20 BOTANY: PRINCIPLES AND PROBLEMS 



to combine carbon dioxide (coming from the air) and water (com- 

 ing from the soil) into grape sugar, a simple carbohydrate food. 

 This process of photosynthesis is entirely confined to green plants. 

 By it is manufactured all the food which sustains the lives of 

 plants and animals and of man himself, for all the complex foods 

 with which we are familiar have been built up by progressive 

 modifications of grape sugar produced in green leaves. 



The Stem and Its Functions. — The stem normally holds aloft 

 in the air the leaves and reproductive organs, and serves as a 

 highway for transportation of water and nutrient materials from 

 the root to the leaf, and of manufactured food from the leaf to 

 other parts of the plant body. Stems may be comparatively 

 small and soft, as in herbaceous plants, or stout and woody, as in 

 trees and shrubs. They are occasionally modified for special 

 functions, such as food-storage or photosynthesis. 



The Reproductive Organs and Their Function. — The sole 

 function of these organs is the production of offspring, through 

 which the life of the plant may be transmitted to succeeding 

 generations. The flower, typically composed of sepals, petals, 

 stamens, and pistil, is concerned with effecting fertilization, or 

 the union of male and female sexual cells; the fruit protects the 

 growing seeds and often aids in their dispersal, and the seeds are 

 young plants themselves in embryo, protected by a coat, provided 

 with a supply of food, and ready to begin their independent 

 growth and development whenever favorable conditions appear. 



Metabolic Processes. — Certain physiological activities of the 

 plant are not confined to any one organ but are characteristic 

 of living substance or protoplasm wherever it may be. Notable 

 among these are digestion, whereby food is rendered soluble; 

 assimilation, whereby such digested food is incorporated into 

 protoplasm, and respiration, whereby the supply of energy 

 necessary for the plant's activities is released through the break- 

 ing down of living tissue, with the consequent absorption of 

 oxygen and liberation of carbon dioxide. 



QUESTIONS FOR THOUGHT AND DISCUSSION 



28. There are almost as many species of Thallophytes as of Seed 

 Plants, but the latter are much more familiar to most people than are 

 the former. Why? 



29. Which of the four main divisions of the plant kingdom do you 

 believe is the oldest? Why? 



