104 MINUTE STRUCTURE OF ORGANS. 



one made in the middle of the course of a bundle with one near 

 its extremity. 



311. The cribrose part of the bundle may also be termed its 

 liber-portion or bast-portion ; the tracheal, its woody portion. 

 These terms are not liable to be confounded with any others, 

 since it is with the cribrose portion that the well-known bast- 

 fibres or liber-fibres are associated, Avhile it is in the tracheal 

 portion that all the constituents of wood are found. 



312. For the first term (bast-portion), Nageli has introduced 

 the word Phloem ; for the second (wood-portion), Xylem. In 

 this treatise these terms will be used interchangeably with the 

 others. But the woody portion of a bundle is sometimes very 

 far from being conspicuously lignified, and the bast-portion may 

 be much reduced. 



313. The three principal ways in which the elements of bun- 

 dles are arranged are : 1 . A single strand of liber has one side 

 in contact with a single strand of wood, the two running side by 



c? c/ */ 



side, - - the collateral bundle. This mode of arrangement is com- 

 mon in the stems of phtenogams. A variety of the collateral 

 bundle has a strand of liber on each side of the wood, or, con- 

 versely, a strand of wood on each side of the liber, - - the bicollat- 

 eral bundle. 2. The strands of liber and wood are in different 

 radii, - - the radial bundle. This is the most common mode of 

 arrangement in roots. 3. A strand of one element is wholly en- 

 veloped by the other element, - -the concentric bundle. These 

 modes of arrangement will be further discussed under " Roots ' 

 and " Stems." 



314. The bundles are surrounded by parenchyma ; but this is 

 very frequently limited at the periphery of the bundle by a cylin- 

 der formed of closely united parenchyma cells, which contain 

 considerable starch. The endodermis is a special case of this 

 structure, in which the cells are more or less distinctly cutinized. 

 When this enveloping cylinder is well defined, it is known as the 

 bundle-sheath. 1 



315. At first, each bundle consists of similar cells (procam- 

 bium), some of which differentiate into fibres, vessels, etc. 

 Bundles in which all the procambium cells become permanent 

 cells are closed; those which retain an inner portion (cambium) 

 capable of further differentiation are open. 



1 In a great number of instances it is convenient to refer to the same struc- 

 ture the long and firm bast-cells which are found at one side of the bundle ; 

 but the subject, when examined from the point of view of development, espe- 

 cially when the vascular cryptogams are. taken into account, presents so many 

 difficulties that it may be here left without further treatment. 



