200 THE EVOLUTION OF PLANTS 



We are, unfortunately, without any petrified 

 specimens of Mesozoic Horsetails, so our knowl- 

 edge of their structure is confined to the coarser 

 features. In the Palaeozoic there is an immense 

 wealth of material of all kinds petrifactions, 

 impressions, and casts, so that our knowledge of 

 structure is extensive, though there remains the 

 inevitable difficulty of piecing the fragmentary 

 specimens together. However, we can give quite 

 a sufficiently good idea of the group as it was 

 then, for our present purpose. 



We may conveniently call the Palaeozoic tree- 

 Horsetails "Calamites," from their most im- 

 portant genus Calamites. The name was origi- 

 nally given them from a supposed resemblance to 

 Reeds (Calamus, a reed), but Suckow, in 1784, 

 for the first time compared them with Equisetum. 

 The Calamites thus form a division of the great 

 class Equisetales, of which our modern Horse- 

 tails and the Mesozoic Equisetites represent the 

 later development. Some of them, if not all, were 

 trees, perhaps not much inferior in height to the 

 giant Lycopods. The tall upright stems appear 

 to have sprung, in many cases, from rhizomes 

 which crept in the mud. The habit seems to 

 have been not altogether unlike that of some of 

 the living Horsetails on an immense scale; the 

 freely branched forms must have been most 

 graceful trees, crowned with a multitude of 



