128 ON THE DIRECTION OF THE RADICLE 



in direct opposition to the immediate action of gravitation : nature, as 

 usual, executing the most important operations by the most simple means. 



I could adduce many more facts in support of the preceding deductions, 

 but those I have stated, I conceive to be sufficiently conclusive. It has 

 however been objected by Duhamel, (and the greatest deference is 

 always due to his opinions,) that gravitation could have little influence 

 on the direction of the germen, were it in the first instance protruded, or 

 were it subsequently inverted, and made to point perpendicularly down- 

 wards. To enable myself to answer this objection, I made many experi- 

 ments on seeds of the horse-chestnut, and of the bean, in the box I have 

 already described ; and as the seeds there were suspended out of the 

 earth, I could regularly watch the progress of every effort made by the 

 radicle and germen to change their positions. The extremity of the 

 radicle of the bean, when made to point perpendicularly upwards, gene- 

 rally formed a considerable curvature within three or four hours, when 

 the weather was warm. The germen was more sluggish ; but it rarely 

 or never failed to change its direction in the course of twenty-four hours ; 

 and all my efforts to make it grow downwards, by slightly changing its 

 direction, were invariably abortive. 



Another, and apparently a more weighty, objection to the preceding 

 hypothesis, (if applied to the subsequent growth and forms of trees,) 

 arises from the facts that few of their branches rise perpendicularly 

 upwards, and that their roots always spread horizontally ; but this 

 objection I think may be readily answered. 



The luxuriant shoots of trees, which abound in sap, in whatever 

 direction they are first protruded, almost uniformly turn upwards, and 

 endeavour to acquire a perpendicular direction ; and to this their points 

 will immediately return, if they are bent downwards during any period 

 of their growth ; their curvature upwards being occasioned by an 

 increased extension of the fibres and vessels of their under sides, as in 

 the elongated germens of seeds. The more feeble and slender shoots of 

 the same trees will, on the contrary, grow in almost every direction, 

 probably because their fibres, being more dry, and their vessels less 

 amply supplied with sap, they are less affected by gravitation. Their 

 points, however, generally show an inclination to turn upwards ; but the 

 operation of light, in this case, has been proved by Bonnet * to be very 

 considerable. 



The radicle tapers rapidly, as it descends into the earth, and its lower 

 part is much compressed by the greater solidity of the mould into which 



* Rccherches sur l'Us;ige des Feuilles dans les Plantes. 



