SECONDARY XYLEM 659 



most portions of the extra-cambial tissue which usually consist of 

 functionless, dead or moribund cells every year. This process of 

 desquamation is most conspicuous where the entire secondary phloem 

 of the preceding year is cast off ( Vitis) ; in such cases the large 

 wastage of tissue is made good by a correspondingly generous annual 

 addition of secondary phloem. 



C. THE SECONDARY XYLEM OR WOODY CYLINDER (INTRA-CAMBIAL 



SECONDARY TISSUE). 351 



1. The component clematis of the secondary xylem. 



The secondary xylem, woody cylinder, or intra-cambial secondary 

 tissue comprises both mechanical and conducting elements. In their 

 primary condition, Dicotyledonous stems are often strengthened by means 

 of an extra-cambial fibrous cylinder (bast-cylinder) ; when secondary 

 growth in thickness becomes active, the provision of mechanical 

 strength largely devolves upon the intra-cambial wood fibres (libriform 

 fibres). The reasons underlying this abnormal disposition of the 

 mechanical elements are fairly evident. In the case of an organ 

 which is undergoing secondary growth in thickness, the stereome 

 cannot remain a permanent part of the organ, unless it is situated 

 on the inner side of the cambial cylinder. But the cambium also 

 produces the water-conducting elements, as well as a part of the carbo- 

 hydrate-conducting tissue, on its inner side ; hence the mechanical and 

 conducting systems amalgamate to form the composite structure known 

 as the woody cylinder. Since the two systems are no longer sharply 

 separable topographically and anatomically, it is not surprising that 

 their physiological individuality should also be lost to some extent. 

 The leading anatomical features of the various tissues, in fact, no 

 longer correspond exactly to their several functions ; one tissue-system 

 may undertake, as a subsidiary function, the work which properly belongs 

 to another. Ultimately, indeed, it may become impossible to discriminate 

 between principal and subsidiary functions ; anatomically, this physio- 

 logical compromise finds expression in the appearance of various cell- 

 forms which are intermediate between typical skeletal and conducting 

 elements. 



Typically, the secondary wood is made up of the following diverse 

 elements 352 : first, mechanical cells in the shape of wood-fibres (libri- 

 form cells) ; secondly, relatively thin- walled vessels and tracheides, 

 which represent the water-conducting tubes : and, thirdly, xylem- 

 parenchyma, serving partly for the storage and partly for the conduction 

 of non-nitrogenous plastic materials. Closely associated with these 

 constituents of the wood proper are the cells of the xylem-rays. 



