6 
INTRODUCTION 
Sensation (Irritability). — Animals are sensitive to heat, 
to pain, to light, and to other outside influences (stimuli). 
Plants, too, respond to light and other stimuli. The response 
of animals and plants to stimuli is called sensation or irri¬ 
tability. It is an important life process or function, for it 
enables them to make the most of their location. Because 
of this function animals and plants are able to adapt them¬ 
selves to their surroundings. 
The foregoing life processes have to do with the life of the 
animal or the plant itself. There is another life process that 
is important in keeping alive the races of animals and plants, 
namely, reproduction. 
Reproduction. -— All animals and plants produce young 
or become extinct. The old die and the young carry on the 
work of the race. The number of animals and plants is 
increased on the earth by means of the life process known as 
reproduction. 
These six fundamental functions or life processes are to be 
found in all animals and plants. In some cases it is difficult 
to study them, for they are hidden or masked by other 
processes, or the animal or plant may be so small that we 
cannot easily make them out. 
Biology is the science that deals with the lives of animals 
and plants. It seeks to understand how they are adapted to 
the kind of life they lead and how they carry on their life 
processes by means of various structures. 
The Parts of Bodies. — These life processes tell us what 
the parts of bodies do, but they tell us nothing about these 
parts themselves. There are five words which are used in 
biology to describe these parts. They are: cell, tissue, 
organ, organ system, and organism. 
1. The Cell. — When the biologist takes apart the plant 
or animal he finds that he can separate the parts until he 
comes to a unit so small that a microscope is necessary to 
see it. These microscopic parts are called cells and are alike 
