PHOTOSYNTHESIS AND RESPIRATION. 91 



(d) Make and compare " epidermis prints " and " leaf -prints " of the 

 leaves of plants growing naturally in open and in shaded places, plants 

 with shiny cuticle (e.g. Holly, Cherry, Laurel), leaves with hairy 

 oovering, etc., at the same time making ordinary " light prints " as a 

 standard for comparison. 



125. Which Light Rays are concerned in Photo- 

 synthesis ? It is fairly easy to test this by comparing the 

 effects of exposing plants to light of different colours i.e. 

 allowing only certain rays to fall upon the leaves. It is found 

 that the rays at the red end of the spectrum are more active 

 than any of the rest in promoting photosynthesis, and that 

 for most plants the curve obtained when the results of experi- 

 ments are plotted on squared paper shows two " humps " or 

 maxima, a higher one in the orange and a lower in the blue, 

 with the lowest intermediate part (minimum) in the green. 



(a) A rough comparison may be made by setting a plant, or a leaf 

 with its stalk dipping into a bottle of water, in a box, one of whose 

 sides is replaced by a sheet of red glass, another in a box with a side of 

 green glass, another with blue glass. After several hours' exposure to 

 light, test each for starch with iodine solution. However, coloured 

 glass is hardly ever pure, in the sense of allowing only rays of one 

 colour to pass through it. This can easily be seen by testing coloured 



flass with a spectroscope (an excellent direct- vision spectroscope can be 

 ad for 25s.), or with a lantern and prism. 



* (6) A better method is to use a pair of double-walled bell-jars, the 

 space between the two walls being filled with a coloured solution. One 

 should be filled with watery solution of potassium dichromate ; the 

 other with watery solution of copper sulphate, to which ammonia has 

 been added. The first solution allows red and orange rays to pass 

 through, the second one blue and violet rays. In this way we can at 

 least divide the spectrum into a red-end half and a blue-end half. Set 

 each bell-jar on a folded cloth, or in a saucer of dry sawdust, so as to 

 shut out any white light, place under each a plant in a pot or a seed- 

 ling that has been dug out and had its root set in a bottle of water. 

 Set both bell-jars in diffused light ; in direct sunlight the temperatures 

 in the two would not be the same (why ?). The plant in the red-orange 

 light will be found after exposure to light (let the experiment last for 

 two days) to have formed abundant starch, that in the blue-violet light 

 will be almost free from starch. 



(c) Watch the bubbles of oxygen arising from a submerged water- 

 plant, and time the rate of bubbling. When this is fairly regular, 

 cover with the blue bell-jar, and notice that the bubbling becomes 

 slower after a short time. After about five minutes (take several 

 readings during this time) take off the blue jar and put on the red- 

 orange one, taking records of the rate of bubbling as before, noting the 

 increase in red-orange as compared with blue- violet light. 



