35.] 



CHAPTER II. THE TISSUES. 



203 



parenchyma-cells, bast or wood ; thin-walled fibrous cells) : but 

 more commonly the product differs very materially from the 

 desmogen-cell, being very much wider (e.g. tracheae), or very much 

 longer longitudinally (wood- and bast-fibres), or very much longer 

 radially (e.g. cells of medullary ray) ; that is to say, the develop- 

 ment of the desmogen-cell into permanent tissue is generally 

 accompanied by very considerable growth. 



The radial and tangential divisions of 

 the cambium-cells and of the desmogen- 

 cells take place in such a manner that the 

 products are, at first, arranged in very 

 definite radial rows (Fig. 145). When 

 the resulting tissue consists of elements 

 which are for the most part essentially 

 alike, this regular radial arrangement per- 

 sists in the permanent tissue ; for instance, 

 in the wood of Conifers (Fig. 150), which 

 consists alinost exclusively of tracheids ; 

 but where some of the elements (as gener- 

 ally in the wood of Dicotyledons) attain a 

 much greater size (as seen in transverse 

 section, Fig. 139), the original radial ar- 

 rangement is lost. 



In those cases in which the permanent 

 tissues consist of very long or very wide 

 fibres or vessels, it is evident that the 

 relative position of the original desmogen- 

 cells must have undergone considerable 

 change in the course of development ; the 

 long fibre is in contact, longitudinally, 

 with a greater number of cells than was 

 the original desmogen-cell ; and similarly, 

 the wide trachea touches, at its circum- 

 ference, a larger number of cells than did 

 the desmogen-cell, originally, from which 

 the segment of the vessel was developed. 

 This gradual change of relative position 

 constitutes what is termed sliding -growth ; it is the expression of 

 the independent growth of each desmogen^cell, in the course of its 

 development into the particular element of the permanent tissue 

 which it is destined to form. This process is by no means confined 



FIG. 153. A Desmogen-cells 

 seen in tangential section. S 

 Tracheid seen from outside. 

 C woody-fibre ; and D vertical 

 row of wood - parenchyma, 

 cells seen in section, from the 

 Oak ; isolated by maceration. 



