184 EVOLUTION OF PLANTS 
by the presence of a single primary leaf, or cotyledon, 
which usually arises from the apex of the embryo, the 
stem-apex of the young sporophyte in most cases being 
formed laterally (Fig. 45, G). Of the Pteridophytes, 
Isoétes shows the nearest approach to the conditions 
found in typical Monocotyledons. There is much uncer- 
tainty at present as to which of the Monocotyledons are 
to be considered as the most primitive, and their relation 
to the other Spermatophytes is also a question about 
which there is much disagreement. These points can be 
settled only after much more is known than at present 
about the development of the flower and embryo in the 
simpler types of the group. The principal disputed 
point at present is whether the forms with the simplest 
flowers are really the most primitive, or whether this 
simplicity is a reduction from a more specialized type. 
The Monocotyledons which possess the simplest 
flowers are aquatics, the simplest of all being probably 
the genus Naias (Fig. 43). In this genus, which is com- 
posed of completely submerged aquatics, the flowers 
are reduced to a single carpel or stamen, the latter usu- 
ally showing but a single pollen-sac or sporangium, 
produced directly from the transformed apex of a 
shoot; the ovule originates in precisely the same way. 
Both kinds of sporangia are remarkably alike in their 
early stages, and the origin of the sporogenous tissue 
is the same in both, and suggests that of many Pterido- 
phytes. Whether or not this simple structure of the 
flower in Naias is the result of reduction from a more 
specialized type, it is certainly more like the sporangia 
of the Pteridophytes than is that of any other Angio- 
sperm. A similar type of flower, but somewhat more 
