CHAPTER IX 



THE FOOD-MAKING OF PLANTS 



What is the work of the plant ? We have seen how the root 

 has to go in search of food material which the protoplasm 

 receives and passes on. In order to grow and sustain life, the 

 plant must have food. Animals depend upon plants for food. 

 Plants must not only manufacture their own food, but they 

 entertain a gay society of bees and butterflies and their 

 relatives, while the needs of the poor and humble guest must 

 be supplied. Moreover, the plant must provide for its large 

 family. 



Not all the water which passes to the leaves is given off as 

 vapour. Some of it is kept to make food for the plant. Food 

 can be made only in daylight by green plants or those that 

 contain chlorophyll. It is made faster than it is needed, and 

 the surplus is stored during the day in the leaves as starch. 

 During the night, when most rapid growth takes place, the 

 surplus is used. Food is not used by the plant in the form of 

 starch, but it is changed into sugar, and then combined with 

 other materials brought up from the soil before it becomes 

 part of the living plant. Starch is simply a convenient form 

 for storing food until it is needed. It is formed not only for 

 daily needs, but biennials and perennials store enough one year 

 to give the plant a good start the next. It is usually stored 

 under ground. Potatoes are almost entirely filled with starch 

 which the leaves have made. Some plants store their food in 

 the form of oil or sugar. To build up protoplasm, both animals 

 and plants require food containing nitrogen. 



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



