i62 BOTAXICAL GAZETTE [September 



but which seem to be significant from the standpoint of phylogeny; 

 also to bring together the salient points concerning the structure of 

 the other genera of the family, so that if possible their relationships 

 may be worked out. A few genera belonging to allied families have 



been included in the account. 



I wish here to record my obligations to the following, who have 

 very kindly supplied me with material: Professors G. F. Atkinson 



Mr 



J 



North Carolina Experiment Station, Mr. A. Jeffrey of TorontOj Dr. 

 R. G. Leavitt and ]\Ir. A. A. Eaton of the Ames Botanical Labora- 



m 



tory, Mr. C. P. Smith of Carmel Bay, Cal., and especially to Professor 

 M. L. Fernald of the Gray Herbarium, , who has been kind enough 

 to determine most of the species employed in the research. 



Potamogetonaceae 



In this family the following genera have been accessible for study 

 from presen-ed material: Potamogeton (thirteen species), Ruppia, 

 Zostera, Phyllospadix, Cymodocea, Zannichellia. Celloidin was 

 found to be the most favorable imbedding material for all except the 



smallest forms, for which paraffin was used. Serial sections through 

 the nodes and other critical regions were in all cases studied. 



POTAMOGETON 



The LEAFY SHOOT. — The mode of origin of this member has been 

 carefully worked out by Irmisch (12), who distinguishes between 

 main shoots and reserv^e shoots. The internal structure of the upper 

 internodes of such shoots varies somewhat widely in the different 

 species, and upon the basis of the characteristics as revealed by a 

 cross-section, together with the leaf structure, Raunkiaer (21) has 

 proposed a classification of the species and devised a convenient key. 

 The differences between the species consist in the presence or absence 

 of cortical bundles, and in the degree of union of the strands which 

 compose the vascular part of the central cylinder. Fig. 11 shows at 



ma 



upper nodes of the leafy shoots. At the upper side of the figure is the 

 section of the petiole with its median and lateral traces. Below this 



