FUNCTIONS OF THE ESSENTIAL ELEMENTS 41 



6.5 per cent, of hydrogen. This is because of its extreme 

 lightness, being the lightest of all the elements. 



39. Oxygen. -This element is second to carbon as a 

 constituent of the dry matter of plants, occurring to the 

 extent of 42 per cent., and entering into the composition 

 of most plant compounds. In addition it is necessary for 

 respiration, or for the oxidation of material, which results 

 in energy for the growing plant (Section 20). Combined 

 with hydrogen it forms water which may be considered 

 here with respect to its physiological functions, although 

 it is not an element. 



40. Water. — This compound serves a variety of purposes 

 in the plant, (a) It provides a medium in which plant food 

 is dissolved and by which food can be transported through- 

 out the plant. (6) It is a reagent which helps break down 

 insoluble compounds to form soluble compounds (Sections 

 2, 24, and 31). (c) By filling the cells and pressing against 

 their walls, it gives rigidity to the plant structure and keeps 

 it erect. This filling out of the cells is called "turgor." 

 (d) By its evaporation from the leaves, water reduces the 

 temperature of plants and thus prevents overheating due 

 to the sun's rays. 



41. Phosphorus. — (a) As a necessary constituent of some 

 proteins phosphorus is required for the building up of 

 certain plant compounds. Cell nuclei, for example, contain 

 phosphorus. Hence this element is necessary for cell division 

 and new growth, (b) It stimulates the growth of seedlings 

 markedly. This is to be noted in the practice of drilling a 

 soluble phosphate with corn. Although they did not know 

 it, it was also the cause of better corn growth when the early 

 New England Indians planted a dead fish in each hill, and 

 so advised the Pilgrims, (c) Phosphorus is important in the 

 ripening of grain. A plentiful supply hastens maturity and 

 also increases the yield of grain over straw or stover (Fig. 7). 

 (d) A plentiful supply of available phosphorus in the soil 

 increases root growth. This increase in root growth is 

 important for crops, especially during a dry season when a 

 generous root system permits plants to get subsoil water 

 more easily. This fact has been noticed particularly in the 



