■ 9 8 



ZONES AND REGIONS [Pt. Ill, Sect. V 



only crustaceous Algae, such as Ralfsia, or carpet-like forms, such as the rock-form 

 of Corallina officinalis, appear to thrive, until light and heat permit the recommence- 

 ment of the annual cycle.' 



Whilst in the Mediterranean Sea the dissimilar winter-forms and summer- 

 forms of many Algae represent adaptations to the differences in the illumina- 

 tion, on the other hand the much more marked differences in the winter- 

 vegetation and summer-vegetation of colder seas depend on the difference in 

 the temperature. Numerous short-lived species of Algae, for instance Chorda 

 filum, complete their life-cycle in the North Sea and the Baltic during 

 the summer months, whereas there are only a few purely winter-forms. The 

 majority of perennial species exhibit vegetative activity during the warm 



months, and reproductive activity in 

 the cold months^. Only a few Algae, 

 such as species of Fucus, are in this 

 respect independent of the season, and 

 those that are actively reproductive 

 in summer only are very few, for 

 instance Polysiphonia elongata and 

 P. nigrescens, according to Kjellman. 



The most striking difference be- 

 tweena summer-state and a winter-state 

 is exhibited by species with deciduous 

 fronds, and particularly by those that 

 are frondless during winter. Thus, 

 according to Kuckuck, Desmarestia 

 aculeata, Cladostephus spongiosus, and 

 C. verticillatus among Phaeophyceae, 

 shed their assimilating branches at the 

 beginning of the cold season, so that 

 the first-named is reduced to a prickly 

 framework (Fig. 485). The frond- 

 less plants become covered with re- 

 productive organs. Various Rhodo- 

 phyceae behave in like manner. Thus, 

 Delesseria sanguinea has its large leaf-like members in an intact condition 

 only in early summer (Fig. 488) ; later on they are lacerated, so that in 

 winter the plant consists of the bare midribs, which then for the first time 

 produce antheridia, cystocarps, and tetrasporangia. The new period of 

 vegetative development begins after the release of the carpospores in 

 January and February, and is not arrested by the circumstance that 

 the winter temperature of Heligoland is at this time at its lowest. 



1 Regarding the favourable influence of low temperature on the reproductive functions 

 in terrestrial plants, see p. 48. 



Fig. 48S. Delesseria sanguinea. 

 natural size. 



Half 



