SECONDARY THICKENING IN LILIIFLORAE 687 



The cambial zone gives rise, on both sides, to a variety of secondary 

 tissues ; as in the case of Gyninosperms and Dicotyledons, these may 

 be classified topographically under the heads of extra-cambial and intra- 

 cambial tissues. The extra-cambial portion appears relatively late and 

 is never voluminous ; it consists of thin-walled parenchyma, containing 

 a certain number of crystal-sacs. The woody cylinder, which is much 

 more bulky and complex, represents a peculiar modification of the primary 

 Monocotyledonous vascular arrangement, and thus differs very markedly 



A 



, 



Fig. 2S3. 



A. Part of a T.S. through the trunk of Dracaena marginata, illustrating the forma- 

 tion of secondary vascular bundles ; v. cambium ; r, cortical parenchyma. B a, B b. 

 Early stages in the development of the bundles. 



from the analogous structure in Dicotyledonous stems ; it is com- 

 posed of numerous secondary vascular bundles embedded in a mass of 

 secondary parenchyma (Fig. 283 a). The demarcation of the vascular 

 bundles begins within the cambial zone, where repeated longitudinal 

 divisions give rise to procambial strands of various sizes ; as the cambial 

 zone progresses outwards, these procambial strands are left behind and 

 become transformed into permanent vascular tissue in centrifugal order. 

 The smaller procambial strands may be seen to be derived from single 

 cells in any particular cross-section (Fig. 283 b, a): the larger ones, on 

 the other hand, originate from several mother-cells, belonging to at 

 least two different radial series. In the formation of a procambial 

 strand, the initial divisions are always tangential; these are first succeeded 

 by radial divisions (Fig. 283 b, a, b), but, once sliding growth begins, 

 walls may arise in any direction. The primary leaf-trace bundles 

 present no peculiar features ; ordinarily they are collateral, but in 



