652 A TEXTBOOK OF THEORETICAL BOTANY 



The Relationships between Phylloids and Phyllomes 



If now we turn to the Ferns we find leaves which are so much larger than 

 phylloids, in relation to the axis which bears them, that they might well be 

 structures of a different order. That this is in fact the case has been suggested 

 by several morphologists, notably Lignier and Tansley. Stripped of detail 

 the theory depicts these large leaves, which dominate the form of the plant, 

 as having evolved from branches of the microphyllous type, complete with 

 their phylloids, so that they are homologous with syntelomes, not with single 

 telomes. Several anatomical facts support this view. Firstly the fern leaf 

 has massive traces, which leave gaps in the stem stele, as we have pointed out 

 above. In this they correspond to the branches rather than to the leaves of 

 Lycopsida, and it is this which has caused them to be labelled megaphyllous. 

 Secondly they show apical growth, sometimes greatly prolonged. Thirdly 

 the leaves of some of the most ancient fossil Ferns have a vascular structure, 

 especially at the base, which closely resembles that of a stem, while in some 

 cases they were not flattened but radially symmetrical. The flattened form 

 and expanded lamina of the fully evolved megaphyllous leaf are supposed to 

 have been produced by broadening and fusion of the phylloids, like the 

 " webbing " of a duck's foot. The amount of fusion which has contri- 

 buted to the formation of the lamina seems to have varied greatly, and 

 in the most complex pinnate types the pinnules are probably still separate 

 phylloids. 



As the original syntelome must have been of the mixed type the sporangia 

 naturally find a place on the megaphyll, and the distinction drawn by Bower 

 between Fern types with marginal and with superficial sporangia may have 

 originated in a dift'erence in the branching of the original syntelome. Possibly 

 the indusium, which is a somewhat late development in evolution, may 

 represent a modified phylloid structure. 



The indications are strongly in favour of this view of the megaphyll as 

 the homologue of a branch system, and against the idea that it has developed 

 from the microphyll by simple increase of size. It is a structure of higher 

 order morphologically than the phylloid and is called, distinctively, a 

 phyllome. The leaves of all the Spermatophyta agree with this pattern and 

 are phyllosiphonic, which is the justification for Jeffrey's extension of the 

 term Pteropsida to cover everything from the Fern upwards. There is, in 

 fact, no doubt that the Spermatophyta have come from megaphyllous fern- 

 like ancestors, and may be ranked as belonging to the Pteropsida rather than 

 the Lycopsida, in spite of the early evolution of seed-like structures in the 

 latter group. 



The root, like the leaf, is no doubt also a specialized portion of the 

 primitive axis. Even in the Liverworts there occur leafless downward 

 extensions of the axis below the ground. This is also found in the Psilopsida 

 and in some Lycopsida, and between these leafless rhizomes and true roots 

 no clear line can be drawn. As is well known, the stelar anatomy of the stem 

 in Lycopodhtm is essentially that of the root in higher plants, which suggests a 



