118 H. JONSSON 



Monostroma Grevillei. Cladophora rupestris. 



Ulothrix flacca. Acrosiphonia. 



Monostroma groenlandicum. Enteromorpha intestinalis. 



Pylaiella littoralis. Porphyra umbilicalis. 



Chordaria flagelliformis. Chaetomorpha tortuosa. 



The Under-vegetation. This is found commonly distributed, 

 and varies considerably, both as regards luxuriance and the species 

 composing it. The luxuriance seems to increase with the degree of 

 exposure (except perhaps in the most exposed places), and then the 

 composition of the species is also changed, as species which pri- 

 marily belong to a lower belt extend higher up, probably on ac- 

 count of the frequent movements of the sea which cause the de- 

 siccation-period to last but a short time. The undergrowth must 

 be regarded as a kind of shade -vegetation; during low-tide it is 

 completely covered by the Fucacece, and at high-water the intensity 

 of the light is also subdued by them, as they float on, or rise with, 

 the water, and are moved backwards and forwards by its ripples. 

 The greater part of the species of the undergrowth do not occur 

 at the height of the Fucus-belt in places open to the light, but 

 occur frequently and abundantly in shaded localities, although these 

 may be found at the upper boundar)' of the Fucus-belt as, for in- 

 stance, in depressions in the talus of debris, upon the under side 

 of overhanging blocks of stone. This vegetation may therefore be 

 justly termed the shade-vegetation of the littoral zone. The species 

 are normally developed and cannot be compared with the shade- 

 forms of light-plants belonging to the land-vegetation. On the other 

 hand, the shade-vegetation of the littoral zone actually corresponds 

 with the shade-vegetation of the land, such as the fern-vegetation 

 and the liverwort-vegetation in the lava-fissures. 



The under-vegetation belongs in part to the shade-vegetation 

 (see p. 123) and is divided into several associations; here it is dealt 

 with under the Fucus-belt as its strictly littoral distribution almost 

 coincides with that of the latter, and it is just as dependent on 

 the upper-vegetation, or even more so, as is the undergrowth in a 

 coppice. As regards the relation between the upper-vegetation and 

 the undergrowth in the Fucus-belt the main advantage is, I think 

 with the undergrowth; although during low-tide, the upper-vegeta- 

 tion may gain some advantage from the fact that the layer of 

 living plants under it retains more water than does the bare sur- 

 face of the rock. 



