142 EVOLUTION OF PLANTS 
as to form a cone orstrobilus. 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 
twenty 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. e 
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 Equisetineze were the species of 
Calamites, 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 
