22 THE NATURE AND WORK OF PLANTS 



offsets, and other structures necessary for the process. 

 In addition, the plant must use a large amount of 

 energy and material in protecting all of its parts 

 from injury or destruction by the climate, other 

 plants, or by animals. 



20. Organs. In the lower or simpler plants, 

 such as algae and bacteria, all of these different 

 things or functions are carried on by a very small 

 body, often consisting of a single cell that could not 

 be seen by the unaided eye. The whole body is used 

 in carrying on nearly all of the various kinds of 

 work. The " higher " plants are those which have 

 developed separate portions of their bodies, especially 

 suited to one or a few kinds of work. The part of 

 a plant thus devoted to one or to a group si functions 

 is termed an organ. Thus the root, stem, leaf, and 

 flower are each responsible for certain kinds of work 

 necessary for the welfare of the plant, and they are 

 the principal organs of the higher plants. 



21. Tissues. If the structure of any of the 

 organs is examined, it will be found that it is made 

 up of a number of different kinds of material. Thus 

 in an elder stem may be found pith, wood, cambium, 

 and bark. The different masses are made up of dif- 



