NUTRITION AND METHODS OF FEEDING i8i 



absorbing over their whole surface the mineral salts, organic 

 acids, etc., from the water with which they are bathed. 

 The so-called ** roots " of the fixed seaweeds are merely 

 hold-fasts, and apart from this function have nothing in 

 common with the roots of land-plants. 



The foodstuffs utilised by marine plants are substantially 

 the same as those which subserve the nutrition of plant-life 

 on land. Following Johnstone {op. cit), we may enumerate 

 them as follows : — 



(i) Nitrogen compounds, in the form of nitrates, nitrites, 

 ammonia, and possibly amines. The amount of these is 

 exceedingly small and their precise estimation a matter of 

 great difficulty. 



(2) Carbonic acid, present in the atmosphere as CO2 

 and in simple solution in the sea-water or in solution as 

 bicarbonates of calcium and other bases. Carbon dioxide 

 results from animal respiration, and is an end-product of 

 the decomposition of organic matter whether brought 

 about by putrefactive or fermentative organisms. Plants, 

 on the other hand, utilise the CO2 for the elaboration of 

 starch. The amount of CO2 in solution or, in other words, 

 the degree of alkalinity or acidity of the sea- water, varies 

 with the relative activities of vegetable and animal organisms 

 and serves, in fact, as an index to these activities. As illus- 

 trating this reciprocal activity of plant and animal with regard 

 to CO2, we may mention the fact brought out by Benjamin 

 Moore (1913-14), namely that " almost all the CO2 provided 

 by a lobster can be reconverted into O by the vegetation 

 which forms upon its own carapace." If marine animals 

 are to remain in health, the water must have a slightly acid 

 reaction ; too great a variation in the direction of alkalinity 

 or acidity is likely to prove fatal. 



(3) Phosphoric acid, in the form of soluble phosphates of 

 calcium. 



(4) Silica, which may exist in solution in the sea, either 

 as colloidal silicic acid or as particles of clay (aluminium 

 silicate) in suspension. In spite of the relatively minute 

 quantities in which it occurs, silica is of great importance in 



