26 THE WORK OF THE LEAF [chap. 



leaf is starch. We have already learnt that starch 

 consists of the element carbon combined with the 

 elements of water, from which it follows that the plants 

 must be able to manufacture starch and oxygen out of 

 carbon dioxide and water, just as in the reverse way 

 starch and oxygen will burn together to produce water 

 and carbon dioxide. 



Assimilation. 

 Carbon ) _ , ^. . , Carbon^ ^ , _ 



Oxygen J ^^^^°"^^°^^^^ + ^^^^^ = Water | Starch + Oxygen. 



Of course the process of making starch is not so simple 

 as this diagram makes out, because there must be several 

 intermediate stages within the plant, still for our pur- 

 poses we only need to recognise the relationship between 

 the final products of the change in order to interpret 

 the various experiments which follow. It is first of all 

 necessary to learn to identify starch within the leaf; 

 because the leaf is alive and strongly coloured, it dis- 

 guises the blue colour which iodine usually produces 

 with starch. The green leaf must be dipped for a 

 moment into boiling water to kill it, and then soaked in 

 alcohol or methylated spirit, until the green colour is 

 dissolved out. If now the bleached leaf is dipped into a 

 very weak solution of iodine, the colour of pale sherry, it 

 will be turned very dark blue, practically black, because 

 all the starch grains that are diffused throughout the 

 leaf are intensely stained by the iodine. The leaf can 

 then be examined under the microscope to see how the 

 rounded grains of starch are imbedded in all the cells 

 composing the interior tissues of the leaf. 



The next step is to show the dependence of starch 

 formation upon the exposure of the leaf to light ; a 

 dwarf tropaeolum plant is very suitable for experiment, 

 if it can be covered completely with a box so as to 

 exclude the light. The next morning but one, remove 



