cn. III. DOWNWARD GROWTH OF THE ROOTS. 



81 



intus-susception. I have observed this in plants which 

 I have introduced through the window-sill, and trained 

 against the shutter, by marking the position and up- 

 ward progress of the stalk of each leaf where it joins 

 the stem. Duhamel first pointed out this fact. With 

 the exception of the parts of the shoot of the current 

 year, no other part of a tree makes any upward pro- 

 gress. The downward growth of a tree, that is, the The 



^ downward 



elongation of the roots, and the OTowth in girthing of growth of 



o ' o t) fc) the tree, 



every part of the tree, may by comparison be said to ening^o?' 

 resemble the growth of minerals by juxta-position ; that simply by 



' f growth at 



IS, roots are leno^thened only by the deposit of new the end of 



^ ^ J ^ r the root. 



growth at their ends, and they do not progress bodily 

 through the earth. I have never remarked accurately 

 how this is with 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 diffi- 

 culty of forcing itself through the earth ; though, no 

 doubt, this peculiar organisation is a contrivance^ and a 

 very beautiful one, to overcome the mechanical diffi- 

 culty. 



The root has as strong a first tendency downward By what 



agency is 



as the stem has upward, though it puzzles our philo- ^^^^^^^^ 

 sophy to account for either. The first downward ten- th^rJot^^^ 

 dency of the root is, however, soon counteracted by ^^^^^^^'^'^ 

 circumstances, such as the necessity of atmospheric 

 aeration, the goodness of upper compared with lower 

 soils, the intervention of rocks, chalk, &c. ; and the 



