en. II. 



COURSE OF THE SAP. 



41 



stream whicli follows, that the heart-wood is a conduit 

 for the sap ; and the existence of plasliers proves that 

 the sap-wood is a conduit for the sap. How I wish 

 that Dr. Lindley would allow us this one bit of un- 

 doubtful ground in vegetable physiology to catch our 

 breath on ! 



Certainly, in respect to first getting the sap into the By what 



Tn G cll ti HI C cl 1 



tree, we have had doubt and difficulty enough. Now f^^^^p^^fj^j-^j 

 for the next doubt and difficulty. Now for getting the ^ap raised? 

 sap u]3 the tree. I was going to say, that no one has 

 an idea of the cause, and laws, and mechanism by 

 which the sap is forced up. But there I should be 

 wrong. Every one has his own idea, and every one's 

 idea differs from that of his neighbour. To show on 

 what extraordinarily loose ground these ideas stand, I 

 will quote two countenanced by perhaps the keenest 

 intellect that ever wrote on the subject. Sir Humphry 

 Davy thought that one cause of the ascent of the sap 

 was the motion caused in trees by wind ; that another 

 cause was the contraction and expansion of the wood 

 from alternation of heat and cold. Look into the hot- 

 house and the hot-bed. In these neither of these 

 causes exists. Not a breath of wind enters ; nor is any 

 alternation of heat and cold allowed. Yet in these the 

 ascent of the sap is freest. And if we look 'out of 

 doors, I should say that the sap would be a slow 

 traveller if its ascent depended on wind and cold. 

 Here, then, I cannot back the favourite, and have a sort 

 of blind leaning for Turgescence^ or Swelling. A dark 



