PARTS OF AN EXOGENOUS OR DICOTYLEDONOUS STEM. 79 
intervening spaces of parenchyma, but as growth proceeds they 
continue to enlarge, while at the same time new fibro-vascular 
elements are developed between them, so that they ulti- 
mately form at the end of the first 
year’s growth a ring of vessels and 
wood-cells round the central mass of 
parenchyma, m, interrupted only at 
certain points by projections of this 
parenchyma in the form of radiating 
lines, r. This ring is also surrounded 
by an external layer, b, of parenchy- 
matous and liber tissues, which is 
connected with the central parenchy- 
ma by the radiating lines, r, already 
alluded to. The stem then presents 
the following parts (jig. 182): 1. A 
central mass of parenchyma, m, 
which is called the Medulla or Pith ; 
2. Aninterrupted sheath of spiral ves- 
sels, t, called the Medullary sheath ; 
3. An interrupted ring of wood- 
cells and vessels, forming the Wood ; 
4. A layer of very delicate thin- 
walled cells, the Cambiwm or Cam- 
Fre. 182. 
Fig. 182. Transverse section of the 
first year’s stem of an Exogenous 
or Dicotyledonous stem. m. Pith. 
r. Medullary rays. ¢. Spiral 
vessels forming the medu'lary 
sheath on the outside of which 
are the other elements of the 
wood and the liber. 0, Cellular 
layers of the bark, 
biwm layer; 5. Radiating lines, r, connecting the pith with 
the cambium layer and bark, the Medullary rays; and 6. The 
Bark, b, a mass of parenchyma surrounding the whole stem, 
Fia. 183. 
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Fig. 183. Transverse section from the centre to the circumference of the 
stem of the Maple, three years old, m. Pith. ¢. Spiral vessels. 4, v, v. 
Pitted vessels. f, 7, 7. Wood-cells. 
c. Cambium or cambium layer. s. 
Corky layer; within which may be observed the other cortical layers, 
marked / pl, p l, p l. 
me. Newly forming bark. The figures 1, 2, 3, refer 
to the three successive years’ growth of the wood. 
and containing in its interior liber-cells, &c., and invested on 
its onter surface by the Epidermis. 
The stems of plants which live more than one year, as those 
