88 
UPWARD GROWTH OF THE HEAD, AND [Part II. 
The downward 
growth of the 
tree, or length¬ 
ening of the 
root, is simply 
by growth at 
the end of the 
root. 
year, to a certain degree, grow bodily upwards, 
or by what (by comparison with the downward 
growth) may be called 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 upward 
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 progress. The downward growth 
of a tree, that is, the elongation of the roots, and 
the growth in girthing of every part of the tree, 
may, by comparison, be said to resemble the 
growth of minerals by juxtaposition; 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 re¬ 
marked 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 difficulty of 
forcing itself through the earth; though, no 
doubt, this peculiar organisation is a contrivance , 
and a very beautiful one, to overcome the 
mechanical difficulty. 
