Manchester Memoirs, Vol. xli. (1896), No. !• 7 



Hence it may be regarded as the innermost layer of the 

 cortex, or the phloeoterma of Strasburger.* 



The Central Cylinder or Stele. 



If the layer of cells just referred to be really the 

 phloeoterma, then all the tissues within it must be those 

 which constitute the stele. On this view we can dis- 

 tinguish, more or less readily according to the state of 

 preservation of the specimens, (i.) a pericycle, (ii.) phloem, 

 and (iii.) xylem. 



The pericycle is seldom sharply defined, but when it 

 is, it consists for the most part of a single layer of thin- 

 walled cells, as large as those of the endodermis, but 

 more variable in size and less regular in shape and 

 arrangement. The cell contents have altogether dis- 

 appeared or are too vague for determination. 



The phloem is composed of thin-walled, empty, and 

 irregularly-shaped elements of different sizes, in which 

 little differentiation has so far been detected. It forms 

 a complete zone round the xylem, but the breadth of 

 the zone is not always uniform. The reference of this 

 zone to the phloem is based more upon its topographical 

 position than upon its structure, but in one preparation, 

 No. 104 (Fig. 1), larger elements are embedded in it, 

 which in form, position, and arrangement resemble the 

 sieve-tubes seen in transverse sections of recent Ferns. 



The xylem occupies the centre of the stele, and, as 

 was pointed out by Williamson, the larger elements are 

 usually at the periphery, while the smaller are in the 

 centre (Figs. 1 and 2). In Williamson's figuret the larger 

 and smaller elements are well differentiated, and this is 

 not an unusual state of affairs, though in some sections 

 the difference is less sharply marked (Fig. 1). In the 



* Histologische Beitrage, III. 

 f Phil. Trans. 1878. Plate XXIV., Fig. 80. 



