114 Knud Jessen. 



From Kjellman's classical work "Ur polarväxt- 

 ernas lif" it has long been well known that the Arctic 

 summer begins very suddenly and that a great many 

 Arctic plants live through the winter with leaves which 

 need only elongate slightly in order to attain full 

 power of assimilation. The majority of the true Arctic 

 species mentioned here live through the winter in this 

 way; some even have small, green, half-opened leaves 

 [Dryas). They have moreover no closed winter-bud sur- 

 rounded by scale-leaves (except, e. g. Rubus chamæmorus) 

 and with but very little room for the new foliage- 

 leaves; but the "buds" are open, and the new organs 

 — leaves and flowers — have ample room and grow out 

 quickly, as soon as ever external conditions permit their 

 doing so, and begin work in the short summer^. 

 B. In the following species the flowers are formed during the 



^ As regards light-conditions in Arctic regions I would remark, that 

 from Porsild's "Actinometrical observations from Greenland" 

 (Arbejder fra den danske arktiske Station paa Disco, No. 4, 

 Medd. om Grönland, XLVII, Kobenhavn, 1911) it is seen (from 

 Tables II and III) that the mean light-total for the months 

 May — August is about the same on Disco as in Denmark (An- 

 holt). On comparing the assimilation-conditions of the plants on 

 Disco and Anholt it is seen that the Greenlandic plants must, 

 besides a lower temperature, also suffice with less hght for their 

 summer work than the Danish, the latter having longer time at 

 their disposal. So far as I know it is an open question what use 

 the plants make — if any — of the light of the midnight sun. 

 The investigations made by С urt el (Rev. générale de botanique, 

 Tome II, 1890) and Porsild (Meddel, om Grönland, XXV, 1902, 

 pp. 25 and 26) regarding this point gave contradictory results. 

 Besides Pors ild, Wiesner has also studied the light-conditions 

 in Arctic regions (Norway and Spitzbergen) and he finds by com- 

 parison with the Alpine regions of Central Europe that these 

 are characterized not only by a greater light-intensity, but also 

 by a greater amount of light in the period of vegetation. (Beitr. 

 z. Kenntniss des photochem. Klimas im arct. Gebiet. Denkschr. 

 math. nat. Gl. d. К. К. Ak. Wien. LXVI I, 1908, and Beitr. z. 

 Kenntniss des Lichtklimas von Tromsø, etc. Tromso Museums 

 Aarshefter, 24. Tromsø, 1901). 



