90 
UPWARD GROWTH OF THE HEAD, AND [Part II. 
Turgescence, 
and the action 
of light on tur¬ 
gescence, pro¬ 
bably direct the 
growth of the 
head of the 
plant. 
Cellular forma¬ 
tion of the 
bark of green 
shoots. 
of elms, cut a deep ditch between the elms and 
his field. The roots, however, were not to be 
done. They of course followed the surface of 
the ground, whether down, horizontal, or up 
hill, and took the ditch “ in and out clever ” 
into the philosopher’s field. Duhamel thinks 
these were very cunning roots, and that they 
had an instinctive notion of the treat they were 
to have on the other side of the ditch, and grew 
at it. Had the philosopher built a wall in the 
ditch, and then filled it in, he would have beaten 
the roots, on account of their inability to leave 
the surface and get under the wall. 
Were we to add one step to the beautiful 
theories of Knight and Dutrochet respecting the 
growth of adhesive plants, and the tendrils of 
climbing plants, from the light, we might attri¬ 
bute the upright growth of the stem and the 
aberration of its branches from the perpendicular 
to the mere swelling (turgescence) of the cellular 
tissue of the new green shoot, and to the action 
of light on the swelling. 
It is asserted, on microscopic observation, 
that the bark of all new green shoots is entirely 
composed of two layers of cellular tissue. The 
cells of the outside layer of tissue decrease in 
size from within outwards; the cells of the inside 
