166 THE EVOLUTION OF PLANTS 



well marked, and this is no wonder, for we have 

 direct evidence that this group has been re- 

 produced in the same way ever since Palaeozoic 

 times; indeed, its present highly -developed con- 

 dition had been fully attained before the close of 

 that period. 



The other living genus of heterosporous Lyco- 

 pods, Isoetes, is totally different in habit from 

 Selaginella. There are about fifty species, which 

 are mostly water-plants; one is fairly common in 

 the lakes of our own country, growing entirely 

 under water; another species, one of the two 

 which grow on dry land, extends to the Island of 

 Guernsey, and so is reckoned as a British plant. 

 On the whole, there is not much difference in 

 structure between the aquatic and the terrestrial 

 species; the genus seems to have long hovered 

 between land and water. 



In all the species, the stem is extremely stunted, 

 running to breadth more than height. The 

 crowded leaves are long and quill-like. There is 

 no constant distinction between vegetative and 

 fertile leaves, and consequently no formation of 

 a cone. 



The stem is deeply grooved on its under sur- 

 face, the grooves dividing it into two, three, or 

 occasionally four lobes; the roots arise from the 

 sides of the grooves. 



The most interesting feature in the anatomy 



