GENERAL RESULTS— SPHENOPSIDA 625 



to an indefinite extent, precisely as in a Gymnospermous 

 tree, and replaced their primary cortex by a broad 

 zone of secondary periderm. It was one of Williamson's 

 greatest services to science that he always, in the face 

 of much opposition, insisted on the true " exogenous 

 growth " of the Calamites, while maintaining with equal 

 decision their Cryptogamic nature. He thus established 

 one of the most striking instances of homoplastic 

 modification, for the close agreement in these anatomical 

 characters between certain Cryptogams and Phanero- 

 gams is not, in itself, any proof of affinity. 



As has already been pointed out, it is probable that 

 the microphyllous character is not a primitive one in 

 Equisetales, and that the leaves are reduced from a 

 larger and more complex type, as seen in Archaeo- 

 calamites and Pseudobornia. 



The old opinion of the French school that some 

 Calamariaceae were seed-bearing plants has not been 

 substantiated by later work. Heterospory, however, 

 appears in a perfectly well-marked form in some of 

 , the fructifications, though the differentiation of the two 

 kinds of spore was not so extreme as in heterosporous 

 Lycopods or the recent Water-ferns. In other Cala- 

 marian strobili the evidence is all in favour of homo- 

 spory ; the abortion of certain spores in sporangia 

 of this type may have prepared the way for the 

 heterospory of the more advanced members of the 

 group (p. 57). In this - respect the Equisetales seem 

 to have reached a higher level than the Sphenophyllales, 

 among which only doubtful indications of incipient 

 heterospory have so far been detected. 



Considering the high development of the Palaeozoic 



