60 THOMAS HICK, B.A., B.SC, A.L.S. 



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 disappeared 

 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 figure 1 

 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 majority of the transverse 

 sections I have seen, the central elements include one or more groups 

 of small ones, which resemble in appearance the protoxylems of recent 

 vascular bundles. When two such groups are met with, the appearance 

 is as if a single group had, in some way, been divided. When three, 

 four, or five groups are met with, as is often the case, they are arranged 

 symmetrically round the centre, and look as though they had 

 originated by division from a smaller number. 



As to the nature of the elements of the xylem, the sections at my 

 disposal do not enable me to speak altogether without reservation. The 

 larger are probably tracheides, as good longitudinal sections, No. 127 

 for instance, show. In this section the wall markings are scalariform, 

 the pits stretching for the most part from one angle to another. But 

 in some slightly oblique transverse sections, represented by No. 114, 

 the pits appear to be much less elongated and approach an elliptical 

 form. 



The case of the smaller elements is not quite so certain. In the 

 longitudinal section just referred to, they present themselves as 



iPhil. Trans. 1878. Plate XXIV., Fig. 80. 



