280 PLANT PHYSIOLOGY 



In some plants it is quite impossible for enough food to be 

 manufactured each year for a good crop of flowers and fruit. The 

 American agave or century plant (Agave americana) may spend 

 20-30 years storing up food materials and water in the thick 

 fleshy leaves before producing any blossoms whatever. Then a 

 thick heavy cluster of flowers is quickly produced, after which 

 the parent plant dies. The desert shrub, Olneya, produces seeds 

 only about once every three or four years, but this production 

 seems to depend more upon an adequate supply of stored food, 

 which is impossible under dry conditions, than upon the immediate 

 water supply. 



Storage Foods. — The principal storage foods are starch, sugars, 

 proteins, fats and oils, inulin, and reserve cellulose. Starches are 

 by far the most common. In the storage organs the starch is 

 built up from the soluble carbohydrates by means of plastids 

 called leucoplasts. In Pellionia, the formation of starch in the 

 leaves and stems by the green chloroplasts can be easily followed 

 with the microscope. The leaf starch is then digested, translocated, 

 and built up again in the form of starch in the storage regions. 



The starch occurs in the form of grains which are characteristic 

 for the species and which are used in the detection of adulterants. 

 For further properties of this important group of foods see Chap- 

 ter XII. Throughout the world plants and organs containing 

 starch are found, many of which, including the cereal grains, 

 buckwheat, beans, potatoes, sweet potatoes, turnips, cassava, 

 sago, tapioca, breadfruit, and bananas, are much used by man for 

 the starch they contain, and serve as the staple articles of diet in 

 the regions where they are found. The potato, which contains 70- 

 80% starch by dry weight, and the cereals, containing 60-80% of 

 starch, are our chief sources of starch in the north temperate zone. 



Sugars are found especially in fruits, but are obtained in large 

 quantities from the root of the sugar beet and from the sterna 

 of the cane and sugar maple. 



Proteins are found among our vegetable foods chiefly in legu- 

 minous seeds (peas, beans, and lentils) and among the nuts. The 

 gluten of wheat is protein (Chap. XIV), and some fruits, for 

 instance the banana, contain appreciable amounts of this impor- 

 tant reserve, although fruits in general are low in protein content. 



The fats and oils are found especially in certain types of seeds. 

 In some members of the lily family, oil replaces starch as the 



