ECOLOGICAL FACTORS 407 



also seems to depend very largely upon the pre-development of a 

 green algal felt {Enteromorpha\ whilst the establishment of For- 

 phyra tenera sporelings on the bamboo poles in Japanese harbours 

 (see p. 450) is dependent upon the development of a diatomaceous 

 film. 



One may also include here variations that can occur within the 

 different organs of a single plant. In the case of large plants, such 

 as the oarweeds, these may be considerable. Reference (p. 393) has 

 already been made to the difference in apparent photosynthesis 

 between juvenile, medium dissected and basal entire leaves of deep 

 water plants of Egregia laevigata. The same phenomenon, even 

 more pronounced, is exhibited by different parts of Macrocystis 

 pyrifera (Table 21). 



Table 21 



Photosynthesis I Respiration ratios for parts 0/ Macrocystis 

 plants {from Clendenning and Sargent) 



Plant Part PjR value 



Naked Stipe 4-1 



Pneumatocy St (biilb) 7 • i 



Sporophyll 9-2 



Apical blade to blade 20 1 1 • i (6-5-1 8 7 range) 



Blade 2i-blade 61 18 -6 (10-29 range) 



Blade 62-blade 120 22-0 (13-34 range) 



For this species it is quite evident that the rate of photosynthesis 

 increases with age of the blade. In the case of Egregia (p. 393) it is 

 the middle-aged dissected appendages that exhibit the highest rate. 



Summary 



Despite the great variety of factors that can and do operate on a 

 maritime shore, nevertheless there is the uniformity of the belts 

 occupied by the major organisms. Furthermore if the number of 

 upper and lower limits occurring at any one level are calculated, it 

 is found that there are certain levels at which the number of limits 

 is greater than elsewhere. This suggests that there is some change 

 in a major factor or factors at that level which affects a number of 

 organisms. Such levels have been termed critical levels, and whilst 

 a number have been established for different places there is general 

 agreement that the following critical levels are widespread : 



