522 INTRODUCTION TO PLANT GEOGRAPHY [CHAP. 



I. Among physical factors we have dealt in some detail with 

 temperature and illumination, though it should be emphasized that 

 both these can vary greatly between tide-marks and in pools and over 

 shallow bottoms. Temperatures may even be elevated sufficiently 

 to cause death of stenothermic Algae, at least when these are exposed 

 for long periods by low tides, while Professor V. J. Chapman has 

 informed the author that he has witnessed the killing of Chondrus 

 at low spring tides by frost. Moreover, seasonal variations can 

 induce ' migration ' of Algae from one level to another. Thus in 

 warm regions some of the Algae which in winter occur at high levels 

 are found only at much lower ones in summer, while in the North, 

 owing to the unfavourably low winter temperatures of surface 

 M'aters, the upward extension of sensitive species occurs instead in 

 summer. Such ' migration ' normally takes place through young 

 disseminules becoming established at the desired level {cf. p. 524). 

 In unusually favourable circumstances, light may be sufficient to allow 

 Algae to live at depths as great as 200 metres, a depth of 180 metres 

 being reached by quite large numbers off the island of Minorca in 

 the Mediterranean Sea. 



Whereas on land the chemical nature of the substratum is com- 

 monly important to the rooted plants, in the sea it is its physical 

 nature which usually matters most. For nutrients are furnished 

 through the sea-water in which the plants are immersed, the sub- 

 stratum serving in most instances merely as a place of attachment. 

 Consequently the degree of hardness, or of smoothness or, alterna- 

 tively, irregularity of the surface, plays the most important role, each 

 taxon being apt to evince some (often exclusive) preference for solid 

 rock, smoothed boulders, gravels, sand, or mud. 



Hydraulic pressure, which increases regularly with depth, appears 

 to have little effect upon benthic Algae except in limiting the exten- 

 sion into deep waters of types with gas-filled bladders. Thus in 

 the widespread Brown Alga AscophyUum nodosum, although the 

 increased pressure of unusually high tides can cause escape of gas, the 

 thickness of the bladder-wall is a function of the depth at which 

 these bladders develop, and an individual transported to a much 

 lower depth loses the gas in its bladders and dies. 



2. As regards chemical factors, we have dealt with the importance 

 of variations in salinity and noted how a marked lowering thereof, 

 as for example at the mouths of rivers or in such ' continental ' 

 seas as the Baltic, is commonly accompanied by marked changes 

 in the algal flora and vegetation. Thus the numbers of species of 



