KUNGL. SV. VET. AKADEMIENS HANDLING AE. BAND 55. N:0 5. 



261 



for this purpose. This arrangement is not to be found in any other Potamogeton- 

 species, but it reminds much of the leaf of P. Rohhinsii, which, however, has a true 

 sheath and ligules proper. The stipules of the densus-le&i are as to their inner mar- 

 gin connate with this vaginal part of the leaf (Fig. 119, D), while the external bor- 

 der is free. — It is not fully known what var. stipulata Arc. may be. 



The poUengrains are of middle size and similar in shape to those of P. perfo- 

 liatus, but P. densus is not likely to hybridize either with this or any other Pot.- 

 species. The foliar appendage of the stamen (perianth-leaf) is triangular, whereas 

 all the other species have this organ broadly rounded or truncate upwards. 



The style is narrow and short with a backwards produced beak above, not 

 quite covered by the low stigma. 



The fruit of this species differs from that of all the foregoing ones. Already 

 F. Hegelmaier has called attention to its thinness (Bot. 

 Ztg, 1870, p. 317). Since that time nobody has, as far 

 as I know, made the fruit a subject of investigation. 

 In order to make the difference clear it is necessary to 

 remind of the following, pertaining to the fruit. The 

 single fruit is a drupelet. The drupelet consists of two 

 parts, the pericarp and the seed. 



A. The pericarp has an external fleshy or la- 

 cunous part (the epicarp) inside the usually large-celled 

 epidermis, and an internal hard layer of stone-cells 

 (endocarp), bordered inward by elongate prosenchyma- 

 tous stone-cells following the direction of the spire. 

 The endocarp forms, from the ventral side inward, a 

 partition along the seed. On the back it forms a lid or 

 operculum. Between the epicarp and the endocarp there 

 are a few layers of narrow transition cells, often with 

 rings or pores. Even the species with the thinnest peri- 

 carp, for instance P. dimorphus, have an external fleshy and an internal 2 — 3 celled 

 part of stone-cells. P. densus, again, is destitute of both the fleshy and the stone- 

 celled part. Its pericarp has only the outer epidermis, 2—4 layers of transition 

 cells, and the inner, sclerotized epidermis-cell. The partition-wall, further, is thin, 

 especially upwards, and relatively short, and only slightly curved depending on the 

 particular form of the seed. In the other species this lamella is more curved and 

 upwards enlarged. P. densus has, finally, the ventral cavity of the pericarp, in 

 cross-cut view, conspicuously wider than the dorsal one, whereas the condition 

 otherwise is the reverse. 



B. The seed is surrounded by a membranous, brown testa, formed, in all the 

 species, by somewhat elongate parenchymatous cells. It either surrounds the embryo 

 tightly along the whole spiral, following the spiral to the tip of the cotyledon which 

 it encloses, or it encloses the hypocotyledonic part to the plumule, whereupon it forms 

 a sack-like enlargement, enclosing the spirals of the cotyledon. The former is the case 



Fig. 119. P. densus L. A, Pistil, 

 lat. view, V°- B, Fruit, lat. vie'w, f. C, 

 Perianth-leaf, f. D, Stipule, '{. E, Trans- 

 verse section of the peduncle, ^^, vbj 

 vascular bundles, ejj, epidermis, h^, pseudo- 

 hypoderma. 



