Evolution of Plants 347 



are surviving ancient types, representative of a large assembly 

 of now extinct forms, but which once were spread over a great 

 part of the world. 



While some of the Lycopodinese remained of low herbaceous 

 or semi-shrubby habit, and started lines of growth that have 

 culminated in the club mosses and selaginellas of our existing 

 flora, others attained to giant stature and bulky habit, as in 

 Lepidode?idron, Sigillaria, and allied genera On the principle 

 of steady evolutionary progression, the origin of these must 

 be looked for well back into silurian times, for during the 

 devonian period they were already abundant. They reached 

 a remarkable climax, however, alike in size, in number of 

 uidividuals, and in widespread distribution, during the later 

 carboniferous epoch, since it is to their softened and pulpified 

 remains that we mainly owe our large beds of coal. 



Several noteworthy points in their structure deserve special 

 mention. First, in the evolution of their large stems, a close 

 similarity or parallelism and yet a varying diversity were 

 established, that caused them to simulate, and also to differ 

 from, those of polyciliate Cycadofilices, Cycadacese, Pinacese, 

 Gnetaceae, and also dicotyledons, while far removed apparently 

 from these in systematic affinity. Second, in their leaf struc- 

 ture a close agreement is showTi between them and existing 

 lycopodineous plants. Third, in sporangial formation they 

 all evolved elongated "strobili" that closely resembled those of 

 living lycopods. 



But here, as in the great polyciliate groups above cited, 

 advance from a homosporous state, such as is retained in 

 Lycopodium still, to a heterosporous condition occurred, so that 

 microspores and megaspores were produced as in Lepidodendron. 

 Fourth, in those carboniferous types that produced the fruits 

 known as Lepidocarpon, a further evolutionary advance took 

 place, so that structures fundamentally resembling seeds were 

 built up. In this respect again a parallelism with the above 

 polyciliate groups is sho\\ni. 



The genus Selaginella, that like Lycopodium is now world- 

 wide as a terrestrial group, is represented by about five hundred 



