— 261 - 



and biggest one lies nearer to the endodermis and in the circle of 

 the other bundles. 



In a good many species the vascular bundles of the axial 

 cylinder are more or less completely united into three bundles each 

 of them with a channel. The axial cylinder is in such species 

 more compressed and the two bigger vascular bundles are standing 

 before the middle of each of the longest sides of the transverse 

 section meeting across the pith, uniting into one big bundle with 

 a channel in the middle; on each side of this middle-bundle the 

 other vascular bundles are uniting into one, and, consequently, we 

 find a big bundle in the middle and a smaller one in each end of 

 the section, separated from each other by two more or less broad 

 belts of pith (fig. 3 G—M). 



. In a third group of species, finally, the vascular bundles of the 

 axial cylinder are united into a single bundle with a channel in 

 the middle and surrounded with phloem in which more or less 

 discernible groups of sieve-tubes are sometimes to be seen, indi- 

 cating that the bundle has come into existence by the conjunction 

 of several bundles (fig. 10). 



The characters taken from the leaves which I have used by 

 stating the groups of species are almost without exception morpho- 

 logical; and being the same that have been used hitherto, I shall 

 not here speak particularly of them, especially because elsewhere I 

 have given the morphological facts of the leaves as well as those 

 of the anatomical structure (1. c. pag. 54—73). 



Synopsis of the groups of species. 



I. Leaves all with sheaths, and all submerged and 

 linear. 



1. Leaves with (3 — )5 principal, of vascular bund- 

 les formed nerves, between which several fine 



nerves from the bast only P. Robbinsii-group. 



2. Leaves with 1 — 5 nerves from the vascular- 

 bundles, without nerves from the bast P. pectinatus-group. 



II. Leaves all, or at least the uppermost (the floating- 

 leaves), without sheaths. 



A. Leaves not linear, at least not the uppermost. 



a. Leaves all submerged, sub-opposite. . .P. densus-gvowp. 



b. Leaves alternate, with exception of the 

 involucral-leaves. 



