234 THE COXDITIONS OF LIFE IX THE SEA [PART III 



What then is the function of the alimentary canal in such 

 animals as these ? Putter suggests that the surface of the latter 

 is one which is instrumental in the absorption of dissolved food 

 matter from the sea. That it should also take in and digest solid 

 food- particles, such as the organisms of the plankton, may be a 

 secondary function acquired after the alimentary canal has been 

 evolved for the absorption of liquid food-stuff. Thus the capture 

 and assimilation of diatoms by a mussel or a holothurian may be 

 compared with the capture and assimilation of insects by an insecti- 

 vorous plant ; which latter process we may regard as being strictly 

 secondary in importance to the process of feeding by means of 

 photo-synthesis of starch by the green parts of the plant. In 

 many micro-crustacea we find that pumping movements of the 

 intestine are regularly carried on, water being taken in, and ex- 

 pelled per anum. This is usually called " anal respiration," it is 

 just as likely (and it is far more probable a priori) that the 

 process represents the circulation of water through the intestine, 

 in order that the surface of the latter may absorb the dissolved 

 food-stuffs which are contained in the water. 



(2) The Law of the Minimum. 



We are indebted to Justus von Liebig for the statement of the 

 " Law of the Minimum." A plant requires a certain number of 

 food-stuffs if it is to continue to live and grow, and each of these 

 food-substances must be present in a certain proportion. If one 

 of them is absent the plant will die ; and if it is present in 

 minimal proportion the growth will also be minimal. This will 

 be the case no matter how abundant the other food-stuffs may be. 

 Thus the growth of a plant is dependent on the amount of the 

 food- stuff which is presented to it in minimum quantity. We 

 have seen that marine plants require certain things — carbonic 

 acid, nitrogen compounds, silica, phosphoric acid, and certain 

 mineral salts. The carbonic acid and the mineral salts are present 

 in relatively large amount, but the proportions of nitrogen com- 

 pounds, silica, and phosphoric acid in the water of the sea are very 

 small. The density of the marine plants will therefore fluctuate 

 according to the proportions of these indispensable food-stuffs. 



