DIV. II 



PHYSIOLOGY 



239 



water, on the other hand, while the plant begins to grow normally, 

 the growth soon ceases entirely, and only a very dwarfed plant is 

 produced. 



Culture solutions of various composition are used ( 19 ). KNOP'S solution 

 contains water 1000, calcium nitrate 1, magnesium sulphate 0'25, acid potassium 

 phosphate 0'25, potassium nitrate 0'25, and * 



a trace of ferric chloride. The solution of 

 v. D. CROXE, with almost completely insoluble 

 compounds of phosphoric acid and iron, appears 

 in some cases to give better results (water 

 1000, potassium nitrate 1, potassium sulphate 

 0-5, magnesium sulphate 0'5, tertiary potassium 

 phosphate 0'25, ferrous phosphate 0*25). 



From such water cultures it results 

 that the typical land plant succeeds satis- 

 factorily if supplied with the elements 

 K, Ca, Mg, Fe, and H, O, S, P, N, if 

 in addition and C (the latter as carbon 

 dioxide) are available in the atmosphere. 

 There are thus in all ten elements 

 which must be regarded as indispensable 

 food -materials. Of these the seven 

 which remain after excluding H, O, and 

 C concern us here, since the plant obtains 

 them as nutrient salts from the soil or 

 water. Six of these seven are found in 

 the ash, while the nitrogen escapes on 

 combustion in the form of volatile com- 

 pounds. That these seven elements are 

 completely indispensable is shown by 

 the fact that if a single one is wanting 

 its loss cannot be made good by an 

 excess of the others, or by the presence 

 of a related element. 



FIG. 245. Water cultures of Fagopyrum 



'turn. I, In nutrient solution 



containing potassium ; II, in nutrient 



solution without potassium. Plants 



reduced to same scale. (After XOBBE.) 



Thus, for example, potassium cannot, as 

 a rule, be replaced by sodium, lithium, or 

 rubidium. Lower organisms (Algae, Bacteria, 

 Fungi) are able to do without Ca. The absence 

 of a single necessary element is shown either 

 by the feeble and dwarfed development of the 



plant (Fig. 245, //, absence of potassium) or by characteristic changes in the plant. 

 The best known of these is the effect of absence of iron, in which case the plant 

 does not become green (chlorosis). Injurious effects of poisoning are shown when 

 calcium is lacking. 



More accurate consideration shows that it is not correct to speak 

 of definite elements which are indispensable to the plant. Just as a 



