VOLUME XLIV 



NUMBER 3 



Botanical Gazette 



SEPTEMBER igoj 



THE STRUCTURE AND RELATIONSHIPS OF THE POTA- 



MOGETONACEAE AND ALLIED FAMILIES^ 



MiNTIN ASBURY CHRYSLER 



(with plates xiv-xviii) 



In the alluring search for primitive monocotyledons which shall 

 throw light on the vexed question of the origin of the group, the Helo- 

 biae have received much attention, including as they do such a wide 

 range of forms, reaching from great simplicity to considerable com- 

 plexity. It is generally agreed that those monocotyledons which 

 possess a stocky rhizome or bulb have reached a degree of specializa- 

 tion which renders them less favorable for a study of primitive feat- 

 ures than are the forms which have a more elongated axis showing 

 distinct nodes and internodes. The Potamogetonaceae are prevail- 

 ingly of the latter class, and the genus Potamogcton has been con- 

 sidered by ScHENCK (27) and others to represent a number of stages 

 in the evolution of aquatic monocotyledons from ancestors which 

 inhabited marshy ground. It has been more than once suggested 

 that the monocotyledons have been derived from dicotyledonous ances- 

 tors through adaptation to an amphibious mode of life, so that the 

 genus Potamogeton deserves study from all standpoints. A quite 

 different view of the origin of monocotyledons has been proposed by 

 Campbell (3), who seeks to link the group with heterosporous Fili- 

 cineae, especially Isoetaceae, through such forms as Naias. 



It is proposed in the present paper to discuss certain features of 

 the structure of Potamogeton which have apparently been overlooked, 



I Contributions fTOm the Phanerogamic Laboratories of Harvard University, 

 No. 9. 



161 



