CONTENTS. 
IX 
CHAPTER III. 
Page 
Upward growth of the head, and downward 
GROWTH OF THE ROOT - - - - 87 
Upward growth of the head, and downward growth of 
the root, considered together - - - 87 
The upward growth of a tree, or lengthening of its 
shoot, is by the enlargement of all the parts of that 
shoot; and all these parts progress bodily upward - 87 
The downward growth of the tree, or lengthening of 
the root, is simply by growth at the end of the root - 88 
By what agency is the head directed upward and the 
root downward ? - - - - 89 
Turgescence, and the action of light on turgescence, 
probably direct the growth of the head of the plant 90 
Cellular formation of the bark of green shoots - 90 
How turgescence acts on the cellular formation - 91 
How light acts on this cellular turgescence - - 91 
Dutrochet, Knight, Davy, &c. think that gravity directs 
the growth both of the head and roots of trees. Ex¬ 
periments in proof - - - - - 94 
Upward growth of first gemmule when deprived of 
light - - - - - -97 
Crane-neck growth of first gemmule of a double-seeded 
plant, to shield it while forced through the earth - 98 
Roots grow through the earth almost without force - 98 
Provision to enable the gemmule of a single-seeded 
plant to thrust itself through the earth - - 101 
Probable distinct organisation of the first gemmule - 101 
Strong downward determination of tap-root, and pro¬ 
bable distinct organisation of it - - - 104 
Tap-root only proper to seedlings, and a contrivance for 
fixing them - - - - - 107 
That the growth of the head and root of a tree is di¬ 
rected by gravity disputed - - - -112 
General growth of head towards light; general growth 
of root to wherever it can find good soil - - 118 
A new layer of bark is formed every year - - 120 
