142 EVOLUTION OF PLANTS 



as to form a cone or strobilus. In their development, the 

 sporangia are much like those of the eusporangiate ferns, 

 but in their method of opening they are more like the 

 sporangia (anthers) of the flowering plants. The early 

 stages in the development of the spores follow the reg- 

 ular type found in all Archegoniates, but the ripe spore 

 is very peculiar, being provided with curious appendages 

 (elaters), formed by a splitting of the outer membrane 

 (Fig. 36, F). 



The existing species of Equisetum differ a good deal 

 in size, varying from small forms not more than ten to 

 twent}^ centimetres in height, to the giant of the genus, 

 E. giganteum of tropical America, which may reach a 

 height of ten metres, with a stem diameter of two or 

 three centimetres. In spite of these differences in size 

 they all agree closely in the structure of the sporo- 

 phyte. 



While all the living members of the class can be 

 placed in a single genus, it is different with the numer- 

 ous fossil forms which are known. Especially during 

 the Carboniferous epoch was there a rich development of 

 this peculiar group of plants, which formed a conspicu- 

 ous feature in the vegetation, where they were repre- 

 sented by numerous genera and species. The modern 

 genus Equisetum probably extends back to the coal- 

 measures, where it was associated with numerous extinct 

 types which reached a far greater size and complexity. 

 The largest of the fossil Equisetinese were the species of 

 Calaraites, which attained tree-like dimensions and whose 

 remains show evidences of a secondary thickening of 

 the vascular bundles of the stem, like that in the trunks 

 of existing trees. It is interesting that a trace of this 



