86 



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



the pitch of all the small- and narrow-leaved species, so the P. confervoides goes to 

 the contrary extremity by developing no mechanical tissue or exceptionally a single 

 subepidermal strand or so. On the other hand the development of such a tissue in 

 the central cylinder is very lively, by which at all events the stem grows both very 

 strong and flexible. A one-layered pseudo-hypoderma covering the epidermis inside 

 also contributes to strengthen the stem. Most similar species instead of this stratum 

 are furnished with a complete circle of subepidermal bast-bundles. But in P. ochreatus, 

 as we have seen, both these reinforcements occur combined also. The central cylinder, 



surrounded with a strong 0-endoder- 

 mis, has the bundles arranged in 

 the usual three groups, here often 

 separated by bast-cells and by ligni- 

 fied or for mechanical purposes trans- 

 formed pith-cells. The median bundles 

 are two with common xylem cavity. 

 The lateral ones are two or only 

 one on either side (oblong diagram). 

 In the floral axis the median 

 bundles run near to each other, 

 their xylem parts dissolve, forming 

 a common channel with great lumen 

 and their bast-sheaths unite into a 

 common endodermis-like sheath, out- 

 side which the two lateral bundles 

 are placed one on either side (fig. 

 34, B). A one-celled pseudo-hypo- 

 derma is also to be found here. Both 

 the stem and the peduncle have a 

 rounded crosscut-form, and although 

 elongate like that of a Colegeton- 

 species, the latter, however, is stiff 

 as usual. 



The fig. 34, G, shows the ana- 

 tomy of the leaf. In the middle of the leaf there runs a vascular bundle with its 

 bast-sheath, round which a circle of small lacunae is arranged. On both sides there 

 appears a row of ample lacunae and in the borders sometimes a row of narrow 

 channels, and always a little bast-bundle. One or a few subepidermal bast-bundles 

 in the middle part strengthen the leaf-structure being nevertheless rather frail, since 

 the epidermis is destitute of a supporting hypoderma. The blade of this species can 

 consequently be said to be reduced to the very lacunar middle portion of the leaves 

 of other species. Nothing is left of the lateral parts but the faint border-strand, and 

 the reduction has evidently culminated. By this the species presents a great resem- 

 blance, or a transition, to the Coleogeton species in their most slender forms as is seen 



Fig. 34. P. confei'i'olfles Echb. A, Transverse section of stem (see 

 the text!), ^; B, Transverse section of peduncle, a, the median fused 

 bundles, h, the lateral ones, 'j^; C, Transverse section of a stem-leaf, ?n, 

 vascular bundle, sir, mechanical strands, ^; D, Middle portion of a 

 stem-leaf, showing the arrangement of the lacnnfe, "^^ \ S, and F^ Tops 

 of leaves, ^; G, Tario, n, stem, b, scar of a leaf, 2-^S interaod'es and 

 leaves of the turio, ■}. 



