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J/ jy U Tli S TJi UC T URE. 



tissue, the wide tubes (vessels) with variously marked 

 walls, formed by the disappearance of the cross-partitions 

 between cells placed end to end, and more or less short- 

 celled tissue or parenchyma. The phloem is likewise 

 made up of three constituents: the long, thiek-walled, 

 tlexible cells called bast cells, which correspond to the 

 fibrous tissue of the xylem ; the wide, thin-walled sieve- 

 cells corresponding to the vessels ; and a certain amount 

 of thin-walled parenchj^ma. 

 The fibro-vascular bundles, as thej' are called, have their 

 origin in the mei'istem of the growing point. This meri- 

 stem is at first uniform, but soon groups of long cells 

 arise in it, and these are then known as procambium, to 

 distinguish them from the surrounding ground-tissue. 

 This procambium is gradually converted into the fibro- 

 vascular bundles. 

 In dicotjdedonous plants, the fibro-vascular bundles are more 

 or less wedge-shaped, as shown in Fig. 279. The inner 

 part of each, bundle consists of xylem and the outer of 

 phloem, and between the xylem and the phloem there is 

 a layer of meristem, known as the cambium. The soft 

 cells of the cambium divide, and the new cells thus con- / 

 tinually being formed become modified on the one hand 

 into tissues which increase the thickness of the xylem, 

 and, on the other, into tissues which are added to the 

 phloem. Later on cambium cells are formed in the 

 ground-tissue between the bundles, thus linking together 

 the cambium-layers of the various bundles, and forming 

 a continuous ring. The links are then known as inter- 

 fascicular cambium, that of the bundles themselves 

 being the fascicular. Bundles of this kind, character- 

 ized by the cambium-layer, and so caj^able of continuous 

 enlargement, are called open bundles. Fig. 280 illus- 

 trates the structure of the dicotyledonous stem. M is 

 the pith ; E, is the cortex ; x, xylem, and i>i phloem of 

 each bundle ; //?, wood formed by fascicular cambium ; 

 ifh, wood formed by interfascicular cambium ; ifp, inter- 

 fascicular phloem ; 6, b, b, bast-fibres ; fc, fascicular, and 

 ic, interfascicular cambium. The external ring repre- 

 sents the epidermis. 

 In monocotyledons, on the other hand, there is no cambium- 

 layer, and consequently the bundle when once formed is 

 incapable of further increase, and so is said to be closed. 

 Fig. 281 is a repi-esentation of the cross-section of an 

 endogenous stem in which many of these closed bundles 

 are visible. Of course in such stems no bark is formed. 

 It has been explained that in the exogenous stem the xylem 

 occupies one side of the fibro-vascular bundle, while the 

 phloem occupies the other. In the closed bundles of 



Fis. 279. 



Fiir. 280. 







Fiir. L'm. 



