Ijo BOTANICAL GAZETTE [September 



charis is figured by Plowman (19), and is stated by him to be char- 

 acteristic of the Cyperaceae. A similar condition is found in Tri- 

 glochin (see below). The resemblance of these axes to the stems of 

 dicotyledons can hardly fail to be noticed, and I am inclined to attach 

 phylogenetic importance to the feature, rather than explain it on 

 physiological grounds, as is suggested by.ScHEXCK (28)- The per- 

 sistence of ancestral features in the reproductive axis has been pointed 

 out by Solms-Laubach (31) and by Scott (29), and the status of 

 this as a general principle has of late received strong confirmation, 

 especially from the work of Jeffrey, who in his memoir on the 

 Abietineae (14) sums up the evidence, in favor of the principle- In 

 Potamogeton we seem to have another case in point, for the peduncle 

 shows the primitive dicotyledonous arrangement of bundles, which 

 moreover are collateral throughout their course. 



GENERAL.^The affinities of the genus Potamogeton may best be 

 discussed later, but we may here consider the relations of the species. 

 The main question at issue is as to whether the species with large 

 floating leaves or those with small submersed leaves are primitive, or 

 whether, as Raunktaer (21) believes, the species with large submersed 

 leaves are to be so regarded. One criterion for the decision of the 

 question is the structure of the central cyh'nder in the various species. 

 In his admirable memoir on submersed plants Schenck (28) has 

 described and figured the central cylinder of a series of species, starting 

 with P. perjoliatus^ in which the bundles of the internodal stele are 

 separate, and ending with P. pectinatus, in which the stele consists of 

 thin- walled cells surrounding a central cavity, and he concludes that 

 the series represents a process of fusion and reduction of the vascular 

 tissues. In another work (27) the same author expresses the view that 

 the pondweeds have sprung from terrestrial plants which passed 

 through the amphibious, then the swimming, and finally the sub- 

 mersed stage: "P. natans wiirde nach dieser Ansicht die ursprung- 

 lichste Form unter den heutigen Arten noch vorstellen." With the 

 general principle of reduction here stated the present writer is in 

 accord, but it seems to him that the place of honor should be awarded 

 to some such species as P. pulcher for the following reasons: (i) P- 

 natans has some of its submersed leaves specialized as phyllodes, 

 while in P. pulcher the submersed leaves are broad and differ from 



