64 LYCOPODIALES [CH. 



The early appearance of the cambial activity on the edge of the 

 vascular cylinder is shown in fig. 133, C, which represents part 

 of a transverse section of a young stem. A leaf-trace, It, is in 

 connexion with the primary xylem, x', which consists of short 

 tracheids, often represented only by their spiral or reticulately 

 thickened bands of lignified wall, and scattered parenchyma. 

 Some of the radially elongated cells on the sides of the leaf- 

 trace are seen to be in continuity on the outer edge of the 

 stele, at st, with flattened elements, some of which are sieve- 

 tubes. The position of a second leaf-trace is shown at It'. 

 External to the sieve-tubes the tissue consists of radially 

 arranged series of rectangular cells, some of which have already 

 assumed the function of a cambium (c). The tissue produced 

 by the cambium on its inner edge consists of a varying amount 

 of secondary xylem composed of very short spiral tracheids; 

 a few of these may be lignified (fig. 133, A, cc') while others 

 remain thin. 



Phloem elements, recognisable by the presence of a thickened 

 reticulum enclosing small sieve-areas (fig 133, B, s) are fairly 

 abundant, and for the rest this intracambial region is composed 

 of thin-walled parenchyma. In longitudinal section these 

 tissues present an appearance almost identical with that 

 observed in a transverse section. Fig. 133, B represents a 

 longitudinal section, through the intracambial zone and the 

 edge of the stele, of a younger stem than that shown in 

 fig. 133, A. Most of the radially disposed cells internal to the 

 meristematic region are parenchymatous without any distinctive 

 features ; a few scattered sieve-tubes (s) are recognised by their 

 elliptical sieve-areas and an occasional tracheid can be detected. 

 The cambium cuts off externally a succession of segments 

 which constitute additional cortical tissue (fig. 133, A, cr) 

 of homogeneous structure, composed of parenchymatous cells 

 containing starch and rich in intercellular spaces. As the stem 

 grows in thickness the secondary cortex reaches a considerable 

 breadth and the superficial layers are from time to time 

 exfoliated as strips of dead and crushed tissue (fig. 133, A, b). 

 The diagrammatic sketch reproduced in fig. 133, A, serves to 

 illustrate the arrangement and relative size of the tissue-regions 



