309 



1. с.)- The structure of the assimilating tissue varies probably 

 according to the outer conditions of the leaves, especially accor- 

 ding to the light directly or indirectly influencing it. The in- 

 fluence of these on the form of the leaf has been studied by 

 Noll (1902 p. 59). In a dark room, which was damp but not 

 steamy, the Hippuris leaves, apart from their yellow colour, 

 assumed the appearance of water-leaves without reaching the 

 size of these however, but 

 they did not develop into this 

 form, neither in a dry, dark 

 room nor in a light, almost 

 steamy room. 



On the ordinary form of 

 leaf there are numerous sto- 

 mata; as is common among 

 water-plants (Costantin 1884, 

 Schindler 1904) they seem to 

 be present in greater numbers 

 on the upper side than on the 

 under side, both in Arctic 

 and in Danish plants. On 

 the ribbon-leaves they are 



found only rarely and then V^ L ' J^-^^ J 



jooooo^ 



but few, and on the short, ^ig- 7. Hippuris vulgaris L. (X ca. 70). 



Upper leaf, transverse section. 



broad, lower leaves there are 



only for example 2 — 4 on a leaf; here they are smaller and 

 with smaller slits than those of the "air-leaves" (cf. figs. 6^6 

 and c). PoRscH (1905 p. 84) found that the stomata in H. vul- 

 garis L., just as in Callitriche venia L., were the reverse of 

 what is usual, as the central slit closed, even on contact with 

 water and under favourable conditions of light. 



Flowering, which occurs in July — August in Greenland, in 

 June in some cases in subarctic South Greenland, seems general 

 and occurs even at 76° 77' N. L., N.-East Greenland; in the 



