DEVELOPMENT OF THE STEM. 795 
strains, such as those produced by the wind, are the sole cause 
of the formation of wood, which is developed to resist the strains. 
His experiments were anticipated by Knight so far back as 
18038 ; but his results must te taken with modification. It is 
probably true that such a conservative formation of wood does 
occur to meet unusual strains ; but the want of correspondence 
in nature between great exposure to such strains and large 
deposit of wood, and the numerous examples of great wood- 
formation in ligneous twiners and nailed-up trees, must prevent 
us from considering it an all-sufticient explanation. In the cases 
where no strains can have occurred, ‘ the natural selection of 
variations can have only operated’ to form wood, according to 
Spencer. 
c. The Medullary Rays.—The functions which these rays 
perform is, probably, to as-ist the diffusion of a portion of the 
elaborated sap from the bark and cambium layer through the 
wood, in which certain of the secretions it contains are ultimately 
deposited. 
d. The Bark.—The bark acts as a protection to the young and 
tender parts within it. The inner part is generally believed to 
convey the elaborated sap from 
the leaves downwards, in order Fira. 1146. 
that new tissues may be deve- < > 
loped, and the different secre- 
tions deposited in the wood and 
in its own substance. The bark 
frequently contains very active 
medicinal substances, and others 
which are useful in the arts, &e. 
B. Development of the Stem 
(Caulome).—The stem is deve- 
loped from the apex or growing 
point (punctum  vegetationis), 
where is situated the apical cell 
or apical groups of cells. In most 
of the Cryptogamia growth is ef- 
Fig. 1146. Longitudinal section through 
fected by the division of a single 
apical cell (fig. 1146, t), which 
is generally large, and divides 
into two daughter-cells, one of 
which becomes the new apical 
cell, while the other, the segment 
cell, by further division, forms 
the permanent tissue. In the 
stems of the higher plants, in- 
the apical region of three primary 
shoots of Chara fragilis. t. Apical 
cell, in which segments are formed 
by septa, each segment being further 
divided by a curved septum into a 
lower cell not further divisible, which 
develops into an internode, g, g’, g’’, 
g’, and an upper cell which produces 
a node, m, m’, and the leaves, b, b’, b’’, 
b”’, which also undergo segmentation. 
(After Sachs.) 
stead of a single apical cell, there are generally several such cells 
(fig. 1147 s, s), which differ from the like cells of roots in having 
no special cap, and from leaves in the fact that the cells last 
formed are at the apex. (See Development of Roots, page 792 ; 
aud of the Leaves, page 811.) 
