PTERIDOPHYTA 155 



ing type of Pteridophytes, which has largely crowded out 

 the more primitive Eusporangiates. Certain genera, 

 like Osm unda, are probably intermediate in character 

 between the two. The Leptosporangiatae have diverged 

 further and further away from the parent stock, reach- 

 ing their highest expression in the heterosporous forms 

 like Marsilia and Salvinia. It is doubtful whether the 

 latter have given rise to any higher types. 



It is possible that another important group of plants, 

 the Angiosperms or highest of the flowering plants, has 

 arisen from the Eusporangiatse. There are numerous 

 striking resemblances in the structures of the two 

 groups, and it is possible that the peculiar genus Isoetes 

 may represent a transitional condition. The relation of 

 the latter to the ferns is by no means admitted by all 

 botanists, but on the whole it seems to be more nearly 

 related to these than to the Lycopods. If this is true, 

 it is not impossible that from some similar forms the 

 lower Monocotyledons have arisen. 



Another group of flowering plants, admittedly the 

 lowest of all, shows almost certain affinity with the 

 eusporangiate ferns. These are the Cycads, whose 

 recently discovered spermatozoids break down the last 

 barrier between ferns and flowering plants. 



The Equisetinese, so far as we can judge, never de- 

 veloped beyond the large heterosporous forms found 

 fossil, but the Lycopods, through forms like Selaginella, 

 and. much larger but similar fossil types like Lepidoden- 

 dron, may perhaps have been the ancestors of a part at 

 least of the Gymnosperms, or lowest group of seed-bearing 

 plants. The similarity of the tissues of the sporophyte, 

 and especially the remarkable resemblances in the gam- 



