112 PLANT PHYSIOLOGY 



possibly other acids, which help to dissolve the mineral salts and 

 place them in an available form. Roots also probably secrete 

 oxidizing enzymes which assist in rendering more available the 

 minerals of the soil. 



The same species of plant will vary in its ash content with the 

 nature of the soil. Potatoes grown on a loamy soil rich in potas- 

 sium have a different ash content from those grown on a sandy 

 soil poor in this element. An idea of these variations is shown by 

 the following figures (after Molliard): 



Plant 



Cabbage 

 Clover 



CaO 



Limy soil Sandy soil 



28% 13.6% 



43.3 29.7 



SO a 



Limy soil Sandy soil 



3.6% 4.6% 



3.1 3.9 



Similarly potatoes grown near the seashore contained 12.6% 

 of chlorine in the ash while in the interior, away from the coast, 

 they were found to contain only 8%. 



3. The age of the plant and its development. If the various organs 

 have differing proportions of the elements, it is to be expected 

 that in the course of the development of the entire plant, as cer- 

 tain organs become more active than others, the proportions of 

 the ash constituents will change accordingly. Thus wheat needs 

 more magnesium when young than when older and barley absorbs 

 much more nitrogen and potassium the first half of its growth 

 period than the last. This is illustrated also by the following table 

 for the potato : 



Date J e ff mt P 2 5 S0 3 Si0 2 CI K 2 Na 2 CaO MgO Fe 2 3 



July 1 3.36 14.9 6.4 2.2 3.0 56.8 4.9 5.6 4.8 1.2 



July 29 2.27 13.7 8.0 2.5 4.8 62.7 1.1 2.9 5.0 0.6 



Aug. 28 2.66 15.1 8.5 3.4 2.7 60.1 1.2 3.3 5.1 1.3 



Oct. 2 2.68 19.7 12.4 1.5 1.8 53.1 2.6 2.9 5.9 0.6 



Still more striking changes are seen in the leaves of the beech as 

 the season advances (Palladin). 



Date pe/c^ash™ Ca0 M *° Fe *° 3 P *°* Si ° 2 



May 16 4.1 42.1 13.8 4.3 0.8 32.4 1.6 



July 18 4.7 17.1 42.3 5.6 1.4 8.2 21.3 



Oct. 15 7.1 7.1 50.6 4.1 1.3 5.1 30.5 



In reading these figures it must be borne in mind that the relative 

 percentage may decrease while the total amount of a constituent 

 increases. 



