624 STUDIES IN FOSSIL BOTANY 



in certain lines of descent, and that the organ is thus 

 of considerable morphological importance. 



On the whole of the evidence there can be no doubt 

 that a real affinity exists between the Sphendphyllales 

 and the Equisetales, the former being clearly the more 

 primitive class of the two, or rather the representatives 

 of a more primitive stock, for the known Sphenophylls 

 are evidently specialised in various directions. The 

 arguments for the aquatic habit of Sphenophyllum do 

 not hold good, as already pointed out (p. 88) ; the 

 great development of the wood relatively to the size of 

 the stem is the reverse of what one would expect in a 

 water-plant ; on the other hand Professor Seward's sug- 

 gestion that Sphenophyllum may have been " a slender 

 plant which flung itself on the branches and stems of 

 stronger forest trees for support," : agrees well with the 

 habit and structure. The occurrence of extensive 

 secondary growth in this genus is interesting, as show- 

 ing that this character was not necessarily correlated, 

 even in Palaeozoic times, with an arborescent habit. 



The enormous development of the Equisetales in 

 early geological periods, compared with their reduced 

 condition at the present day, is a striking fact. It is 

 of interest to consider in what respect the gigantic 

 Palaeozoic Horsetails differed from their humbler 

 successors. 



One great and obvious difference was the formation 

 of secondary wood and bast and of periderm. Starting 

 with a structure in stem and root essentially like that 

 of a recent Equisetum, the Calamariaceae, by the activity 

 of a normal cambium, produced new wood and phloem, 



1 Fossil Plants, vol. i. 1898, p. 392. 



