THE WATER-RELATION 93 



It thus appears that those salts are landed by the Transpiration-Stream 

 at the very place where they will be chiefly used, and in the daytime, 

 when, as wc shall see later, the constructive process is most active. 

 Thus the Transpiration-Stream is not a mere method of making up 

 what the plant has lost. It is actually the means of providing certain 

 of those substances which are essential for the constructive nutrition 

 of the Plant. 



Food of Plants. 



The next question will naturally be what substances they are which 

 are transferred ? The soil-water contains the substances in solution 

 in it in a very dilute form. It would therefore be difficult to arrive by 

 direct test at any conclusion as to the actual substances absorbed by 

 the root. They can best be recognised by an indirect method, viz. 

 by analysis of the plant-body, so as to show the actual substances of 

 which it is composed. The elements involved may be grouped accord- 

 ing to their constancy of occurrence, thus : 



Constantly present : Carbon, Hydrogen, Oxygen, Nitrogen, Sulphur, Phos- 

 phorus, Potassium, Calcium, Magnesiun, and Iron. Usually present : Sodium, 

 Silicon, Chlorine. Occasionally present : Manganese, Aluminium, Copper, Zinc, 

 Cobalt, Nickel, Strontium. Present in Seaweeds : Iodine, Bromine. 



It will be unnecessary to follow the occasional elements further. 

 The chief physiological interest lies in those which are constant, for 

 in them there is a probability of special physiological use. To obtain 

 information on this point experiments have been made by water- 

 culture (Fig. 70), the roots being grown in water in which the food- 

 substances are dissolved. A full culture solution giving all the sub- 

 stances which are constant in the plant, excepting Carbon, may be 

 made up according to various recipes. That of Sachs is as follows : 



Water 1000 cc.m. 



Potassium nitrate - - - i.o gramme. 



Sodium chloride - - - 0.5 



Calcium sulphate - - - 0.5 ,, 



Magnesium sulphate - - - 0.5 ,, 



Calcium phosphate - - - 0-5 ,. 



Ferric chloride - - - - a trace. 



The common calcium phosphate should be finely pulverised, and as 

 it is only sparingly soluble it forms a sediment during the experiments. 

 This recipe contains all the constant elements excepting Carbon. It 

 also contains Sodium and Chlorine, but not Silicon. Such a solution 

 supplied under certain precautions in a glass jar to the roots of a seed- 

 ling may suffice for its full growth, and even for its flowering and 



