32 THE FLOWERING PLANT. 
question. Two rectangularly-shaped cells making a radial row 
would then be formed. Further divisions would give a radial 
row with more numerous cells, This is the kind of process which 
goes on in cambium. It is, in fact, a meristem, or actively-dividing 
formative tissue (cf. p. 17), with abundant protoplasm and thin 
cellulose cells, as might be expected. It is secondary meristem, 
because it abuts externally and internally upon permanent tissues, ° 
and the cells formed by its division undergo various changes, 
finally becoming bast and wood elements. Bundles which con- 
tain cambium are termed open, because they are able to increase 
in size by its means. Such bundles characterize gymnosperms 
and dicotyledons. The entire ring of bundles is enclosed by a 
sheath or layer of small starch-containing cells, which crosses 
over the medullary rays. This is the bundle sheath or endodermis. 
By studying a radial longitudinal section we shall get clearer 
ideas regarding the elements of the bundles. Beginning as before 
at the outside, we shall find the hard bast composed of thick- 
walled fibres, bast fibres, with tapering ends, by means of which 
they dovetail together. A tissue with elements united in this 
way is said to be prosenchymatous. The bast fibres, moreover, 
are not cells, but cell-derivates, z.c., they have been derived from 
cells. The presence or absence of protoplasm is the main test of 
cell nature or the contrary. What then has been the history of 
a bast fibre? It was originally a small cell, not specially elon- 
gated, with thin cellulose walls, and completely filled with proto- 
plasm. Growth in length then rapidly took place, and at the 
same time alterations both of the wall and contents went on. 
The cellulose wall, kept on the stretch by the turgidity of the 
cell, had layer after layer of woody matter deposited on its inner 
side by the agency of the protoplasm. ‘Thin places or pits, in 
this case resembling canals, were, however, left. ‘The layers in 
the thickened and lignified cell-wall can be recognized in the 
cross-section, as well as the original party-walls between the adja- 
cent cells, now known as the middle lamella. The pits appear as 
streaks running across the thickened wall. ‘They run from the 
interior of the fibre to the middle lamella, which closes them, so 
to speak, and forms a pit membrane. It will be seen from the 
above that woody matter is not limited to the wood or xylem, 
though mainly characteristic of it. As regards the protoplasmic 
contents of the original cell, these became vacuolized (cf. p. 8), 
1.e., small vacuoles were formed which then coalesced so that the 
protoplasm became limited to a parietal layer. This layer gradu- 
ally used itself up in the thickening of the wall, and the bast 
fibre became complete. Such a fibre is not therefore living, and 
it plays a purely physical part in the organism. The fibres belong 
