70 



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



thicker, their leaves broader, and the inner leaves always more or less (± 10 mm) 

 surpassing the others with free tops. The apexes of these leaves of both species 

 roundish, or in P. zosterifolius truncate, with a very short point. 



P. acuiifolius has the most flattened stem of all species hitherto known. The 

 rhizome, however, is even in this species almost terete (fig. 26, B). The epiderm- 

 cells somewhat Avider than those of the stem, the cortical lacunae smaller and the 

 endodermis and subepidermal bastcells more thinwalled than in the stem (= reduced 



floating apparatus and meoha- 

 A ^>^ ;^.^^v^^r-<' ' r.r nical system). 



^ For the purpose of show- 



ing the coherence partly be- 

 tween the stem, ligule and leaf, 

 partly between the vascular and 

 bast-bundles of the stem, ligule 

 and leaves I have figured four 

 serial transverse sections through 

 a node of a young shoot. Fig. 

 27, A, represents a transverse 

 section of the stem through the 

 point from where the leaf is 

 growing forth. The border 

 strands of the stem have unite 

 into a bigger strand on either 

 side (hstr), which by partition 



gives rise to the marginal 

 strands of the ligule -and leaf 

 (B, hstr). The subepidermal 

 strands of the stem [str) have 

 moved somewhat inwards from 

 the epidermis, and the lacunar 

 system has become reduced 

 consisting now principally of 

 small rounded rooms (l). A 

 lateral vascular bundle of the central cylinder (to the right in fig.) is sending its 

 vessels to form the lateral nerve {n') of the leaf. A dotted line indicates the line of 

 demarcation between the stem and the leaf. Oc, oil-cells, cc, central axis. ^. B, 

 section immediately above A, showing a further progression of development, the 

 two lateral bundles and the opposite bundle of the stele having curved outward in 

 order to enter the leaf as its three nerves (x — X — x); the dotted line indicates the 

 line of demarcation between the leaf and ligule, and between the ligule and the stem, 

 which now is deprived of its mechanical strands. C, The development more advan- 

 ced, the borders of the leaf and ligule detached from each other and from the stem, 

 beginning of a branch in the leaf axil is indicated by a dotted line (fer). D, The 



Fig. 27, P. acittlfoUiis Link. Explanation, see ttie text. 



