1917] Ayres : Nutrient Solutiiui on Plants Grown in Sand Cultures 371 



greater in group D, the calcium must have been used with greater 

 economy in this group where the total concentration was low. 



The large number of variables which are present in each of 

 the solutions make accurate deductions concerning the exact re- 

 lation between any element and the growth of the plant almost 

 impossible. In fact, growth has been shown to be influenced not 

 by one factor alone but by combinations of factors. 



GENERAL DISCUSSION 



It is evident, as noted above, that the large number of vari- 

 ables present in an experiment of this character so complicates 

 the situation that definite conclusions are drawn only with con- 

 siderable difficulty. Inorganic salts can be used by the plant 

 only from solution. The complexity of this solution increases 

 with the number of ions, which must be rather large since the 

 plant cannot make normal growth unless certain ions are present. 

 To further complicate the situation, all the salts may be avail- 

 able which are required for growth, but the unbalanced condition 

 of the solution may cause injury to the growing plant.^* This 

 condition of balance, in turn, seems to be related to the total 

 concentration of the solution as well as to its qualitative compo- 

 sition.^^ In this connection it is important to note that if it 

 were possible to keep the balance in the solution constant by 

 renewal of salts, growth differences would be less marked than 

 when the plant grows in a solution in which the balance is con- 

 stantly changing due to absorption of ions by the plant.-''' All 

 of the above points must be taken into consideration in any ex- 

 perimental work which is done in this field of investigation. The 

 exact influence which the concentration of the solution has upon 

 the complicated physiological processes concerned in plant nutri- 

 tion is a problem which can be solved only by the gradual ac- 

 cumulation of a mass of evidence bearing upon the subject. The 

 complexity of the whole problem is such as to require more than 



34 Loeb, Archiv. ges. Physiol., vol. 88, pp. 68-78, 1902, and Amer. Jour. 

 Physiol., vol. .3, pp. 327-338, 1900; Osterhout, Science, n. s.. vol. 35, pp. 

 112-115, 1912, and Jour. Biol. Chem., vol. 1, p. 3fi3, 1906. 



35 Gile, loc. cit. 



36 Brenchley, Ann. Bot., vol. 30, pp. 77-90, 1916. 



