414 VEGETATION OV SEEBS^ 



were it subsequently inverted, and made to point perpendicu- 

 larly downwards. To enable myself to answer this objection, 

 J made many experiments on seeds of the horse-chesnut, and 

 of the bean, in the box I have already described : and as the 

 seeds there were suspended out of the earth, 1 could regularly . 

 watch the progress of every effort made by the radicle and 

 germen to change their positions. The extremity of the radi- 

 cle of the bean, when made to point perpendicularly upwards, 

 generally 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 ijf twenty-four hours ; and all my efforts to make 

 it grow downwards, by slightly cb .nging its direction, were in- 

 variably abortive. 



Another, and apparently a more weight}', 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 perpem'icularly upwards, and that their roots always spread 

 horizontally ; but this objection, I think, may be readily 

 answered. 



CaHseswhy rpj^g luxuriant shoots of trees, which abound in sap, in 



the blanches ot _ . . ^ 



trees do not whatever direction they are first protruded, almost uniformly 

 exactly obey ^^^^ upwards, and endeavour to acquire a perpendicular di- 

 tais operation. ' •, r 



rection ; and to this their points will immediately return if 



they are bent downwards during any period of their grotvth ; 

 their curvature upwards being occasioned by an increased ex- 

 tension 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 shew 

 an incHnation to turn upwards ; but the operation of light, in 

 this case, has been proved by Bonnet * to be very consider- 

 able. 

 Why the roots The radicle tapers rapidly as it descends into the earth, 

 horizonteJly? ^^^ ^^^ lower part is much compressed by the greater solidity 

 of the mould into which it penetrates. The true sap also 



* Reciierches sur I'Usage dcs Feuilles d^ns Ics Flsyites. 



continues 



