20 THE MORPHOLOGY OF PTERIDOPHYTES 



phloem, pericycle and endodermis, as if these tissues had 

 'invaded' the central parenchymatous region (though, need- 

 less to say, the developmental processes do not involve any 

 such invasion). Fig. 3E illustrates the structure of a dictyo- 

 stele in which three leaf gaps are visible in the one transverse 

 section. Frequently it happens that each leaf gap is associ- 

 ated with the departure of several leaf traces to the leaf, but 

 in this example, for clarity, only one trace is shown supply- 

 ing each leaf. The remaining portions of the stele are 

 referred to as meristeles and, although in transverse section 

 they appear to be unconnected, when dissected out and viewed 

 as three-dimensional objects they are seen to form a network. 

 Figs. 3D and 3F are perspective sketches of a solenostele 

 and a dictyostele, respectively, from which the surrounding 

 cortex and ground tissue have been removed in this way. 



It must be pointed out at this stage that some morpholo- 

 gists use a different system of terminology and group to- 

 gether the medullated protostele and the solenostele as 

 varieties of so-called siphonosteles, on the grounds that 

 each has a hollow cylinder of xylem. The former they des- 

 cribe as an ectophloic siphonostele, because the phloem is 

 restricted to the outside of the xylem, and the latter they 

 describe as an amphiphloic siphonostele, because the phloem 

 lies both outside and inside the xylem. This practice, how- 

 ever, has disadvantages. First, it tends to exaggerate the 

 difference between the solenostele and the dictyostele — a 

 difference that reflects little more than a difference in the 

 direction of growth, for where leaves arise at distant inter- 

 vals on a horizontal axis their leaf gaps are unUkely to over- 

 lap, whereas leaves on a vertical axis are often so crowded 

 that their leaf gaps must overlap. Secondly, it overlooks the 

 fundamental distinction between the solenostele and the 

 medullated protostele — a physiological distinction depend- 

 ing on the position of the endodermis. 



When gaps occur in a stele without any associated leaf 

 traces, they are described as perforations and the stele is said 



