THE VARIETY OF PLANT LIFE 



99 



abundant remains accumulated in vast prehistoric swamps to 

 form the organic deposits which later turned to coal and thus 

 became important sources of fuel. 



Fig. 69. — The horse-tails are fern relatives with segmented green stems, 

 lacking leaves. Reproductive stems on left, terminated by cone-like structures 

 producing the spores. 



The Naked-seed Plants 



All of the preceding phyla of plants, irrespective of the com- 

 plexity of their maintenance organs, reproduce by structures 

 which are protected little or not at all against environmental 

 factors. But in this last phylum we are to consider — the Sper- 

 MATOPHYTA — the cmbryo is always developed within that protec- 



