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 starch -layer. 

 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 prosenchymatons. The bast fibres, moreover, 

 are not cells, but cell-derivates, i.e., 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), 

 i.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 



