l8 THE MORPHOLOGY OF PTERIDOPHYTES 



vegetative organs are nevertheless of considerable import- 

 ance in classifying pteridophytes, for the shape, size, arrange- 

 ment and venation of leaves (and even presence or absence 

 of leaves) are fundamental criteria. It so happens that it is 

 difficult, if not impossible, to devise a definition of the term 

 'leaf that is entirely satisfying, but, for practical purposes, 

 it may be said that among pteridophytes there are two very 

 different types of leaf, known respectively as megaphylls and 

 microphylls. The famiUar fern frond is an example of the 

 former; it is large, branches many times and has branching 

 veins. By contrast, microphylls are relatively small, rarely 

 branch and possess either a limited vascular supply or none 

 at all ; the leaf trace, if present, is single and either remains 

 unbranched within the microphyll or, if it branches at all, it 

 does so to a limited degree and in a dichotomous manner. 



As might be expected, the leaf traces supplying micro- 

 phylls cause little disturbance when they depart from the 

 vascular system (stele) of the parent axis, whereas those 

 supplying megaphylls are usually (though not invariably) 

 associated with leaf gaps. A stele without leaf gaps is termed 

 a protostele, the simplest type of all being the soUd proto- 

 stele. Fig. 3A illustrates its appearance diagrammatically as 

 seen in transverse section. In the centre is a solid rod of 

 xylem which is surrounded by phloem and then by pericycle, 

 the whole stele being bounded on the outside by a con- 

 tinuous endodermis. Another variety of protostele is the 

 medullated protostele, illustrated in Fig. 3B. In this the 

 central region of the xylem is replaced by parenchyma. Yet 

 other varieties of protostele will be described as they are 

 encountered in subsequent chapters. Steles in which there 

 are leaf gaps are known as dictyosteles, if the gaps occur 

 frequently enough to overlap, and as solenosteles if they are 

 more distantly spaced. Fig. 3C is a diagrammatic represen- 

 tation of a solenostele as seen in transverse section passing 

 through a leaf gap. The most remarkable feature is the way 

 in which the inside of the xylem cyhnder is lined with 



