20 Dr Harvey's Observations on the 



the flannel is alive. No more are we entitled to affirm, that 

 the old stems and roots of a tree are alive, because of the 

 sap moving through them to the growing parts above. They 

 may be merely the medium or channel of its transmission, 

 and aid in effecting this in the same way that the flannel 

 does. 



Unquestionably, the movement of the sap is a vital action, 

 and due to vital agency. This agency, however, has its seat 

 in the living buds, and in the living structures proceeding 

 from them, and actually growing. It is directly connected 

 with, and dependent on, the vital processes going on there 

 during the spring and summer.* The first movement of the 

 sap in spring is in the immediate vicinity of the buds. The 

 fluid there, previously at rest, is the first to be set in motion, 

 and its movement is determined by the act of vegetation be- 

 ginning in the buds under the influence of heat and light. 

 The subsequent increase in the activity of that process de- 

 manding additional and greater supplies of sap, an agency is 

 exerted which operates downwards in the direction of the 

 soil, and causes the nourishing fluid to ascend. And it is 

 farther important to remark, that the movement of sap from 

 the soil upwards through the trunk to the parts where vital 

 actions are undoubtedly going on is, the whole season through, 

 regulated by the activity of these actions. Of all this we 

 have several decisive proofs. If a branch of a tree, standing 

 in the open air, be introduced into a hot-house at a time when 

 no vegetation, and no circulation of sap is going on in the 

 tree, the buds of that branch will vegetate, and sap will cir- 

 culate through it, while as yet nothing of the sort is in pro- 

 gress in any of the other branches of the tree.f It is quite 

 inconceivable that the roots and stems should exert so exclu- 

 sive an agency, or have any share in producing so partial a 



* ** It is evident, then, that the force, whatever be its nature, by which the con- 

 tinued movement is kept up, must be developed by the processes to which that 

 movement is subservient ; in other words, that the changes involved in the acts 

 of nutrition and secretion, are the real source of the motor power." — Carpenter, 

 Manual of Physiology, p. 315. 



t Alison, Outlines of Physiology, p. 70; Carpenter, Manual of Physiology, 

 p. 313. 



