PROCESS OF NUTRITION. CHAP. III. 



the leaves, even when the branch was bent down 

 whether by nature or art, so as to point to the 

 earth,* in which case the power propelling the 

 proper juice is acting not only in opposition to that 

 of gravitation, hut with such force as to overcome 

 it. This is an unanswerable argument ; and yet it 

 seems to have been altogether overlooked, or at 

 Accord- least undervalued in its importance by Mr. Knight, 

 Knight. w h nas more recently investigated the subject of 

 the descent of the proper juice; but without 

 having been able to offer any thing that can be at 

 all regarded as satisfactory. He endeavours, how- 

 ever, to account for the effect by ascribing it to the 

 joint operation of the four following causes : gra- 

 vitation, capillary attraction, the waving motion of 

 of the tree, and the structure of the conducting 

 vessels ; but, like charity among the virtues, the 

 greatest of these is gravitation. 



Gravita- Gravitation. A vertical shoot of a Vine was 

 forcibly bent down in nearly a perpendicular di- 

 rection, and its succulent extremity introduced into 

 a pot as a layer, without wounding the stem or 

 depriving it of any of its leaves. Two circular 

 incisions were made in the bark of the inverted 

 part and the intervening portion of bark was stript 

 off. But there was more wood formed at the lip 

 now uppermost, than at the lip opposite to it ; 

 which in the opinion of Mr. Knight was owing to 

 the force of gravitation, since the result would have 



* Phys. des Arb. iiv. iv. chap. v. 



