96 Introduction to Botany. 



intervention of plants. The starch and proteids stored up 

 in seeds, and particularly in the grains of cereals, are in 

 quite stable condition, and are well adapted to carry plant 

 life through months and even years of adverse conditions. 

 Their stability and condensed condition also make them 

 fit for the food of mankind through the months between har- 

 vests and through years of famine. It is this stable and 

 condensed form which enables cereals to be transported to 

 all quarters of the globe so that the people of one conti- 

 nent may be fed from the granaries of another. 



74. Amount of Work done by Leaves. The amount of 

 work done by the leaves of plants in a single year over 

 the entire earth's surface is beyond computation ; but 

 perhaps some idea of its immensity may be gained by 

 calling to mind that in the state of Kansas alone, in the 

 year 1900, over 134 million bushels of corn and over 

 77 million bushels of wheat were harvested. Or, to take 

 examples with smaller numbers, it has been estimated that 

 one square meter of sunflower leaf may, in a single summer 

 day, produce twenty-five grams of starch, and a single stalk 

 of corn may in the same time send out into the ears from 

 ten to fifteen grams of reserve food material. 



75. Duration of Work of Leaves. In some kinds of 

 plants, such as tulips, peas, wheat, and corn, the leaves 

 finish their work before the close of the growing season 

 and die, having first given over to the seeds, bulbs, etc., 

 most of their materials which could be used as reserve 

 food. In other plants, such as our common trees and 

 shrubs, the leaves continue their work until the close of 

 summer. They then send over to the body of the plant, 

 which is to survive the winter, much of the useful material 

 contained in them ; and then sever themselves from the 

 branches by producing a layer of delicate tissue, so that 



