48 INTRODUCTION 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.! 
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. 
} Dependency among lower plants is discussed at length in later chapters. 
