THE WORK OF STEMS 253 



keeping the other well watered. Then place the covers 

 on them and observe the growth as before. 



(c) Light. Repeat the experiment, giving both 

 pots of plants plenty of water and warmth (the same 

 amount to each) , but keeping one covered with an 

 opaque cover while the other is exposed to strong 

 light. Do plants grow faster during the day or during 

 the night I 



The growth of the stem requires a great deal of 

 food. Test the growing portion (especially the forma- 

 tive region) for food substances (for fat, use the 

 alcanna test, page 259). Study especially the behav- 

 ior of starch in buds (e. g., buds of' Hawthorn, 

 Maple, Linden, Lilac, etc.). In general the embryo 

 leaves contain no starch in the fall, although there is 

 plenty in the tissue just beneath them. In the spring 

 it wanders into the young leaves and furnishes material 

 for their growth. Buds furnish nutritious food to 

 many kinds of animals. How are the food substances 

 brought here from the leaves ? See if you can trace 

 their paths through the stem. This will probably be 

 easier in the case of starch than in the case of the 

 other substances. Starch cannot pass through the 

 walls of the cells which compose the plant, but it is 

 readily changed into sugar, which can pass from cell 

 to cell and so reach the growing region; when it 

 arrives there it may be changed into fats, oils, or even 

 into proteid (by combining with nitrogen, sulphur and 



