cb:. nr. DOWNWARD GROWTH OF THE ROOTS. 



85 



determination of the root, and upward vertical deter- 

 mination of the stem, the annual vertical growth in 

 elongation of a tree, either upward or downward, is 

 nothing in comparison with the growth of it, which 

 forms angles with a vertical line : that is, out of all the 

 numerous points of elongation of the head of a tree, 

 there is but one which can go vertically upward in 

 continuation of the line of the stem ; and out of all the 

 numerous points of elongation of the root, there is but 

 one which can go vertically downward in continuation 

 of the line of the stem ; and I believe that one ceases 

 to do so very soon. 



If the leader of a tree is killed, the light falling 

 equally from above on many buds, a multiplicity of 

 leaders may be developed ; though if one grows more 

 vigorously than the others, by overshadowing them 

 from above with its own side-growth, it will force 

 them to grow sideways to the light, and the tree will 

 again become single-leadered. Without pruning, a 

 tree may become round-headed from the merest acci- 

 dent ; for instance, an insect, or a bird, or the wind 

 destroying the top bud. A tree on the side of a steep 

 hill may be seen to grow from the opaque side above 

 it, and, after it has reached a certain height, to curve 

 back again. 



So far light would appear to be the principal agent Dutrochet, 

 in directing the growth of the heads of plants. Yet Davy, &c., 



, , think that 



the majority of physiologists attribute the direction of fj^r^J^tsVe 

 the growth both of the head and of the roots of plants both of the 



