THE FIBRO-VASCULAR SYSTEM. 121 



bundle, Avhich lias its tissues arranged concentrically around 

 one another ; this is the bundle of the stems and leaves of 

 ferns (with a few exceptions), Selaginellae, and a few excep- 

 tional cases in Phanerogams (Figs. 96, 97, 98, 100) ; (3) the 

 Radial bundle, which has its tissues arranged radially about 

 its axis ; such a bundle occurs in the stems of Lycopodium, 

 and it is the primary bundle of the roots of most Pterido- 

 phytes and Phanerogams (Figs. 101, 104, 105, 108). 



160. The development of the fibro-vascular bundle takes 

 place in this wise: in the previously uniform Primary Meris- 

 tem there arises an elongated mass of cells, constituting 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 the 

 fibro-vascular bundle of the stems and leaves of Gymno- 

 sperms and Dicotyledons 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. In some cases (e.g., in the leaves) 

 all of the procambium is changed into permanent tissue, 

 forming what is termed the closed bundle; in other cases 

 there is left between the phloem and xylem a narrow zone 

 of the procambium (now called the Cambium), forming 

 ivhat is known as the open bundle. 



151. In the stem and leaf bundles of Monocotyledons 

 the development of procambium into permanent tissue is 

 essentially as in Dicotyledons and Gymnosperms, with this 

 difference, that here they all become closed. In Pteridophytes 

 .and the roots of Phanerogams the development, while agree- 

 ing in general with the foregoing, is quite different as to de- 

 tails ; all are closed, unless those in. the roots of Dicotyledons 

 and Gymnosperms should be shown to be exceptions. , 



152. The fibro-vascular bundles of leaves and the re- 

 productive organs are quite generally reduced by the absence 



so-called phloem portions. Such bundles are considered by De Bary to 

 be variations of the collateral form, and he designates them as bi-col- 

 Jateral bundles. 



