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184 BOTANICAL GAZETTE [September 



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pedicel of the floAver, and the young leaves. In contrast to this sim- 

 plification, the genus Zostera leads to a state of greater complexity 

 in Cymodocea and Halodule, where the two cortical strands of Z, 

 marina are replaced by two arc-shaped groups, each of several strands 

 but with a similar relation to the leaf traces and central cylinder. 

 Phyllospadix differs so little from Zostera in its vascular tissues, that 

 if this were the only criterion both might be included in the same 

 genus. Triglochin and Scheuchzeria appear to be only remotely 

 related to the 'foregoing genera. Both are much more typical mono- 

 cotyledons, as appears from the rhizome and flowers, and are more 

 naturally related to the Alismaceae. Aponogeton seems to be more 

 closely related to Alismaceae and Juncaginaceae than to Potamogcton- 

 aceae. The following diagram represents in a graphic way certain 

 of the relations suggested in the foregoing account : 



Halodule Naias 



Phyllospadix Cymodocea Zannichellia 



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Zostera Ruppia 



Potamogeton 



Summary 



1. The course of the leaf traces in Potamogeton is not typically 

 monocotyledonous, for in the more robust species the three traces run 

 down through one internode at the periphery of the central cylinder 

 before entering the medulla, where they descend through a second 

 internode, finally fusing with cauline strands. 



2. At the points of fusion the bundles are found to be amphivasal^ 

 while they are collateral throughout their internodal course. 



3. The phloem of the bundles at the base of a branch shows a 

 marked swelling. 



4. The cortical bundles present in some species of Potamogeton 

 are not regarded as a primitive feature, since they do not occur in the 

 fertile part of the floral axis and occur only sparingly if at all in the 

 creeping stem. They enter the stem as small strands from the stipules 

 and petioles and are not as a rule connected with the Drincipal leaf 



