OROVPS OF TISSUES, OR TISSUE- SYSTEMS. 49 



is that the tissues have a concentric arrangement: the tra- 

 cheary tissue is encircled by a layer of parenchyma; this 

 by one of sieve-tissue; this again by fibrous tissue; and 

 80 on. 



90. De Bary's classification of fibro-vascular bundles is 

 useful in designating their general plan. He includes all 

 forms under three kinds, viz., (1) the Collateral bundle, 

 which has one mass of xylem by the side of a single mass 

 of phloem; (2) the Concentric bundle, which has its tissues 

 arranged concentrically around one another; (3) the Radial 

 bundle, which has its tissues arranged radially about its 

 axis. 



91. The development of the fibro-vascular bundle takes 

 place in this wise : in the previously uniform primary 

 meristem there arises an elongated mass of cells, consti- 

 tuting the Procambium of the bundle; as it grows older 

 the cells, which were at first alike, become changed into 

 the vessels, fibres, and other elements of the bundle-tissues. 

 In most higher flowering plants this change begins on the 

 two sides of the bundle — i.e., on the outer edge of the 

 phloem and the inner edge of the xylem; from these points 

 the change into permanent tissue advances from both sides 

 toward the centre of the bundle. 



92. In some cases all of the procambium is changed into 

 permanent tissue, forming what is termed the closed bun- 

 dle; in other cases there is left between the phloem and 

 xylem a narrow zone of the procambium (now called the 

 cambium), forming what is known as the open bundle. 

 Closed bundles are thus incapable of further growth, while 

 open bundles may continue to grow indefinitely. 



93. The fibro-vascular bundles of leaves and the repro- 

 ductive organs are quite generally reduced by the absence 



