178 WATER CULTURE, PHOTOSYNTHESIS, RESPIRATION. 



235. Methylene Blue Method (Prof. Farmer's). Dissolve a 

 little methylene-blue in water, so that the solution is well coloured 

 yet quite transparent when placed in three large test-tubes. Keep 

 one tube (A) as a control and standard of colour. Into B put some 

 cut branches of Elodea (Prof. Farmer recommends Chara as giving 

 a quicker result). Into C put some soaked Peas ; cut a slice from 

 each, so that the solution may have access to the cotyledon tissue. 

 Place the three tubes in darkness for two days. A remains un- 

 changed ; in B and C the solution is decolorised, owing to the dye 

 having been deprived of oxygen. Transfer (with a pipette or glass 

 tube) some of the decolorised solution from B and C into two test- 

 tubes and shake each tube up ; the blue colour reappears owing to 

 the access of atmospheric oxygen. Now place the tubes in the 

 light ; in B the blue colour is restored, owing to the giving-off of 

 oxygen by the leaves in the process of photosynthesis, while A and 

 C remain colourless. Turn out the Peas from G into a saucer, 

 rinse them with water, and slice some of them with a knife or 

 razor ; the cut surfaces turn blue as the oxygen of the air enters the 

 tissues. 



236. That Light is essential for Photosynthesis is 



strikingly demonstrated by the effects of excluding light 

 from portions of leaves otherwise exposed in the ordinary 

 way; this may be done by a variety of methods. For 

 instance, set a Tropaeolum or Primrose plant in darkness 

 for two days, and on the morning of the third day pick off 

 a leaf (A) and place it in a tube of alcohol, then treat 

 different leaves on the plant as follows: fix a strip of 

 tinfoil across a leaf (J5) ; cut out in tinfoil or a card some 

 pattern or letters (e.g. the word LUX) and fix this stencil 

 over a leaf (C) ; expose the plant to light for several 

 hours. Take off the leaves B and 0, also a leaf (D) that 

 has been left untouched ; decolorise with alcohol, and test 

 the four leaves, A, B, C, D, with iodine. 



237. Light Screens. The preceding experiment is 

 open to the objection that while light is excluded, the 

 other conditions are not kept as nearly as possible the 

 same as should be the case in all physiological experi- 

 ments in which the influence of a single factor on any 

 process is being studied. By covering a leaf with a band 

 or stencil of tinfoil, we not only darken the leaf but also 

 exclude air from it, besides causing changes in temperature. 



