THE STEM OF PLANTS. 49 
in circles towards the circum- 
ference of the trunk. This is 
shown in Fig. 56, representing 
the horizontal section of the 
stem of a Tree Fern. 
The reed-like bundles tra- 
verse the stem of the Tree Fern 
from top to bottom, presenting 
a dark edge, formed of very 
tough fibres, impregnated with 
adark browncolour, containing 
cellular tissue and a few vessels 
of different sorts. Among 
these vessels we shall particu- 
larly notice some prismatic 
tubes, which show on each of their faces horizontal clefts very 
close to each other, and at equal distances, called 
scalariform vessels. Fig. 57 shows the structure 
and relative arrangement of the scalariform vessels 
in the trunk of a Fern-tree. These vessels are 
represented under the microscope. “Here,” says 
Mr. Dresser, “the woody matter contained in the 
stalks of some leaves is transmitted into the stem, 
retaining the same disposition it did in the leaf 
stalks, in which case it surrounds the central cellular 
axis (Fig. 56); and the cellular apex of the stem 
grows and develops new leaves, the new portion of 
the stem being formed at the summit. Hence 
plants with this form of stem are said to be acroge- 
nous, from axpa, a “summit,” and yevvad, “produce,” 
or summit growers. All ferns have stems of this form. 
Although the Tree Fern has no close resemblance 
to the stem of either exogenous or endogenous plants, 
it seems to be more closely allied to the latter. 
Thus the rind or bark consists of layers of cellular 
tissue only, marked by the cicatrices of leaves or Vi 
fronds, showing that its-leaves are produced at the Pe. 51-—Seal 
summit only. ern 
Fig. 56.—Section of the stem of a Tree Fern. 
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