50 BOTAJHTT. 



they differ only in their less diameter, and in having imper- 

 forate horizontal or oblique partitions. They are doubtless 

 properly classed with the tracheids (see paragraph 54). 



84. On the outer side of the tracheary portion just de- 

 scribed lies a mass of narrow, somewhat elongated, thin- 

 walled cells, which constitute a true meristem-tissue, to 

 which the name of cambium* has been given (c, c, Figs. 

 31 and 33). Next to the cambium lie, in order, sieve-tis- 

 sue and soft tissue (parenchyma); these do not occupy 

 separate zones, but are more or less intermingled, forming 

 a mass called the Soft Bast {y, y, y, Fig. 31, and^. Fig. 

 33). The sieve-tissue includes sieve-tubes and cambiform 

 or latticed cells. In the extreme outer border of the bun- 

 dle is a mass of fibrous tissue {b, b). The layer of starch- 

 bearing cells just outside of the last-named tissue is the so- 

 called "bundle-sheath." 



85. In most higher flowering plants the fibro-vascular 

 bundles of the stems have a structure essentially like that 

 of the Castor-oil Plant just described. In them it is evi- 

 dent at a glance that the bundle is divided into two some- 

 what similar portions, an inner and an outer, by the cam- 

 bium-zone. Nageli, who first pointed out these divisions, 

 named the inner one the Xylem portion, because from it 

 the wood of the stem is formed; the outer he named the 

 Phloem portion, for the reason that it develops into bark. 

 If we wish to be less technical we may call the first the 

 Wood portion, and the second the Bark portion. 



86. In some cases the xylem and phloem are composed 

 of corresponding tissues, (1) Vessels, (3) Fibres, and (3) 



* Cambium, a low-Latiu word meaning a liquid whicli becomes 

 glutinous. The term was introduced when the real structure of the 

 part to which it was applied was not understood. 



