48 INTKODUCTION TO BOTANY 



conditions under which plants produce most abundantly. They 

 must study and improve those plants which are found to 

 perform their nutritive functions in such a way that large 

 amounts of desirable food are produced economically. 



The island of Chung-ming, at the mouth of the Yangtze 

 River in China, has an area of 270 square miles. It has but one 

 large city, yet the whole island has a dense population. The 

 inhabitants have made such a study of the productivity of 

 plants that the island is said to support a population of 3700 

 people per square mile. Our own rural population of 61 per 

 square mile of improved land suggests by comparison the 

 necessity of further study of the food cycle of plants and of 

 the conditions under which our economic plants thrive best. 



48. Independent and dependent plants. So far in this chapter 

 we have spoken only of plants that have chlorophyll and can 

 make their own food from materials that are not ordinarily 

 regarded as nutrient substances ; that is, we have discussed 

 only independent plants. But there are many plants that do 

 not possess chlorophyll, and even some that do possess it, that 

 are dependent for their food upon the chlorophyll work of 

 plants similar to those we have already discussed. Then some 

 plants are dependent, not for food, but in other ways. Some 

 dependent flowering plants, like the woodbine, or Virginia 

 creeper, are almost independent. A woodbine may grow in 

 the open and attain its full size, but in dense woodlands 

 grapevines, woodbines, and many other climbers can only 

 make a normal growth by climbing upon the trunks of trees 

 and so raising themselves into the light. 



49. The food supply and dependency in flowering plants. 1 

 The principal groups into which dependent flowering plants 

 are divided are as follows: 



1. Lianas, or climbers. 



2. Epiphytes, or plants which rest upon other plants. 



3. Saprophytes, or plants which live on the products of the 

 decay of organic matter. 



1 Dependency among lower plants is discussed at length in later chapters. 



