PTERIDOPHYTA 155 
ing type of Pteridophytes, which has largely crowded out 
the more primitive Eusporangiates. Certain genera, 
like Osmunda, are probably intermediate in character 
between the two. The Leptosporangiate 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 Eusporangiate. There-are numerous 
striking resemblances in the structures of the two 
groups, and it is possible that the peculiar genus Isoétes 
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 Equisetinesz, 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- 
