178 
VEGETABLE PHYSIOLOGY. 
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By ARTHUR HENFREY, Esq., F.L.S., Lecturer on Botany at St. George’s Hospital. 
THE ELEMENTARY STRUCTURE OE PLANTS -( Continued ). 
T HE last chapter (page 124) concluded ■with a description of the general character of the con¬ 
struction of the stem of Monocotyledons, and it was then pointed out that the woody skeleton, 
conqiosed of the fibro-vascular bundles, consists of a large number of these lying loosely embedded, 
free from one another, in the soft, spongy, cellular tissue of the stem; it was shown that these bundles 
do not undergo any important alteration after they are completely developed, and that the principal 
changes which take place in Monocotyledonous trunks, such as those of Palms, arise from the consolida¬ 
tion of the general cellular tissue, by its acquiring a woody consistence, through the deposit of thicken¬ 
ing layers in its cells, so that the fibro-vascular bundles are at last hound together, and become part 
of a solid woody mass. But this does not alter the relative position and arrangement of the various 
parts. 
It is very different with the stems of Dicotyledons. We find here that not only is the structure of 
the bundles peculiar, but that there is a new and more definite mode of arrangement; moreover, 
that the history of their growth includes a series of additional developments, in which altogether new 
parts are produced, and which only terminate with the life of the plant. The characters of the 
Dicotyledonous stem should be studied, in the first place, in annual plants, and in the yearling shoots 
of perennials, the conditions being essentially the same. 
When we cut one of these across, we find a more distinctly marked division into regions than exists 
in the Monocotyledonous stem ; the fibro-vascular bundles do not here lie scattered through the cellular 
mass, but present a regular circle at their cut ends, surrounding and cutting off a central spongy region, 
which we recognise as th e pith, from a second external cellular region, belonging to the bark or rind 
structures. If the stem or shoot be very young the ends of the bundles 
will much resemble those of the Monocotyledons, and will form a 
circular row, divided from one another by lines of the cellular structure^ 
reaching out from the pith to the bark; when a little older they will be 
found to have acquired a somewhat triangular shape with their in¬ 
creased size, resembling so many wedges with the narrow ends towards 
the pith, and now by their enlargement they press upon the lines of 
cellular stucture intervening between their sides, so as to reduce these 
to mere plates, which constitute what are called the “ medullary rays.” 
The rind, enveloping the wood, has also acquired further development, 
presenting a division into several regions, each having its peculiar 
character. To make these and their relations to the inner portions 
clear, we will trace the nature of the various parts met with as we 
DIAGRAM ILLUSTRATING THE FORMATION 
OF A DICOTYLEDONOUS STEM. 
examine them, from the pith outwards to the surface, in a stem or 
a, pith; b, rind; c, c, c, plates of shoot at the close of its first year’s growth. 
cellular tissue connecting them, me- The pith ; s composed of t]le ne a rl y unaltered cellu- 
dullury rays ; d, d, d , fibro-vascular, J 
or woody bundles arranged in a circle. tlSSUG 01 Wnicn tllG WllolG stGITL nt first consisted.; 
this is surrounded by a layer of firm woody matter, 
formed of a circle of fibro-vascular bundles, arranged side by side, and closely 
in contact, like the staves of a cask, only parted from one another by slit-like 
openings here and there, filled up with cellular tissue, which are the remains of the 
tissue lying between them while they were small and delicate; and, as I have already 
said, are called the medullary rays. The fibro-vascular bundles, when examined perpendicular sec- 
microscopically, are found to be composed of the following structures:—Next the TI0X of a portion of 
pith lies a layer of the so-called spiral vessels, the delicate elastic strengthening stej^in adirection 
organs, which being the first formed, lie here, immediately on the pith, of which crossing that of 
they were at first the only protecting structures; they were formerly supposed to themedullary 
have some peculiar function connected with the pith, and hence the circular sheath EATS ' 
around the pith they collectively form was called the “medullary sheath;” the «’ poro " sd 1 1 f ets; . 
medullary sheath, however, is merely the first-formed portion of the woody fibres ; c, c, cut ends 
skeleton, developed in the growing end of the stem, while it is young and delicate, of the medullary 
and, therefore, composed of spiral vessels, for the same reason that the ribs of the rays ' 
leaves and leaf-stalks are—namely, to give strength combined with great flexibility. 
