44 BOTANY. [chap. i. 



•development, and doing the usual amount of work, so 

 in proportion will the formation of wood be arrested. 



In rare instances a few fibre- vascular bundles originate 

 in stems quite independent of the leaves ; these are 

 differentiated from the fundamental tissue of the growing- 

 point at a higher level than the origin of the youngest 

 leaves, and are known as caulme bundles. 



In Phanerogams, or flowering plants, a typical fibro- 

 vascular bundle consists of two kinds of permanent tissue, 

 that is, tissue which, once formed, undergoes no further 

 differentiation ; the two kinds, as previously stated, are 

 respectively called phloem or bast, and xylem or wood, 

 and these are arranged collaterally or side by side, the 

 phloem being outermost or nearest the periphery, the 

 xylem innermost or nearest the centre of the trunk. In 

 many of the Vascular Cryptogams, the arrangement of the 

 two parts of a bundle is concentric, the phloem completely 

 surrounding the xylem. 



Depending on the further mode of development, and 

 on the manner of arrangement of the fibro-vascular 

 bundles in the stem, along with other characters derived 

 from the seed, flower, and leaf, Phanerogams are arranged 

 under two subdivisions, Monocotyledons Siai Dicotyledons. 

 (i) Monocotyledons. The fibro-vascular bundles con- 

 sist of the elements phloem and xylem only, and no 

 secondary growth, that is, no additions to the elements 

 existing in the bundle as originally formed, takes place ; 

 such bundles are said to be closed, meaning, as stated 

 above, that no additions in the way of new cells are 

 added at a later stage, hence the bundles remain small, 

 and for the greater part of their length isolated, coalescing 

 by their tapering tips only with other bundles lower 



