70 



J. O. HAGSTROM, CR1TICAL RESEARCHES 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. zosterijolius truncate, with a very short point. 



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

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

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

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



floating apparatns and meclia- 

 nical system). 



For the purpose of show- 

 ing the coherence partly be- 

 tween the stem, lignle and leaf, 

 partly between the vascular and 

 bast-bundles of the stem, lignle 

 and leaves I have fignred fonr 

 serial transverse sectionsthrough 

 a node of a young shoot. Fig. 

 27, A, represents a transverse 

 section of the stem throngh the 

 point from where the leaf is 

 growing forth. The börder 

 strands of the stem have unite 

 into a bigger strand on either 

 side (bstr), which by partition 

 gives rise to the marginal 

 strands of the ligule and leaf 

 (B, bstr). The subepidermal 

 strands of the stem (str) have 

 moved somewhat inwards from 

 the epiderniis, and the laeunar 

 system has become reduced 

 consisting now principally of 

 small rounded rooms (/). A 

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

 rossels to form the lateral nerve (n') of the leaf. A dotted line indicatcs the line of 

 demarcation between the stem and the leaf. Oc, oil-cells, cc, central axis. y, />, 

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

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

 order to enter the leaf as its three uerves (x — x — x); the dotted line indicatcs the 

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

 which now is deprivod of its mechanica] strands. C, The development more advan- 

 oed, the börders of the leaf and ligule detaehed from caeh othcr and from the stem, 

 beginning of a branch in the leaf axil is Lndicated by a dotted line (br), D, The 



Fig. 27. /'. acuiifolius I.ink. Expl.ination, seo the text. 



