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1907] CHRYSLER— POT AMOGETONACEAE ■ 171 



the floating ones mainly in their thinness; (2) It is probable that the 

 separate bundles in the central cylinder of P. piikher represent a more 

 primitive condition than the ''trio" of P. naians. There seems to be 

 every reason to believe that the small-leaved submersed forms with a 

 stem showing a concentric stele are to be regarded as simplified by 

 reduction. Furlhcrj the group of species with stipules adnate to the 

 petiole represented by P, filijonnu^ P. peclinatuSy and P. Rohbinsii 

 are probably quite specialized forms, since they arc connected with 

 such forms as P. natans by a small group including P. spirilliis and P. 

 hybridiis, in which the stipules are slightly adnate and the spikes are 

 of two kinds, the emersed ones cylindrical, and the submersed ones 

 capitate, consisting of four to six flowers. It should also be noted 

 that P. Robbinsii^ etc., have their flowers in an interrupted spike. 



RUPPIA MARITIMA 



This monotypic form has been uniformly placed in the same 

 . family as Potamogeton on account of its emersed inflorescence, per- 



fect flowers^ and alternate leaves. The complex arrangement of 

 branches characteristic of Potamogeton is here represented by a creep- 

 ing axis giving off roots and leafy shoots, as described by Irmisch 

 (12). The structure of the stem is comparatively simple: inside a 

 lacunar cortex is a central cylinder of the type of P. pectinatus, viz.', 

 with a central cavity surrounded by small thin-walled cells. Trache- 

 ids are present at the nodes. Each leaf possesses a central bundle 

 i and two very slender lateral ones. The central bundle enters the 



central cylinder directly, as in Potamogeton; while the lateral bundles 

 descend into the cortex of the stem for a greater or less distance, 

 though they do not reach the next lower node, but dwindle away and 



m 



disappear, as is illustrated in the diagram, fig. g. This condition has 



in all probability been derived by reduction from that in which the 



cortical bundles joined the central cylinder at the next node below the 



insertion of the leaf to which they belong, for it is unreasonable to 



suppose that the leaf traces should originally have had no connection 



r>f tlip c;tpm. Thus the cortical 



channels 



bundles of Ruppia belong to Van Tieghem's second group (32, p. 

 751). The course of these bundles, and the occurrence of xylem at 

 the nodes, as well as the filiform leaves and submergence of all parts 



