524 INTRODUCTION TO PLANT GEOGRAPHY [CHAP, 



Professor V. J. Chapman {in litt.), ' The evidence suggests that it is a 

 matter of the relative ratio between photosynthesis and respiration. 

 In warm waters these are nearly equal and hence there is never a 

 great development. At lower temperatures the respiration rate 

 drops more rapidly than photosynthesis, so that there is a greater 

 excess of photosynthesis over respiration and hence greater growth.' 

 It is to be expected that the nitrate and phosphate content, which are 

 known often to control phytoplanktonic development, have some 

 influence also on benthic Algae ; certainly many of these, such as 

 Prasiola on land, are greatly favoured, from the high-arctic regions 

 southwards, by manuring, etc. 



3. Among biological factors the successional tendencies are notable, 

 as exhibited in the repopulation of denuded rock surfaces. Here a 

 rapid development of Enteromorpha may precede the attachment of 

 eggs of Bladder Wrack [Fucus vesiculosus) and, apparently, facilitate 

 the development of young plants of the latter, which later may oust 

 the Enteromorpha. Again, the building up of silty banks is often 

 aided by algal growth fostering deposition. 



Also important may be the relationships between epiphytic Algae 

 in the sea and the ' hosts ' on which they grow. Thus the epiphyte 

 is often protected by the host against rough seas or excessive illumin- 

 ation ; or the host may benefit from protection provided by the 

 epiphyte ; or the load of epiphytes may be too great and cause the 

 host to be torn away from its point of attachment. In many cases 

 of widely different affinity the epiphyte is reduced to a disk-like 

 form completely adnate to the host, while some epiphytes actually 

 penetrate the host tissues, such penetration usually being accom- 

 panied by some degree of parasitism. Much the same relationship 

 exists between many Algae and marine animals, apparently often 

 resulting in symbiosis. Again, browsing by marine animals such 

 as Molluscs — and, in some parts of the world, harvesting by Man — - 

 may have a noticeable effect on the algal or other marine vegetation, 

 and may even cause the disappearance of particular species from 

 some localities. Finally, diseases can have much the same effect — 

 as in the case of the ' wasting disease ' of Eel-grass (Zostera) off 

 many northern shores. 



The migration of Algae into deeper or shallower water at different 

 seasons is chiefly manifested by quick-growing species having more 

 or less continuous reproduction, following which the sporelings only 

 survive and grow in the most favourable zone for the particular 

 time of year. 



