238 THE MAINTENANCE OF THE INDIVIDUAL 



a greater extent than it tears them down in the metabolic process. 

 Compare the growth of an animal with that of a plant such as a big 

 tree. While the animal is more fixed in size and limited in age, the 

 tree grows for a longer period of time and grows to a greater size. 

 These differences are due to a continuous growth of the meristemic 

 tissue already mentioned, and also to the fact that new tissues and 

 organs grow continuously from this area of meristem that is found in 

 growing buds, stems, and roots. The most important difference 

 between green plants and animals lies in the fact that the green plant 

 can make use of the sun's energy to manufacture foodstuffs on which 

 not only it, but also the animals which eat it, depend. 



In an investigation of a living green plant, two methods of study 

 present themselves. We can rather carefully dissect and study each 

 system of structures which makes up the living organism and direct 

 our attention to the microscopic make-up of each part. In this way 

 a fairly complete picture will be had of the organism in its entirety. 

 But such a picture will lack vitality. If the plant is a living thing, 

 then why not study it from the point of view of function, of what it 

 does and how it lives, using only so much reference to the structures 

 as will make intelligible the work of the parts of the plant? This is 

 the viewpoint adopted for this unit. The plant is to be thought of as 

 a living, working organism, performing the same metabolic functions 

 as any other organism, but in addition doing a different kind of work 

 from that of animals, that of synthesizing organic foodstuffs out of 

 chemical raw materials from the air, the soil, and the absorbed water. 

 This unit, then, will bring up a number of important points. Among 

 them will be such questions as these : What are the adaptations 

 which enable the green plant to do its work? Where does the raw 

 material from which food is manufactured come from and how does 

 the plant get it ? Under what conditions is the work of food manu- 

 facture performed? Where is food made and how does it get to the 

 cells where work is done? Why is light necessary for green plants 

 and why do they bleach in the darkness? Are green plants really 

 as important as is here indicated ? These and similar questions will 

 be answered in the pages that follow. 



Structure of Green Plants 



It is not easy to give a general description of a green plant. In the 

 higher plants it is obvious that there are several well-defined regions 

 which are called root, stem, leaves, flowers, and fruits. In these 



