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A TEXTBOOK OF THEORETICAL BOTANY 



frequently surrounded by sclerotic sheaths, complete or partial, which are 

 quite independent of the phloem but are otherwise analogous to the general 

 pericyclic fibre-sheath in Dicotyledons. It is difficult to see why both types 

 of sheath should not be classed under the same name, as the bundle sheaths 

 in Monocotyledons are particularly characteristic of those stems in which 

 no general pericycle exists. 



Each vascular bundle is a complex of tissues, partly conducting elements, 

 partly storage elements and partly strengthening elements. 



Haberlandt gives the following analysis of the components :— 



Bast Fibres ^ 



' Leptome 



Vascular 

 Bundle 



Composite | 



Conducting < 

 Strand 



Hadrome <J 



f Sieve Tubes and 

 I Companion Cells 



I Phloem 



1^ Parenchyma 



Vessels and 

 Tracheids 



-Phloem 



I^Xylem Parenchyma 



Xylem (Libriform) 

 Fibres 



^Xyk 



Fibro- 

 vascular 

 Bundle 



Conducting 

 Parencyhnia 

 of Bundle 

 Sheath 



While xylem and phloem denote the whole of the two main tissue systems 

 in the bundle, Haberlandt applies the terms hadrome and leptome to the 

 two sets of conducting elements respectively, to distinguish them from the 

 fibres present. 



The most general arrangement of tissues in the bundle is the collateral, 

 in which both tissues lie on the same radius, the phloem constituting approxi- 

 mately the outer half of each bundle and the xylem the inner half. We have 

 described above the first appearance of protoxylem at the inner side of the 

 procambial strand and of protophloem at the outer side. The further 

 differentiation of each tissue from procambial cells is centrifugal in the case 

 of the xylem and centripetal in the phloem. In Monocotyledons generally, 

 and in some reduced herbaceous types among the Dicotyledons, the whole 

 of the procambium is thus differentiated and no further addition of vascular 

 tissue is possible. Such bundles are called " closed." Among Dicotyledons, 

 however, it is usual for a zone of cells about half-way across the procambium 

 to become cambium, which remains meristematic and thereafter adds cells 

 continuously to both xylem and phloem in radial files. This change marks 

 the beginning of what is called secondary thickening. As we have previously 

 pointed out, the organization of cambium may begin so early that every 

 part of the vascular tissue is formed from it {e.g., Helianthiis), and the radial 

 arrangement rules from the beginning. Such cases show that a clear 

 distinction of primary, or procambial growth, from secondary, or cambial 

 growth, is not always possible. 



