146 



PART II. ANATOMY AND HISTOLOGY. 



[ 35 



The development of the young cell into one or other of the 

 various forms of permanent tissue already described, may be 

 either accompanied or unaccompanied by cell-division. In the 

 former case, the divisions may be transverse or longitudinal ; the 

 cell undergoes transverse division when the 

 product is a row of short cells (e.g. wood- 

 parenchyma, Fig. 116 Z>, and Fig. Ill C: 

 bast-parenchyma ; secondary medullary 

 rays, wood-vessels with short segments) : 

 the cell generally undergoes longitudinal 

 division once or twice, by tangential walls, 

 soon after it has been cut off from the 

 cambium ; but this does not take place in 

 the line of the medullary rays, where the 

 radial diameter of the young cells is greater 

 than it is near the bast or the wood : again, 

 the young cell may undergo longitudinal 

 division in a plane other than the tan- 

 gential, as for instance the longitudinal 

 division of the mother-cell, which separ- 

 ates the sieve-tube-segment from the com- 

 panion-cell in the bast of Angiosperms. 



The developing cell may retain its 

 original form and size (e.g. small medul- 

 lary rays ; rows of parenchyma-cells, bast 

 or wood ; thin-walled fibrous cells) : but 

 more commonly the mature product differs 

 very materially from the young 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 development of the young cell into 

 permanent tissue is generally accompanied 

 by very considerable growth. 

 The radial and tangential divisions of the cambium-cells take place 

 in such a manner that the products are, at first, arranged in very 

 definite radial rows. When the resulting tissue consists of ele- 

 ments which are for the most part essentially alike, this regular 

 radial arrangement persists in the permanent tissue ; for instance, 

 in the wood of Conifers (Fig. 113), which consists almost exclu- 



FIG. 116. A Developing 

 vascular cells, derived from 

 the cambium, seen in tan- 

 gential section. JB Tracheid 

 seen from outside. C Woody 

 fibre ; and J) vertical row of 

 wood-parenchyma-cells seen 

 in section, from the Oak ; 

 isolated by maceration. 



