210 THE EVOLUTION OF PLANTS 



difference is important for comparison with other 

 groups, especially the Sphenophyllum family, to 

 which we shall come presently. 



A curious plant, Pseudobomia, has been found 

 in the Upper Devonian rocks of Bear Island in 

 the Arctic Ocean. It was a large plant, though 

 scarcely a tree, with a jointed stem and leaves in 

 whorls of about four. The leaves are much more 

 compound even than in Archceocalamites, for be- 

 sides being repeatedly forked, each division is cut 

 into a great number of fine segments; before their 

 connection with the stem was known these leaves 

 were described as the fronds of a Fern. The fruc- 

 tifications, which were in the form of long, loose 

 cones, are imperfectly known; the plant was al- 

 most certainly allied to the Calamites and its prin- 

 cipal interest lies in the much-divided, Fern-like 

 leaves. Taking Archoeocalamites also into account, 

 it looks very much as if the ancestors of the 

 Horsetail stock had well-developed compound 

 leaves, which later became reduced in some way, 

 first to the narrow simple leaves of the typical 

 Calamite and then to the mere leaf-sheaths of 

 Equisetites and the modern Equisetum. 



We thus see that the Horsetails of our fields and 

 marshes are the last survivors of a great and an- 

 cient race which reached its highest development 

 in Palaeozoic times, when it formed one of the 

 leading families of plants. In certain characters, 



