horizon- 

 tally at will. 



A number of beans were planted upon the circumference of the 

 wheel, in circumstances known to be indispensable to their germi- 

 nation and growth. By giving the wheel a sufficient -velocity, it 

 was easy to make the centrifugal force greater than the centripetal 

 force. In Mr. Knight's apparatus, this happened when the wheel 

 in the vertical plane performed one hundred and fifty revolutions in 

 a minute. The whole of the radicles were then seen to turn theii 

 suckers beyond the circumference in lines which were prolonga- 

 tions of the radii of the wheel, and their growth took place in planes 

 perpendicular to its axis. 



The stems took a completely opposite direction, and after a time 

 their summits attained the centre or axis of the wheel. 



In causing the wheel to revolve in a horizontal plane, the same 

 effects were still observed, when the rapidity of rotation was suffi- 

 cient to annul the action of terrestrial gravitation. But when the 

 motion was so far diminished, as merely to modify or to lessen the force 

 of attraction, without entirely annulling it, the plant took a course 

 comprised in a plane which formed a certain angle with the circum- 

 ference of the wheel. With a certain velocity, the roots were in- 

 clined 10° below the horizontal plane in which the wheel moved, and 

 the stems then formed an angle of the same magnitude above the same 

 plane. The angle of deviation formed in this position of the wheel waa 

 always smaller in proportion as the raj»idity of rotation was greater. 



