MARINE ALGAL VEGETATION 169 



in the following, name some species which I am fairly certain may 

 be included among the perennials: — 



Gigartina mamillosa. Halosaccion ramentaceum. 



Ahnfeltia plicata. Polysiphonia urceolata. 



Euthora cristata. Rhodomela lycopodioides. 



Rhodophyllis dichotoma. Odonthalia dentata. 



Rhodymenia palmata (?). Ptilota plumosa. 



Rhodochorton Rothii. 



The crustaceous algae such as Hildenbrandia, Petrocelis, Cruoria, 

 Peyssonellia and Rhododermis, and by far the greater number of the 

 calcareous algae must also be classed as perennial. 



2. Periodical Changes. 



The Period of Activity. The Period of Rest. The period 

 of activity of the annual species is identical with their period of 

 life and it extends, probably as regards the majority of the species, 

 over the spring and summer months. The perennial species and 

 the species which can live more than one year, have a very long 

 period of activity which extends over the greater part of the year 

 with the exception of the darkest part; consequently these species 

 have a very short period of rest. Although from the observations 

 to hand it is not possible to fix the length of the period of rest, 

 yet they indicate that it must be short. The Fucacece may be men- 

 tioned as an example. Their vegetative growth appears to be very 

 slight during December — January, and in the case of the older 

 individuals there is probably none at all at that time; but although 

 the majority of the individuals of the Fucacece are sterile during 

 winter yet, even in December, reproductive organs are developing 

 here and there. Young plants of the summer or autumn appear to 

 have vegetative growth also during the winter. In the Fucacece-he\t, 

 taken as a whole, the period of rest is consecmently extremely short. 

 In the sublittoral zone I think that entire rest must be of extremely 

 short duration. 



Renewal of the lamina. The young shoots. As is well 

 known, a renewal of the lamina takes place yearly in the Laminaria- 

 species. At what time this takes place in Iceland cannot be slated 

 with absolute certainty, but the observations seem to indicate that 

 the new lamina begins to grow even in the winter time, as the light 

 increases. I believe that in SW. Iceland a general renewal of the 



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