Protoplasm 315 



A cell may assume various shapes in order to adapt itself to 

 its mode of life, or to the pressure of neighboring cells which 

 in many-celled organisms crowd it, and change its shape to a 

 greater or less extent. Cells may be hexagonal, threadlike, brick- 

 shaped, or of various irregular forms. 



Cellular Structure of Living Things. A living thing con- 

 sists either of a single cell or of many cells. The cell which forms 

 a single-celled living thing is wholly independent. In many-celled 

 living things this is not the case. These consist of groups of cells 

 having definite functions to perform that are necessary to the life 

 of other groups of cells. The body of man, which represents the 

 very highest type of life, is made up of groups of cells muscle 

 cells, nerve cells, bone cells so connected and interrelated that 

 each group depends on the activity of the other groups to aid in 

 the performance of its function. Thus, by the co-operation of all 

 the groups, the life of the organism is maintained and kept in a 

 normal healthy condition. 



Functions, such as breathing and digestion, are performed by 

 organs composed of tissues, made up of cells. This make-up of 

 cells, tissues, and organs is known as structure. Since the cell is 

 the smallest part, or unit, of a living thing that can perform func- 

 tions, it is called the unit of function. It is also the unit of struc- 

 ture. 



Environment. In order to sustain life the presence of cer- 

 tain factors or conditions is absolutely necessary. These factors 

 are air, water, food, heat, and light. If it were not for air we 

 could not breathe. Even a short time without water and food 

 shows us how necessary are these factors if we are to keep alive. 

 Heat and light, too, are essential to a healthy existence. Not 

 only human beings, but all living things are absolutely dependent 

 on these factors for their very existence. These, with other less 

 essential conditions, make up the surroundings of every organ- 

 ism that exists. - As our surroundings are subject to great 

 changes we can easily understand that they can lead to equally 

 great changes in the structure or habits of an animal or a plant. 

 These changes may be either physical or chemical, or both. Hence 

 all life involves a constant succession of changes. 



