514 INTRODUCTION TO PLANT GEOGRAPHY [CHAP. 



Stability is also desirable. Thus any surface and other waters of 

 excessive turbulence tend to show only moderate phytoplanktonic 

 populations even when conditions of light and nutrient supply are 

 very favourable. The chief depauperating influence in such circum- 

 stances seems to be removal of organisms by descending currents. 

 Horizontal ocean currents, however, are commonly important in 

 bringing in types from other climatic belts, as is evidenced by the 

 widespread distribution of most planktonic species, and even though 

 living conditions may be similar during only limited periods of 

 the year. Neritic types carried seawards by outgoing currents often 

 persist for a while and may even reproduce, though in time they 

 will perish ; nor can a population resume existence if the water is 

 persistently devoid of vegetative plants and resting spores of species 

 capable of taking advantage of suitable conditions. This has been 

 suggested as the explanation of the poverty of some ofi^-shore com- 

 mimities where the depth of the water, it is thought, may prohibit 

 the ready ' return ' of resting stages. 



Also important to the local vegetation, except in very deep water, 

 is the physical nature of the shore or ocean floor — for example, 

 whether it is hard or soft, fixed and rigid or loose and therefore 

 likely to be moved by waves or other influences. Normally bare 

 rock, which on the whole is very inhospitable for land plants, is the 

 best substratum for the larger Algae that form the vast bulk of 

 eulittoral and sublittoral marine vegetation. Especially are such 

 durable rocks as granites suitable for attachment, whereas the softer 

 schists, shales, and sandstones are insufficiently stable for the attach- 

 ment of large Algae, carrying at best only rather small species. But 

 in general the ocean floor is covered by softish sedimentary deposits 

 that result from weathering and erosion on land or from life in the 

 sea. There is now some reason for supposing that the chemical 

 composition of the substratum may yet hold considerable significance 

 for marine Algae, though this significance is apparently far less than 

 is commonly the case in fresh waters. 



In spite of their rather vague and variable separation, the oceanic 

 (pelagic) and neritic provinces are very different. The oceanic 

 province is itself divided into an upper lighted zone and a lower, 

 dark one : its outstanding features when compared with the neritic 

 are its great area and range of depth, its transparency owing to the 

 usual absence of detritus of terrestrial origin, and the consequent 

 deep penetration of light and resultant blueness. In chemical 

 composition these off-shore waters are relatively stable, with salinity 



