42 PLANT PHYSIOLOGY ' 



individual must sooner or later nourish itself, because food 

 enough for continued living is not given to it by its parents 

 in the seed, spore, or eg,g, or later. 



THE POOD-MATERIALS 



The food-materials of all organisms are fundamentally- 

 alike. This conclusion may be formed from the observation 

 that all animals are directly or indirectly dependent upon 

 plants for food — the Carnivora preying upon the Herbivora, 

 the Herbivora devouring plants. It might be suspected that 

 the nitrogen, sulphur, and iron bacteria, peculiar in their 

 sources of energy, might be peculiar in the foods used for 

 the construction of their bodies. Chemical analysis of the 

 bodies of all organisms, especially of the protoplasmic parts, 

 shows that there are no exceptions. The conclusion above 

 stated is further confirmed by culture experiments in which 

 food-materials and foods of known composition are the only 

 ones employed. 



Analyses and cultures have shown that of the seventy or 

 so chemical elements only ten are absolutely indispensable 

 as food constituents, namely, carbon, hydrogen, oxygen, 

 nitrogen, sulphur, phosphorus, potassium, calcium, magne- 

 sium, and iron. Two other elements are invariably found in 

 analyses of the bodies of animals and plants, namely, sodium 

 and chlorine, which are of universal occurrence combined as 

 common salt. Only with the utmost difficulty is this com- 

 pound excluded from cultures, and there may be doubt 

 of its ever having been done. Owing to the universal 

 presence of common salt, rather than because of its useful- 

 ness, it occurs in all organisms. Besides these twelve ele- 

 ments, others occur in more or less soluble compounds in 

 the bodies of organisms. Some of these may be very useful 

 though not indispensable. The composition of the soil or 

 water in which plants live, will directly affect the composi- 

 tion of their bodies. Plants living in soils containing large 

 quantities of copper, zinc, arsenic, aluminum, or silica will 

 necessarily absorb larger amounts of the salts of these ele- 

 ments than plants living elsewhere. The plants containing 

 most silica— the Equisetums, grasses, and sedges — employ 



