INTRODUCTION 21 



to be perforated. Thus, there may be perforated solenosteles 

 which, at a first glance, might be confused with dictyosteles ; 

 however, as soon as attention is paid to the relationship 

 between leaf traces and perforations, the distinction becomes 

 clear. When more than one stele is visible in any one trans- 

 verse section the plant is described as polystelic. Yet another 

 variant is the polycyclic stele, in which there are two or more 

 co-axial cyUnders of conducting tissue (Fig. 3G). 



All the vascular systems mentioned so far are composed 

 entirely of primary tissues, i.e. tissues formed by the matura- 

 tion of cells laid down by the main growing point (apical 

 meristem). It is customary to draw a rough distinction 

 between tissues that differentiate before cell elongation has 

 finished and those that differentiate only after such growth 

 has ceased. In the former case, the xylem and phloem are 

 described as protoxylem and protophloem. They are so con- 

 structed that they can still alter their shape and can, thereby, 

 accommodate to the continuing elongation of the adjacent 

 cells. Accordingly, it is usual for the Ugnification of proto- 

 xylem elements to be laid down in the form of a spiral, or 

 else in rings. Metaxylem and metaphloem elements, by 

 contrast, do not alter their size or shape after differentiation. 



The order in which successive metaxylem elements mature 

 may be centripetal or centrifugal. When the first xylem to 

 differentiate is on the outside and differentiation proceeds 

 progressively towards the centre, the xylem is described as 

 exarch and all the metaxylem is centripetal. When the proto- 

 xylem is on the inner side of the metaxylem and differentia- 

 tion occurs successively away from the centre, the xylem is 

 described as endarch and all the metaxylem is centrifugal. 

 A third arrangement is known as mesarch, where the proto- 

 xylem is neither external nor central and differentiation pro- 

 ceeds both centripetally and centrifugally. In Figs. 3A-3C 

 the xylem is mesarch, while in Fig. 3G it is endarch. 



In addition to primary vascular tissues, some pterido- 

 phytes possess a vascular cambium from which secondary 



