196 EVOLUTION OF PLANTS 



with the Composite among the Dicotyledons, or with the 

 grasses, which they far outnumber in species, they give 

 the impression of a group of plants in a formative con- 

 dition, which has not yet reached a stage which fits them 

 to compete successfully with their hardier rivals. 



Among the interesting modifications shown by the 

 orchids and not found elsewhere among the Monocoty- 

 ledons (except in the nearly related Burmanniacese), is 

 the adoption of the saprophytic habit by some of them. 

 Such forms, e.g. Corallorhiza, are characterized by a 

 partial or complete loss of chlorophyll, with a corre- 

 sponding reduction of the leaves, which are small and 

 scale-like. 



SUMMARY 



Considering the Monocotyledons as a whole, they are 

 much less numerous than the Dicotyledons as well as 

 simpler in structure, and these points together with cer- 

 tain structural resemblances between them and the ferns 

 seem to indicate that they are the more primitive of the 

 two great divisions of the Angiosperms, and it is not 

 improbable that they have originated directly from 

 pteridophytic ancestors, or possibly through forms 

 related to the Cycads. 



It seems likely that the lowest of the Monocotyledons 

 are the simple aquatic forms like Naias, where the flower 

 consists of a single stamen or carpel, and from these the 

 higher types with hermaphrodite flowers, and later those 

 with a showy perianth, have been derived. It is true that 

 many botanists consider the extreme simplicity of the 

 flowers of the aquatic Monocotyledons to be a reduced 



