34 THE FLOWERING PLANT. 
are applied to tubular cell-derivates that have been formed by the 
more or less complete coalescence of adjoining cells. In the wood 
no special notice need be taken of the wood parenchyma, nor of 
the wood fibres, which closely correspond to the bast fibres. The 
essential parts are the wood vessels, which, like the vessels.of the 
bast, are cell fusions made up of numerous joints. Here, how- 
ever, the cell nature is still more completely lost, for the walls 
are thickened and lignified, and the protoplasm has completely 
disappeared, being replaced by air and water. ‘The cross-walls 
have been almost entirely absorbed, so that a continuous tube 
is formed. The more external vessels are pitted, and obviously 
made up of joints, while next the pith smaller vessels occur in 
which the thickening of the wall has taken place in a spiral or 
ring-like manner. These spiral and annular vessels are the oldest 
parts of the wood, and therefore termed protoxylem. Having 
had more time to develop than the other vessels, their members 
are longer, and the jointed nature is not so obvious. The cam- 
bium is not easy to make out in longitudinal section, but its cells 
appear elongated, and are in fact rectangular prisms with rather 
oblique ends. It has been stated that cells derived from the 
cambium go to increase the bast and wood, and before this can 
be effected must evidently undergo various changes, according to 
the nature of the structures formed. A prismatic cell may become 
a fibre pretty much as described on p. 32, but of course less 
elongation is here necessary. Short parenchymatous cells may 
be derived from such a cell by the formation of transverse walls, 
while the origin of sieve tubes and wood vessels will be under- — 
stood from what has been said above. It must further be added 
that the prismatic cells enlarge and become cylindrical before 
they can be converted into members of sieve tubes or wood 
vessels, The stem, like the root, terminates in a growing-point 
(cf. p. 16), but there is nothing to correspond to the root-cap. 
The above histological description of the sun-flower stem will 
serve pretty well for most herbaceous dicotyledons, but the fibres 
of wood and bast are often absent. When they are present the 
bundle is fibro-vascular. The most important points may be put 
in a tabular form :— 
Hard Bast.—Bast fibres or sclerenchyma. 
I. Bast or Phloém. ; Soft Bast.—Bast vessels or sieve tubes, 
bast parenchyma. 
II. Cambium. 
Wood fibres. 
III. Wood or Xylem. ; Wood vessels. 
Wood parenchyma. 
