CLASSIFICATION OF MONOCOTYLEDONS 229 



A well-marked feature of the group is the protection of the 

 flower-clusters by a prominent leaf-sheath. The development 

 of this sheath as a protecting organ before the appearance of a 

 fully developed perianth is one of the constant features of the 

 more primitive Monocotyledons, and in some of the following 

 groups it becomes highly specialized. 



The hydrophytic Pandanales, therefore, begin in the great- 

 est simplicity, so far as floral structures are concerned, the 

 Pandanaceae being the most primitive forms on account of the 

 indefinite number of the sporophylls and the spiral arrangement 

 of the stamens, and the series has not advanced verv far. It 

 should be remembered, however, that the three existing families 

 probably represent fragments of a formerly much larger alli- 

 ance, so that the association of the temperate Typha and Spar- 

 ganium with the tropical Pandanaceae may not be so unnatural 

 in reality as it appears at present. It is extremely desirable to 

 obtain some accurate knowledge of the essential morphology of 

 the Pandanaceae. 



II. Helobiales. — This includes the Potamogetonaceae, 

 Xaiadaceae, Aponogetonaceae, Juncaginaceae, Alismaceae, Bu- 

 tomaceae, and Hydrocharitaceae, together containing about 235 

 species. Engler has set apart the small family Triuridaceae, 

 containing about 18 species, as representing a distinct series, 

 Triukidales, but this can be disregarded in this very general 

 presentation. 



This is one of the most remarkable of the monocotyledonous 

 lines in its extent, reaching from the greatest floral simplicity 

 in Potamogetonaceae to highly developed flowers in Hydro- 

 charitaceae. It has been called an unstable or plastic line, and 

 may have given rise to higher forms ; in any event it is probably 

 to be regarded as one of the most important phylogenetic lines 

 among the Monocotyledons. For this reason morphological 

 investigation in recent years has specially cultivated this series 

 of forms, particularly the more primitive families. About the 

 only taxonomic character that holds these diverse forms together 

 is the fact that they are exceptional among Monocotyledons in 

 the feeble development of endosperm. They are characteris- 

 tically aquatic, and sheathing bracts enclosing the flower-clus- 

 ters are largely developed. In most of the forms the spiral 

 arrangement and indefinite number of floral parts is very appar- 



