THE ORGANIZATION OF THE INDIVIDUAL PLANT 137 



of its tasks, but it rests upon the same basis — the orderly cooperation 

 of a multitude of living units, the cells, each of which has some small part 

 to play that is related to the functioning of the whole organism. 



The plant is confronted by the same fundamental problems as the 

 animal. These are: (1) The maintenance of the individual, in the first 

 place, by the capture, transformation, storage and utilization of energy 

 and materials; in the second place, through devices for protection against 

 unfavorable factors in the environment, both physical and biotic. (2) The 

 maintenance of the race, through reproduction. The second of these prob- 

 lems constitutes the theme of the next section of this book. 



Some important differences between plants and animals. The most 

 obvious and characteristic features of the higher plants, including the 

 division of the plant body into root, stem, and leaf, its relative immobility 

 and rigidity, and the prevailingly green color of the foliage, are all directly 

 related to the method of food getting. All green plants 1 owe their color 

 to the presence in their cells of the complex substance called chlorophyll, 

 which functions as a catalyst. 2 In the presence of chlorophyll the energy 

 of sunlight causes carbon dioxide and water to combine into the simple 

 sugar, glucose, which is the basic plant food (not the carbon dioxide, water, 

 and minerals taken in by the plant, as is sometimes stated). The process 

 of glucose manufacture is called photosynthesis (Greek, phos, photo, 

 "light," and synthesis, "putting together"). 



Organisms may be divided into two great functional groups according 

 to whether they do or do not possess chlorophyll and in consequence can 

 or cannot manufacture their food. Only the green plants belong to the 

 first group, having chlorophyll. Practically all other organisms, including 

 multicellular animals, Protozoa, and certain plants that have lost the 

 ability to produce chlorophyll, must obtain their food directly or in- 

 directly from green plants. 



A typical animal, in order to obtain food, must be able to move about, 

 to recognize and secure the food, and to rework it into a form suitable for 

 its own use. Furthermore, the existence of other food-seeking organisms 

 necessitates means of protection or escape. Related to these needs is the 

 general development among animals of locomotor and sensory devices, 



1 Some "green" plants appear to be of colors other than green, because of the 

 presence of masking pigments in addition to chlorophyll. The true antithesis of the 

 "green" plants is found in certain plants that lack chlorophyll. 



2 A catalyst is a substance in the presence of which a specific chemical reaction 

 takes place that would otherwise occur slowly or not at all. The catalytic agent takes 

 part in the reaction but does not itself enter into permanent combination with the 

 reacting substances. An enzyme is a catalyst produced as a result of cellular activity 

 but independent of the presence of living cells in its operation. According to this 

 definition, chlorophyll is not an enzyme, since photosynthesis is not produced by 

 chlorophyll extracts nor by isolated chloroplasts but occurs only in intact cells. 



