WATER CULTURE, PHOTOSYNTHESIS, RESPIRATION. 165 



218. Water Culture Jars. Get about 20 large glass 

 jars of equal size if possible to hold 4 or 5 litres, cer- 

 tainly not less than 2 litres. Fit each jar with a wooden 

 cover or a cork, having in the centre a hole for the plant, 

 a slit running from this hole to the edge of the cover (so 

 that the plant can be inserted or removed easily when 

 necessary), and a second and smaller hole for a stick to 

 which the plant can be lashed for support. Take care 

 to keep the cover, as well as the part of the plant in 

 contact with it, quite dry failure in water cultures is 

 often due to damping-off caused by Fungi. If a stick 

 is used to support the plant, it is hardly necessary to put 

 in any packing material round the base of the shoot if 

 any packing is used, soft asbestos is much better than wet 

 cotton-wool. 



It is also advisable to cleanse the jars thoroughly before 

 starting the cultures wash with water, rinse with strong 

 nitric acid, wash again with water, rinse again with water, 

 then with strong solution of mercuric chloride, and lastly 

 with boiled water. 



219. Water Culture Solution. Various recipes have 

 been given for the making up of culture solutions for green 

 plants. The following (Knop's Solution) has the advan- 

 tage that it provides just the essential elements, neither 

 more nor less. Weigh out 



(a) Potassium nitrate, KN0 3 1 gram . 



(&) Potassium phosphate, KH 2 PO 4 1 gram. 



(c) Magnesium sulphate, MgS0 4 1 gram. 



(d) Calcium nitrate, Ca(N0 3 ) 3 3 grams. 



Dissolve a, b, and c in 1 litre of water, then add the 

 calcium nitrate a precipitate of calcium phosphate is 

 formed, but this is gradually dissolved and utilised by 

 the roots. This is a solution of 0'6 per cent, strength, 

 and may be diluted to the extent required ; when a dilute 

 solution is made from this stock solution, shake the bottle 

 well so as to include a right proportion of the precipitate 

 in the dilute solution. 



