20 PLANTS AND MAN 



forms specialized tissues and organs by cell differentiation. When 

 each reproductive cell develops into a new organism by itself, 

 there is only one parent and the process is termed asexual 

 REPRODUCTION. Ascxual reproductive cells, or spores, are far 

 more common among plants than animals for reasons which 

 will be discussed in Chapter 3. On the other hand many organ- 

 isms produce reproductive cells which must unite in pairs in 

 order to live, the fused cells forming a single cell which then pro- 

 ceeds to develop into a new organism. This is the essence of 

 SEXUAL REPRODUCTION, the two cclls being known as egg and 

 sperm. Sexual reproduction involves bringing the sperm and the 

 t^g^ together so that fusion can take place, a process which is 

 possible for animals with their powers of locomotion but obvi- 

 ously difficult for stationary land plants. Except for the seaweeds 

 and simple land plants, sexual reproduction with motile sperm 

 cells is not as highly developed among plants as among animals. 

 With these facts in mind it is possible to better appreciate 

 the fundamental similarities and differences in the plant and 

 animal pattern of living. We have seen that both types of organ- 

 isms have much in common; the physical basis of protoplasm, 

 organized into cells made up of cell walls, nuclei and cytoplasmic 

 parts; the division of labor in the formation of tissues and organs; 

 basic activities such as metabolism, growth and reproduction. 

 Differences between the two kingdoms are not so much differ- 

 ences in fundamental structures or functions as in different ways of 

 accomplishing the same result. Most important are the two types 

 of constructive metabolism — the autotrophic food manufacture 

 of green plants and the contrasting heterotrophic dependence of 

 animals upon ready-made organic compounds. Associated with 

 this basic difference is the presence of chlorophyll, with the result- 

 ing prevalence of the green color in plants; and the more obvious 

 ways in which we think of animals and plants as being unlike — 

 in the ability to move about, to be conscious of the surroundings, 

 to fe^l and to think. As will be evident when the activities of 

 plants are discussed more in detail, this plant pattern of existence 

 has been just as successful in its own way as the more familiar 

 one of the animal kingdom. 



