42 



GROWTH OF THE PLANT 



growth of cereals in South Australia, (e) Plenty of phos- 

 phorus reduces the content of nitrogen in seeds of grains. 



Fig. 7. — Effect of phosphorus on seed development. Corn. The shock 

 on the right came from a phosphate-treated plat. (Soils Department, 

 Wisconsin^Station.) 



42. Potassium. — (a) For the manufacture of carbo- 

 hydrates, potassium is necessary. It seems probable that 

 the condensation of formaldehyde to dextrose takes place 

 only in the presence of some potassium compound, organic 

 in nature. (6) For the hydrolysis of starch, potassium 

 seems necessary, possibly forming a suitable solution in 

 which the amylases can best act. (c) The presence of potas- 

 sium is necessary for the production of cellulose, since it 

 gives rigidity to stems. A lack of potassium is shown in 



