THE LEAF 79 



Starch is a very common substance in plant tissues. 

 It is one of the chief forms in which plants store up 

 reserves of food to be used on some future occasion, 

 when greater demands are made for food than can be 

 supplied by the assimilation of the moment. In the 

 production of fresh shoots from potatoes, and in the 

 germination of seeds, a large amount of growth goes on, 

 entirely at the expense of the food - reserves stored 

 away in the tuber or seed. It is only later, when the 

 new shoot has formed its own green leaves, that it can 

 do anything at all towards making fresh supplies of food 

 for itself. 



It has already been stated, and will later be experi- 

 mentally proved, that assimilation, resulting in the 

 formation of starch, can only go on in the green parts of 

 plants, and only there when they are exposed to sun- 

 light. The question naturally arises then : how do we 

 find starch in tubers, seeds, or other non-green and even 

 underground parts of plants ? The answer to this, too, 

 will be supplied by means of simple experiments. If a 

 growing plant is left exposed to a good light from early 

 morning to afternoon and its leaves tested then, they 

 will be found to be loaded with starch. But, place this 

 same plant in darkness for twelve hours or more, and 

 its leaves will be found to be almost emptied of starch. 

 As a matter of fact, the starch formed in them in the 

 sunlight has been changed into sugar, and in this form 

 carried away from the leaves in which it was made, and 

 either used up in growth or often changed back again 

 into starch and stored up in some other part as a 

 reserve of food. 



