309 



1. c). 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 J 904) 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 

 but few, and on the short, 

 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 b 

 and c). PoRscH |I905 p. 84) found that the stomata in H. vul- 

 garis L., just as in Callitriche verna //., 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 



OOoooooooooooogoo^ 



Fig. 7. Hippuris vulgaris L. (X ca. 70). 



Upper leaf, transverse section. 



