CHAPTER XII. 



THE FOOD MAKING OF PLANTS. 



Besides manufacturing their own food, many plants entertain 

 a gay society of flies, bees, butterflies, and their relatives. 

 Moreover a plant must provide for its large family of growing 

 seeds. In Chapter IV we found that air, water, and soil were 

 the sources of food, and in Chapters X and XI the composi- 

 tion of air and soil were studied. 



Carbonic acid gas, which is exhaled by animals and 

 plants and which occurs in such minute quantities in the air, is 

 a waste product to animals when exhaled but it can be appro- 

 priated by green plants and made into living substance. This 

 process is known as assimilation. In leafy plants this pro- 

 cess takes place chiefly in the leaves. As protoplasm is formed 

 in the leaves it is broken down into simpler forms of food and 

 carried away by the phloem to build up growing parts. Part 

 of it is accumulated as reserve food in the day and used up at 

 night when most rapid growth takes place. Starch is a con- 

 venient form of storing food until it is needed for daily use and 

 biennials and perennials store enough one year to give the 

 plant a good start the following year. Potatoes are almost en- 

 tirely filled with starch. Some plants store food in the form 

 of sugar as the beet, while many composites store inulin. 



We can tell where starch is found by staining with iodine, 

 as we did in case of the Sunflower stem. A tincture of iodine 

 may be obtained or the crystals dissolved in water. Scrape a 

 small portion of potato and place it in a tube of water. Add 

 a few drops of iodine. The liquid at once turns blue. Place 

 some maizena or laundry starch in slightly warm water. Allow 

 it to cool and add iodine. The same blue colour appears. 

 The particles of starch are coloured blue by iodine, 



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