PHOTOSYNTHESIS AND RESPIRATION. 73 



Gather and examine leaves of various kinds, living or 

 fallen, evergreen or deciduous, stalked or unstalked, simple 

 or divided (compound). What is there common to nearly 

 all leaves, however they may differ in detail? In what 

 surroundings do you find plants with thick fleshy leaves? 

 Does the general broad and thin form of the leaf-blade 

 suggest anything as to the functions of the leaf ? Do most 

 leaves seem to expose as much surface as possible to light 

 and air ? We saw that the seed-leaves of Broad Bean and 

 of many other plants contain food, and that if they are 

 removed from a soaked seed which is beginning to germinate, 

 the growth of the young plant is stopped. 



1O7. Do Leaves contain Pood? We have seen that 

 seed-leaves often contain food-materials e.g. starch, proteids, 

 oils. It will be easy to find out whether foliage-leaves con- 

 tain any of these foods. Let us test them for the presence of 

 starch, by using iodine solution. To do this we must try to 

 remove the green colour. 



* Boil in water some leaves taken from a Bean seedling, or some other 

 plant with thin flat leaves, which has been growing in the light. The 

 colour does not come out. Place the boiled leaves in alcohol, and notice 

 that the leaves gradually lose their colour, while the alcohol turns 

 green. Try several different plants : in some the extraction of the 

 green substance takes place very slowly, in others (e.g. Broad Bean, 

 Tropaeolum, Primrose) much more quickly. l When the leaf is colour- 

 less, place it in a saucer and pour dilute iodine solution over it. The 

 depth of colour produced shows roughly how much starch is present. 

 If there is abundance of starch, the colour is nearly black ; if little 

 starch, it is bluish ; if no starch is present, the iodine merely turns the 

 leaf brownish. 



1O8. Does the Green Leaf make Starch ? Where does 

 the starch come from ? Is it made in the leaf ? 



We have seen that when a Bean seed germinates in the 

 dark the seedling has small pale leaves which do not become 

 green, while starch stored up in the cotyledons gradually 

 disappears. If an ordinary green plant were set in darkness 

 for some days, would its leaves contain starch ? 



1 Do not boil leaves in alcohol ; this is dangerous, wasteful, and (if 

 suitable plants are used) unnecessary. 



