108 PROCESS OF NUTRITION. CHAP. III. 



the susceptibility of the plant to the action of heat 

 as necessary to the protrusion of the bud, accord- 

 ing to the temperature in which it has formerly 

 been kept. And should it not have been also re- 

 collected that the phenomenon might have been 

 owing merely to the difficulty attending the ad- 

 mission of the proper juice into any vessels capable 

 of conveying it in the inverted direction ? there 

 being no set of vessels leading from the bud di- 

 rectly upwards, as there are vessels leading from it 

 directly downwards, in the original position of the 

 shoot. So that the entrance of the proper juice 

 could be effected only by lateral communication, 

 which in this case the structure of the vessels may 

 very possibly not admit of. 



Insuffi- Such are the causes assigned by Mr. Knight 

 account f r tne descent of the proper juice. They are each 

 perhaps of some efficacy ; and yet even when taken 

 altogether they are not adequate to the production 

 of the effect. The greatest stress is laid upon gra- 

 vitation ; but its agency is obviously over-rated, as 

 is evident from the case of the pendant shoot ; and 

 if gravitation is so very efficacious in facilitating 

 the descent of the proper juice, how comes its 

 influence to be suspended in the case of the ascend- 

 ing sap. The action of the Silver grain will 

 scarcely be sufficient to overcome it ; and if it 

 should be said that the sap ascends through the 

 tubes of the alburnum by means of the agency of the 

 vital principle, why may not the same vital principle 



