CH. III. DOWWAKD GROWTH OF THE ROOTS. 107 



what causes the branch-root to grow vertically upward. 

 If the vertical upward growth of a root is doubted, 

 I can show it now going on to any one who desires to 

 see it. I have myself put the question to Nature, and 

 I have her autograph answer to it in my possession. 



In this case of roots growing upward, the descensus 

 becomes an ascensus. What is called the ascending 

 sap in the wood of the root, becomes a descending 

 sap ; and what is called the descending sap in the bark 

 of the root, becomes an ascending sap. With submis- 

 sion to Sir Humphry Davy, what has gravity to do 

 with all this ? 



That this our nineteenth century has infinitely 

 more knowledge of vegetable physiology than any fore- 

 going age I have not a doubt. But if any one imagines 

 that we have arrived at a competent knowledge of the 

 science, when a dozen questions of such vital impor- 

 tance as these are open, I think he only shows that he 

 is ignorant of the depth of our ignorance. 



