34 



the elongation of the roots and the growth 

 in diameter of every part of the tree, 

 may by comparison be said to resemble 

 the growth of minerals — by Juxta-posi- 

 tlon; that is, roots are lengthened only 

 by the deposit of new growth at their 

 ends, and they do not progress bodily 

 through the earth. I have never remarked 

 accurately how this is in plants grown in 

 water ; but I believe it to be the same 

 as when they grow in the earth, and that 

 this mode of growth is the result of the 

 organic structure of the root, and not of 

 the mechanical difficulty of forcing itself 

 through the earth. 



It has been hitherto held that the stem, 

 branches, and roots grow in diameter by 

 the deposit of new growth on their out- 

 side, and that there is no increase from 

 within of their internal parts. 



Is it possible that the growth by intus- 

 susception, or by juxta-position, may be de- 

 pendent on the presence or absence of the 



