86 



FOOD ELABORATION AND RESPIRATION 



the cut end of the twig. This gas has been found to be largely 

 oxygen. The water-plant gets its carbon dioxid gas from 

 that which is dissolved in the water. A gas, as well as a 

 solid, may be dissolved in water. Observe the bubbles on 

 pond-scums and water-weeds on 

 bright day. 

 181. Starch is present in the 

 green leaves that have been ex- 

 posed to sunlight; but in the dark 

 no starch can be formed from 

 carbon dioxid and water. Apply 

 iodin to the leaf from which the 

 chlorophyll was dissolved in a 

 previous experiment (174). Note 

 that the leaf is colored purplish 

 brown throughout. Starch gives 

 a blue coloration with iodin. 

 The leaf contains starch (76). 

 Secure a leaf from a plant 

 that has been in the darkness 

 for about two days. Dissolve 

 the chlorophyll, as before, 

 and attempt to stain this 

 leaf with iodin. No purplish 

 brown color is produced. 

 182. Plants or parts of plants that have developed no 

 chlorophyll can form no starch. Secure a variegated leaf 

 of coleus, ribbon-grass, geranium, or of any plant showing 

 both white and green areas. On a day of bright sunshine, 

 test one of these leaves by the alcohol and iodin method 

 for the presence of starch. Observe that the parts devoid of 

 green color have formed no starch. However, after starch 

 has once been formed in the leaves, it may be changed into 

 soluble substances and removed to be again converted 

 into starch in other parts of the living tissues. 



