STRUCTURE AND PHYSIOLOGY OF PLANTS 13 



blanched by excluding the light from the stems. Why does 

 a covering of straw or other thick mulch in a strawberry 

 patch prevent the growth of weeds? What harm comes from 

 the accidental covering with soil of young corn plants by 

 the 'cultivator? 



EXERCISE. Leaves Seek Light. Set some growing plants 

 in the window for a few hours and notice that the leaves 

 turn toward the light. Turn the plants around and see how 

 long it takes for the leaves to face the light again. 



EXERCISE. To Show Presence of Starch. Crack a few 

 kernels of wheat or corn and treat them with a few drops 

 of iodine. The change of color which follows is a proof of 

 the presence of starch. This was stored in the seed for its 

 future use. Starch turns blue when treated with iodine. 

 The same trial made with potato, corn-pith, pith of tree 

 twigs, and many seeds, will show the presence of starch. 



Balance in Nature. Animals use large quantities of 

 oxygen from the air and return carbon dioxide to the air. 

 Plants make use of the waste which animals breathe off. 

 The plants break up this gas, retaining the carbon and giving 

 off the oxygen. A small amount of oxygen is also used by 

 plants. The carbon retained by the leaves and the water 

 which the plants obtain from the soil are combined to form 

 organic matter. This is chiefly starch at first, but it may 

 be readily changed to sugar or other forms suitable for 

 storage. In sugar-beets and sugar-cane the storage matter 

 is largely sugar. 



Tissue for new growth of the plant may be formed either 

 from stored plant-food or from newly-formed plant-food that 

 has not been taken to any storage place in the plant. 



EXERCISE. Balance of Life in an Aquarium. Make an 

 aquarium in a large fruit-jar or other glass vessel, as shown 

 in Fig. 5. Very clear sand is used in the bottom. The plants 

 are started from pieces of water plants commonly growing 

 in ponds or lakes. The animals may be water insects of 



