PROTOPLASM 27 



The Cell. In most plants and animals the protoplasm is divided 

 into very small parts called cells. These are merely the simplest 

 units of protoplasm of which* the plant or animal is composed. 

 A living cell usually consists of a tiny mass of protoplasm surrounded 

 by a membrane called the cell wall. The central portion of the 

 protoplasm, more active than the rest, is called the nucleus. The 

 cell wall gives definite shape to the cell and the nucleus seems to 

 regulate growth and reproduction. Cells are usually very minute, 

 but are of innumerable shapes, varying with the special work they 

 may have to perform. 



Some plants and animals consist of only one cell. In more 

 complicated animals, there are a great many different groups of 

 cells, each fitted for some one purpose, as, for example, the vast 

 number of cells that together make up a muscle and have developed 

 especially the power of motion. 



Tissues. A group of similar cells, devoted to a single use, is 

 called a tissue. There are many kinds of tissues, as wood, bark, 

 and leaf, in plants, and bone, muscle, nerve, etc., in animals. 



Organs. In all the more familiar plants and animals, various 

 tissues are grouped together to form a more complex part, which 

 has some important general use. The stem of a tree, for instance, 

 whose use is to support the leaves, flowers, and fruit, consists of 

 wood, pith, bark, and other tissues, all working together for one 

 purpose. The leg of a cat is made up of bone, muscle, nerve, and 

 other tissues, working together to make locomotion possible. Such 

 groups of tissues are called organs and the purpose or use of any 

 part is called its function. 



So we can say that all living things are composed of protoplasm; 

 the protoplasm is usually divided into cells; the cells are grouped 

 into tissues, and these, in turn, into organs fitted for some particular 

 function or functions. 



Systems. Often in the higher forms, especially among animals, 

 several organs are grouped together to perform related functions. 

 Such groups are referred to as systems, as, for example, the circu- 

 latory system, which includes the heart, arteries, veins, and capil- 

 laries. These are organs, all united in the work of circulation. 



