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. The transverse partitions 

 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 pointed nature is not so obvious. The cam- 

 hium 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 sunflower stem will 

 serve pretty well for most herbaceous dicotyledons, but the bast 

 and wood fibres are often absent. When they are present the 

 bundle is fibro- vascular. The most important points may be put 

 in a tabular form : — 



!IIard Bast. — Bast fibres or sclerenchyma. 

 Soft Bast. — Bast vessels or sieve tubes, 

 bast parenchyma. 



II. Cambium. 



( Wood fibres. 

 III. Wood or Xylem. ^ Wood vessels. 



( Wood parenchyma. 



