332 



— and the specimens of M. spicatum and Callitriche are also 

 less robust than those from Iceland and the Færoes, which show 

 no obvious differences from plants from other temperate re- 

 gions. The shorter period of vegetation might suffice to explain 

 the smaller size, but in the case of the robustness, e. g. of 

 the leaves, other circumstances must also be active, very likely 

 amongst others temperature of the water. But investigations 

 relative to this must be carried out on the spot and combined 

 with experiments. 



Whether the quantitative peculiarities of Hippuris from 

 Greenland — the smaller number and the relatively greater 

 breadth of the leaves together with the fewer circles of air-chan- 

 nels of the stem — which features might possibly all be re- 

 garded as reduction — are constant characters, the arctic Hip- 

 puris thus being a special systematic form, or whether they 

 would alter if exposed to other circumstances, can be decided 

 only by investigations on living and cultivated plants. 



My best thanks are due to Professor Warming for the kind 

 interest taken in this work, to Dr. C. H. Ostenfeld for various, 

 especially phytogeographical items of information and for having 

 controlled the determination of some species, further to the Director 

 of the Danish arctic station at Disco, mag. sc. M. P. Porsild 

 for the above-quoted notes on the biology oï Hippuris in Green- 

 land and for several other communications. 



Additional Note. About C. autumnalis mag. sc. Porsild has given me the 

 following information. "It was growing on gytje on the lee-side of a shallow 

 pond, the windward side being without vegetation. It must be supposed to 

 remain frozen in the ice from the beginning of Octbr. until late in May, 

 when the ice melts at the border, while in the middle it remains until late 

 in June, at least". 



20.— 6.-1910. 



