180 H. JONSSON 



in the case of many species both the vegetative period and the 

 period of fructification is longer in the Arctic Sea and the northern 

 part of the Atlantic Ocean than in the remaining more southern 

 part of the boreal area of the same Ocean. It holds good also 

 for Iceland, that both these periods are prolonged. Iceland, the 

 Faeroes and Greenland agree also in the fact that the summer is 

 richest in fruit-bearing species. 



3. Littoral Winter-vegetation at Reykjavik. 



The littoral vegetation changes its appearance according to the 

 season, and this is especially owing to the annual species. The 

 vegetation is most luxuriant, and richest in species during spring 

 and early summer; in the latter part of the summer the annual 

 species decrease in number and a quantity of them disappear, and 

 in the autumn only a small number of them is left. During winter 

 only a few short-lived species are found, and some of them play 

 either no part, or only an unimportant one in the vegetation which 

 by that time is usually composed of perennial species. Thus, the 

 number of the species of the winter- vegetation is much less than 

 of the summer-vegetation because the Chlorophycece , which is the 

 group in the littoral zone that is richest in species during summer, 

 are few in number during winter; also the number of species of 

 the Phceophycece is greatly reduced in the littoral zone during winter. 

 It is the Fucacece which form by far the greater part of the mass 

 of plants in the littoral zone, during the winter as in the other 

 seasons of the year. 



In December and January, in the winter 1911--1912, the vege- 

 tation of the upper and lower littoral zones was composed as 

 follows : 



Highest of all a Prasioletum stipitatae occurred in patches. 

 Prasiola stipitata grew luxuriantly and had a normal appearance. 

 It was not injured by the winter climate. 



Below that came a well -developed Porphy return umbili- 

 calis, occurring also in patches on account of the surface -form 

 of the coast. The Porphijra was both sterile and in fruit and had 

 an entirely normal appearance. 



Below that again came the Fucacece-comm unity which, as is 

 usually the case there, consisted at the top of a Pelvetia-Fucus 

 spiralis-be\i and at the bottom of the usual Fucus-bell (Fucns vesi- 



