307 



investigated , as the boundary between the submerged and 

 emerged parts can only be exactly determined at the place of 

 growth. But both in arctic and Danish plants the bark has been 

 relatively greater than given by Costantin. 



In longitudinal sections thin sieve tubes can be seen. 



Scattered stomata are present on the stalk, as also the 

 characteristic, shield-shaped hairs closely investigated by Rauter 

 {1871). These hairs occur in large quantities on the leaves, 

 especially on the upper side, and in greater numbers on the 

 higher than on the lower leaves. 



The leaves appear in three forms with transition-forms. 



In the lower, 4 — 6 leaved whorls they are broad and short 

 without lateral veins and with 

 the mid-vein stopping at the 

 tip without hydathodes (fig. 5 a, 

 b, c): they greatly resemble the 

 leaves on some H. tetraphylla 

 specimens, on which however I 

 have not made anatomical in- 

 vestigations. 



The upper leaves, to judge from the Danish specimens 

 air-leaves, and the uppermost water-leaves are longer and nar- 

 rower (fig. 5 e) ; they have branched lateral veins and prolonged 

 midvein, whose tip which falls off later is provided with hyda- 

 thodes (fig. 6 a] and epithema, in which the spiral tracheids 

 sometimes, but not usually, reach right out to the tip (cf. 

 Borodin 1870). These leaves are dorsiventral, the upper part 

 of the green tissue palissade-like, the lower part with more 

 numerous and larger lacunae, sponge-like (fig. 7) (cf. Clements 

 1905 fig. 36). 



Shoots are present both from Greenland and Iceland, which 

 have the long, narrow, flaccid, submerged leaves but not in the 

 most extreme form from deep water {B.fluitans Liljebl. v. fluviotilis 

 Hartm.). PoRsiLD (1902 p. 206) observed the usual dilTerence 



Fig. 4. Hippuris vulgaris (X ca. 200). 

 Epidermis of the stem. 



