GROUPS OF TISSUES, OR TtSSUB-SYSTEMS. 45 



just outside of the last-named tissue is the so-called "bun- 

 dle-sheath." 



83. 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. 



84. In some cases the xylem and phloem are composed 

 of corresponding tissues, (1) Vessels, (2) Fibres, and (3) 

 Soft Cells. The vessels are the tracheary tissue in the 

 xylem and the sieve-tissue in the phloem. The fibrous 

 tissue of the xylem is the variety with the shorter and 

 harder fibres, known as wood-fibres; that of the phloem is 

 composed of the longer and tougher bast-fibres. The soft 

 tissue (parenchyma) of the two portions is much alike. 



85. In the fibro-vascular bundle of the young roots of 

 SWeet Flag there are many radially placed plates of trache- 

 ary tissue {^pp, Fig. 28), which alternate with thick masses 

 of sieve-tissue {ph). Between these alternating tissues, and 

 within the circle formed by them, there is a mass of soft 

 tissue. The whole bundle is separated from the large-celled 

 soft tissue of the root by a well-marked bundle-sheath (s) ; 

 the latter is bounded interiorly by a layer of active thin- 

 walled cells (the pericambium), from which new roots origi- 

 nate. In the older roots the central cell-mass is trans- 

 formed into stony tissue. 



