42 



J. O. HAGSTEOM, CRITICAL RESEARCHES ON THE POTAMOGETONS. 



ctr 



also has the same curvature, but in dried state false keels are often formed both 



dorsally and laterally. 



The epidermis-cells of the stem always consist of long narrow cells which here, as 



in all the Coleogeton-species, increase somewhat in length in the peduncle. The pseudo- 



hypoderma usually consists of a 

 two-celled layer in the basal in- 

 ternodes, in the upper ones it is 

 one-celled. Interlacunarly there 

 always occurs the vaginal circle 

 of more (basally) or fewer (up- 

 wards) vascular and fibrous bund- 

 les. In most cases there are at 

 the middle of the stem 6 — 12 

 bundles, and in the top internode 

 as a rule 4 bundles. In the ped- 

 uncle, on the other hand, they 

 are without exception absent. 

 The endodermis of spike-bearing 

 specimens always consists of well 

 developed ■w-cells of the broad 

 type. Crosscut-form of the stele 

 oblong at the base of the stem, 

 more roundish or circular in the 

 top, fig. 16, A, B,C. The bund- 

 les of the stele are arranged into 

 three groups as usual, the median 

 one with two separate bundles 

 (basally), thus four-bundledly, or 

 two fused bundles (upwards), the 

 oblong type. In the top of the 

 stem and especially in the ped- 

 uncle, the laterally situated bund- 

 les grow more reduced and often 



Fig. 16. P. pectinatus L. A, B, C, Anatomical diagram of the central 

 cylinder, different individuals, serving to show the chief types of the .-xrrange- 

 ment of bundles and channels, ca. \''; A, from the basal part, four-bundled, 

 B, from the middle of the stem, oblong type, C, from the uppermost inter- 

 node, next to the peduncle, more enlarged, oblong type compressed; letters 

 as in fig. 1. D — ff. Transverse sections of leaves showing the arrangement 

 of nerves and the lacunar system of leaves of different width; Z>, Leaf of 

 about 1—2 mm in width; E, Leaf of var. zosteraceus (Fa.), "-f; F, Stem-leaf SCCm tO be grOUpcd rOUUd a COm 

 of var. striatHS (R. & P.), from Jnjuy and Pudaguel, S. America, and a 

 broad-leaved specimen from TrendS, in Denmark, \''; G, Narrower leaf, \^; 

 H, var. mongolicus Benn., Epidermis without pseudohypoderma, ''f . Letters, 

 see the text! 



mon xylem cavity (circular dia- 

 gram-type). In the rachis the 

 four bundles again run separa- 

 tely, the four-bundled type having thus returned. 



The leaves usually present three vascular bundles, m, n} fig. 16, D, E, G; and 

 the lateral bundles run considerably nearer to the border than to the midrib. Broad 

 leaves contain two pairs of lateral nerves arranged as is shown in the fig. 16, F, n^, 

 n^. A reduced condition, again, is to be found in extremely narrow leaves, fig. 16, H. 

 Subepidermally there occurs nearly always at the margin a weak bast-bundle which 



