162 THE EVOLUTION OF PLANTS 



A very general feature in Selaginella is the 

 presence of leafless branches, which grow down 

 from the forks of the stem and produce roots as 

 soon as they touch the ground. Whether the 

 root-bearers are themselves of the nature of 

 stems or roots is still disputed; the existence of 

 these intermediate organs is of considerable 

 interest, and characteristic in one form or an- 

 other of many Lycopods. 



The spore-bearing organs, throughout the 

 genus, are in the form of well-defined cones, 

 the fertile leaves often having a different arrange- 

 ment from that of the ordinary vegetative 

 leaves. 



The structure of the stem is extremely diverse 

 in different species. In the simplest forms 

 (S. spinosa, for example) there is a single cen- 

 tral column of wood, developing from without 

 inwards, and surrounded by bast; in a large 

 number of species, however, the stem contains 

 two or more separate vascular cylinders, of 

 varying form; in some the arrangement becomes 

 highly complex. The leaves, however, have 

 essentially the same simple structure as in 

 Lycopodium. 



A very characteristic feature is the presence, 

 on the upper side of each leaf, near its base, of 

 a tongue-shaped outgrowth, the ligule, which 

 serves to secrete mucilage and keep the young 



