90 PHOTOSYNTHESIS AND RESPIRATION. 



124. Amount of Light received and absorbed by the 

 Leaf. A leaf receives only a small proportion of the light 

 which falls on it, even when exposed to full sunshine, the 

 greater part being lost by reflection and absorption. A 

 smooth and shiny cuticle reflects the light, while much of it 

 is absorbed by a thick cuticle or a dense covering of hairs. 

 In either case the result is the same ; the green tissue (meso- 

 phyll) of the leaf in most cases actually receives only about 

 one-tenth of the incident light. 



However, in most leaves the epidermis absorbs but little of 

 the available (i.e. unreflected) light, and, on the other hand, 

 most of this light is used by the green tissue. Thin leaves 

 transmit a good deal of the light, but such leaves usually 

 belong to plants which grow in shaded places, so that in any 

 case the absorption of the light received is practically com- 

 plete in most plants. 



Only a small proportion probably less than 5 per cent. 

 of the light received by the green mesophyll cells is actually 

 used in photosynthesis ; the remainder is converted into heat 

 and causes vaporisation of the water in the chloroplasts. 



(a) A photographer's "exposure meter" should be used to compare 

 the intensity of light in open and shaded places, etc. This instrument, 

 some forms of which are very cheap, consists essentially of a strip of 

 sensitised paper and an arrangement for exposing a small piece of the 

 paper at a time, the period of exposure in seconds being noted. 



(6) Expose pieces of the sensitive strip to the light for 1, 2, 3, 4, and 

 5 seconds successively, about noon on a bright day, and note the dif- 

 ferences in tint in the five pieces. After exposure put the light-prints 

 into a light-tight box, which should only be opened in ruby light (or 

 gaslight if "gaslight paper" is used). 



(c) Strip a piece of epidermis from a leaf, place it over the opening 

 in the meter, and expose to sunlight. 



Take an exposure, for the same length of time, with the same leaf or 

 a similar one, over the sensitive paper. 



Take a print by exposing the paper itself to the light. 



On comparing the three prints we get a rough idea of the amounts 

 of light shut off by the epidermis, absorbed by the mesophyll, and 

 transmitted through the leaf. 



In the Sunflower the epidermis allows roughly O'l of the incident 

 light to pass ; the paper covered by a piece of epidermis needs to be 

 exposed nearly ten times as long as the uncovered paper to give the 

 same tint. The whole leaf allows very little light to pass, only about 

 O'OOS ; paper covered by a leaf takes three minutes or more to acquire 

 the tint produced by full light in one second. The mesophyll, there- 

 fore, gets 0-095 of the light which falls on the leaf. 



