420 THE FOOD OF PLANTS 



wetted with distilled water. Roots removed from soil to a water-culture are more 

 or less injuriously affected (cf. Sect. 26). 



Good nutrient solutions are obtained by dissolving 4 grms. of calcium nitrate, 

 i grm. potassium nitrate, i grm. magnesium sulphate, i grm. acid phosphate of 

 potassium, 0-5 grm. potassium chloride in 7 litres of water ( = 0-016 per cent, of 

 salt) or in 3 litres ( = 0-025 P er cent - of salt), and adding 3 to 6 drops of the 

 medicinal solution of ferric chloride. Also (a) 20-5 grms. magnesium sulphate 

 may be dissolved in 350 cc. of water and (b) 40 grms. calcium nitrate, xo grms. 

 potassium nitrate and 10 grms. acid phosphate of potassium in 350 cc. Then by 

 adding 100 cc. of (a) and (l>) to 9-8 litres of water a solution is obtained which 

 contains 0-2 per cent, of salts, and to which only the addition of a few drops of 

 ferric chloride, and in some cases of a little potassium chloride is necessary. 

 Tollens gives a method by which concentrated nutrient solutions may be prepared 

 in which no precipitate is formed '. 



The percentage composition of the nutrient solution may vary within com- 

 paratively wide limits, but growth is usually retarded by an excessive preponderance 

 of sulphates or phosphates, and magnesium should be offered in less amount than 

 calcium or potassium (cf. composition of ash constituents). The best nutrient 

 solution for most Phanerogams contains from o-i to 0-5 per cent, of salts. For 

 small plants and for demonstration experiments vessels containing 2 to 5 litres may 

 be employed, but otherwise the use of larger vessels holding more than 10 to 20 

 litres is to be recommended. In all cases the loss of water by evaporation, &c. 

 must be made good, while if small vessels are used an occasional renewal of the 

 saline constituents will be necessary. The nutrient solution given above has an acid 

 reaction, but the addition of a few drops of phosphoric acid is advantageous in 

 order to prevent the fluid becoming alkaline 2 , though a little phosphate of iron 

 is always precipitated. By occasionally blowing air through the culture fluid it is 

 brought within reach of even the smallest roots, and at the same time the water 

 is kept well aerated. Good illumination and a sufficiency of carbon dioxide are 

 necessary for the growth of autotrophic plants, and hence the cultures should be 

 kept in the open as much as possible 3 . Any over-heating of the culture fluid may 

 be avoided by surrounding the culture vessel with sawdust, and this, at the same 

 time, ensures that the roots are kept in darkness or exposed only to feeble light. 



To secure the absence of a particular element, say potassium, sodium salts may 

 be added in its place, or by using nitrates instead of sulphates the absence of sulphur 

 may be assured. When a soluble salt of barium is added, and hence the absence of 

 sulphates is necessary, the plants -may be provided with sulphur in the form of 

 isothionic acid, or taurin, or in the case of fungi by sulphur dioxide 4 . It must- 



1 Knop, Versuchsst., 1884, Bd. xxx, p. 293; Tollens, Bot. Jahresb., 1883, p. 36. 

 a Hellriegel (Unters. u. d. Stickstoffnahrnng, 1888, p. 140) states that lupines are sensitive to 

 acid solutions. On the use of large vessels, cf. Wortmann, Bot. Zeitung, 1892, p. 643, and Nobbe, 1. c. 



3 The pots may be placed on trolleys travelling on rails, and run into a greenhouse when 

 the weather is cold or stormy (Hellriegel, I.e., p. 15). 



4 See Nobbe, Versuchsst., 1870, Bd. xm, p. 331; I.upke, Landw. Jahresb., 1888, Bd. xvn, 

 p. 889; Aschoff, ibid., 1890, Bd. XIX, p. 113; Schimper, Flora, 1890, p. 220. 



