168 WATER CULTURE, PHOTOSYNTHESIS, RESPIRATION. 



II. PHOTOSYNTHESIS. 



222. Iodine Test for Fhotosynthetic Starch in 

 Foliage-leaves, etc. Pick leaves from various plants 

 which have been exposed to light in the usual way. To 

 test the leaves for starch, which is in most green plants 

 the first visible product of photosynthesis, the chlorophyll 

 should be removed by means of alcohol, and the blanched 

 leaves placed in dilute iodine solution. The extraction of 

 the chlorophyll is hastened by boiling the leaves in water 

 for a few minutes before steeping them in the alcohol, also 

 by using warm alcohol or placing the alcohol, containing 

 the leaves, in a large test-tube set in a beaker of water and 

 warming over a sand-bath (to prevent the ignition of the 

 inflammable alcohol or its vapour). It will be found that 

 the leaves of Tropaeolum, Primrose, and Fuchsia are readily 

 decolorised and otherwise well suited for photosynthesis 

 experiments, but other plants should be tried. 



Since the alcohol makes the decolorised leaves brittle, 

 soften them by steeping in hot water for a minute or two ; 

 place them in iodine (dissolved in potassium iodide) for a 

 few minutes ; rinse them in water, then place them in clean 

 cold water in a saucer. A yellow or brownish colour indi- 

 cates absence of starch ; if the leaves contain starch they 

 will turn blue or almost black. The colour obtained depends 

 upon the amount of starch present in the tissue and the 

 strength of iodine solution used. The leaves may be pre- 

 served in alcohol, which destroys the blue colour, and may 

 afterwards be again treated with iodine after being rinsed 

 in hot water but it is always better in class work to start 

 each experiment from the beginning. 



Since the colour given with the iodine test is often by no 

 means blue but a purplish brown, it is a good plan to place 

 the leaf for a few minutes in benzole after treating it with 

 iodine. The benzole removes the iodine from the proto- 

 plasm and the cell-walls, but does not affect the blue 

 " starch iodide," hence this method causes the blue colour 

 to show up clearly, being no longer masked by the 

 brownish colour of the iodine- stained protoplasm and 

 cellulose. 



