1907] CHRYSLER— POTAMOGETONACRAE 165 



of the phloem at this point are narrow cells with very thin walls, quite 

 unlike the well-marked sieve tubes which are seen in other parts of 

 the stem. Such bundles are frequently seen to be amphivasal just 

 before they are inserted on bundles of the main stele. Species with an 

 axial cylinder like P. natans are: P, perjolialiiSj P. NiitlaUH^ P, 

 praelongus, etc. 



P. natans has been chosen for the foregoing description, not because 

 it presents the most primitive structure of any of the species, but 

 because the vascular tissues are well developed, probably on account of 

 the transpiration from the numerous and large floating leaves, What 

 seems to be a more primitive disposition of the vascular bundles of 

 the central cylinder is seen in P. pulcher (fig. ip), also a form with 

 some floating leaves, but with broad submerged leaves instead of the . 

 phyllodes of P. natans. The course of the leaf trace bundles in P. 

 pulcher resembles that already described, with the noticeable differ- 

 ence that the three bundles do not unite to form a ''trio, " but merely 

 approach one another in the medulla of the second intcrnode after the 

 insertion of the leaf, giving the appearance shown in cross-section 

 (fig. 20). A diagram (fig. 2) will make clear the topography of the 

 central cylinder: as before, /j, T^, t^ repre- 

 sent the traces of the next higher leaf; Z^? 

 Pa, t^ those of the second higher leaf; while 

 the remaining bundles are cauline. As in 

 the former species, amphivasal bundles 

 occur at the nodes when two bundles are 

 about to fuse. In spite of the liberal size ^^^ ^ 



of the leaves, and the fact that some of 



them float, no cortical bundles are present in the stem (fig. ig). 

 Numerous smafl bundles are present in the stipules, but they turn 

 abruptly inward and enter the central cylinder, while the median 

 bundle of each stipule as usual Joins the lateral leaf trace. 



The two species already considered represent two types of central 

 cylinder found in the genus. They are evidently closely related, and 

 the natans type may evidently be regarded as derived from the pulcher 

 type by the partial fusion of the three leaf traces during their descent 

 through the central cylinder. At least two other types may be dis- 

 tinguished, in one of which the bundles of the central cylinder have 



