512 INTRODUCTION TO PLANT GEOGRAPHY [CHAP. 



what some students term the pelagic division of the sea (their other 

 division being the benthic, comprising the ocean floor and shore). 

 Often the ' compensation point ' (at which the daily accumulation 

 of food as a result of photosynthesis is just balanced by the break- 

 down during respiration of stored materials) is very much less deep, 

 being sometimes only a few metres from the surface, though of course 

 varying with many factors, including the organisms concerned. 



It is also convenient to separate an uppermost or ' eulittoral ' 

 zone as extending from the highest to the lowest * normal ' tide- 

 levels on shores, and a ' sublittoral ' extending from the base of 

 this down to a depth of about 40 to 60 metres, the lower boundary 

 being set at the lowest limit at which the more abundant attached 

 plants grow. It should be noted that some authors continue the 

 eulittoral down to the lower limit of at all abundant attached plants, 

 their so-called sublittoral beginning here and extending down to 

 a total depth of about 200 metres, and so corresponding with what 

 we have here termed the infralittoral. Below the infralittoral is the 

 deep-sea system, divided into an upper ' archibenthic zone ', extend- 

 ing to a depth of between 800 and 1,100 metres, and the lower 

 ' abyssal-benthic zone ', in which conditions are practically uniform. 

 Here the temperatures are always low (- 1° to +5° C), solar light 

 is lacking, and there are no seasons. The various zones, etc., are 

 shown diagrammatically in Fig. 170. 



The component colours of white light are variously absorbed by 

 sea-water — those of shorter wave-length, such as the very abundant 

 green, being in general less absorbed than the red, and even the blue, 

 though the red may be relatively little affected by stains and sus- 

 pended matter. This differential absorption of different components 

 of the spectrum seems to be one of the main factors behind the 

 ecological preferences of different Algae for different depths, though 

 details are still not clear. Thus in general Green Algae reign in 

 the uppermost layers where red rays are plentiful, such rays being 

 apparently essential for healthy growth of many of these plants, while 

 Red Algae predominate deeper down where green rays still penetrate, 

 although in each colour-group are species belonging to very different 

 depths. Brown Algae tend to be plentiful at all depths except the 

 deepest inhabited by Algae, where Red Algae commonly predominate. 



Apparently both the intensity and the wave-length of the light 

 play a large part in controlling the regional distribution of Algae, 

 the green surface-forms flourishing under conditions of high light- 

 intensity and plentiful red rays, though hardly able to utilize the 



