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, 

 Isoetes 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 



