64 FIEST STUDIES IN PLANT LIFE 



starch is stored up in thick roots, some in bulbs, some 

 in undergroimd stems, some in twigs and some also, 

 — as we saw in the case of wheat, in the seed. When 

 the leaf has made more starch than it can hold, it 

 passes it down to the stem. If you tear a leaf ofif, and 

 put the stalk in water, it will go on making starch, 

 but soon has to stop because it can no longer pass the 

 starch down to the stem. During the night, when a 

 leaf cannot work for want of light, the starch flows 

 down into twig and stem, and so the leaf is almost 

 free of starch when the morning light calls it to 

 another day's work. 



2. An experiment to show that there is starch 

 in a potato. Now, there is an easy way of finding 

 out whether there is starch in any substance. When 

 moist starch is touched by iodine, it becomes black 

 with a blue tinge. You can prove that there is starch 

 in a potato by scraping some of the cut surface into 

 pulp, and then applying a little tincture of iodine.* 

 In a similar way, starch can be found in any quickly 

 growing leaf that has been at work in the sun. This 

 experiment is too difficult for you just now, but you 

 can easily prove that leaves that are not green can 

 make no starch. 



3. Experiment to shew that a white leaf can 

 mai<e no starch. Take a white leaf — you can often 

 find one on a geranium that is in a shady spot — and 

 cut into the surface gently so as to make a network of 

 cuts. Steep the leaf in a tincture of iodine. Then 

 rinse in water and you will see that there is no sign 

 of starch. This shews that a leaf that has no living 



* Tincture of lodmo can be got at a trifling cost from a chemist, or 

 crystals of iodine can be diBsolved in alcohol. Iodine is poisonous. 



