324 EFFECT OF CHEMICAL AGENTS [Ch. XI 



Silicon. — This element is of wide occurrence among organ- 

 isms. In many plants, especially the grains and grasses, it is 

 exceeded in abundance only by potassium. Among the lower 

 organisms whole groups, e.g. diatoms, Radiolaria, and glass 

 sponges, are characterized by the great amount of silica made 

 use of. Even in vertebrates it is found wide-spread in the 

 blood, gall, bones, feathers, and hair. The silicon required for 

 the body is gained by plants and lower organisms from the 

 soluble silicates or silicic acid of the soil and waters ; by verte- 

 brates, probably from plants. 



Although Sachs ('87, p. 271) was able in 1862 to show that 

 growth even of maize (about one-third of whose ash is silica) 

 continues in the absence of silicon, yet some grains do better, 

 according to Wolff ('81), when that element is abundant. 

 It is significant, likewise, that, as Poleck ('50) found, 7% of 

 tlie ash in the albumen of the hen's egg is silicon. 



Copper. — Brief mention may be made of the fact that this 

 metal occurs in a great variety of organisms, but usually in 

 minute quantity. It occurs as a physiologically important con- 

 stituent of the hsemocyanine of the blood of the squid (Feed- 

 EEiCQ, '78, p. 721), of crabs and lobsters, and of certain gas- 

 tropods and lamellibranchs (Feedeetcq, '79). 



2. The Organic Food used by Organisms in Growth. — All 

 organisms may use organic compounds as food ; all organisms 

 which contain no chlorophyll, certain bacteria excepted, must 

 do so. This organic food may consist of solutions of definite 

 composition or it may consist of solid masses of plant and 

 animal tissue composed of varied and indeterminate kinds of 

 organic molecules. The former are made use of chiefly by 

 plants ; the latter, by the higher animals. This distinction is, 

 however, not a necessary and constant one, for on the one hand 

 insectivorous plants and many fungi live upon solid masses 

 which they digest, and on the other hand some of the Protozoa 

 can be fed upon known solutions, and doubtless some of the 

 higher animals could be likcAvise fed, although few experiments 

 seem to have been made in this direction. Even in methods 

 of nutrition there is no sharp line to be drawn between the 

 different groups of organisms. 



a. Fungi. — Our knowledge of the effect of known chemi- 



