198 THE EVOLUTION OF PLANTS 



trace it back, the more important and the more 

 highly organised do we find the race to have been. 

 In other words, the evolution of the Equisetum 

 stock, from Palaeozoic times to the present, has 

 been on the whole a steady descent, and not an 

 advance. 



The Tertiary species are generally like the 

 larger Horsetails of the present day; the same 

 applies to a certain extent to the later Mesozoic 

 species; there is one form, the Wealden Equise- 

 tites Burchardti, which had tubers, like our com- 

 monest living Horsetails. The genus Equisetites 

 had leaf-sheaths like Equisetum, from which, in- 

 deed, it is often scarcely distinguishable. In the 

 Jurassic rocks larger species are met with, and 

 when we reach the Triassic, gigantic plants of 

 this family occur. The stem of Equisetites are- 

 naceus, for example, reached a thickness of eight 

 inches, with 120 leaves in a whorl. In a Rhaetic 

 (Upper Triassic) species (E. scanicus) from 

 Sweden, M. Halle made the interesting observa- 

 tion that the number of vascular strands in the 

 stem was three times that of the leaves in a whorl; 

 in modern Equisetums these numbers are equal. 

 The excess of vascular strands in the fossil shows 

 that the vascular system was more complex, 

 resembling what we find in the Palaeozoic tree- 

 Horsetails. At the same time, Equisetites already 

 had the leaf-sheaths of an Equisetum and the 



