28 COMPOSITION OF LIVING THINGS 



panded blade away from the branch is a little- stalk, the petiole, 

 which extends into the blade of the leaf. Here it splits up into a net- 

 work of tiny veins which evidently form a framework for the flat 

 blade somewhat as the sticks of a kite hold the paper in place. If 

 we examine under the compound microscope a thin section cut 

 across the leaf, we shall find that the veins as well as the other parts 

 are made up of many tiny boxlike units of various sizes and 

 shapes. These smallest units of building material of the 

 plant or animal disclosed by the compound microscope are called 

 cells. The organs of a plant or animal are built of these tiny 

 structures. 



Tissues. The cells which form certain parts of the veins, the 

 flat blade, or other portions of the plant, are often found in groups 

 or collections, the cells of which are more or less alike in size and 

 shape. Such a collection of cells is called a tissue. Examples 

 of tissues are the cells covering the outside of the human body, 

 the muscle cells, which collectively allow of movement, bony 

 tissues which form the framework to which the muscles are at- 

 tached, and many others. 



Adaptations of Structure to Function. If I look at my hand 

 as I write, I notice that the fingers of my right hand grasp 

 the pen firmly; that because of the several joints in the fingers, 

 the wrist, and forearm, free movement can be given to the hand 

 when the muscles attached to the bones move it. The hand is 

 capable of a great number of complicated and delicate move- 

 ments, most of them associated with the work of grasping objects. 

 Because of the peculiar fitness in the structure of the hand for 

 this work, we say that it is adapted to this, its function, that is, 

 grasping objects. Each organ of the plant is fitted or adapted in 

 some way to do certain kinds of work. It is the object of the 

 chapters following to point out how the parts of a plant or animal 

 are adapted to their various functions. 



Problem V. The structure and general properties of living 

 matter. (Laboratory Manual, Prob. V.} 



To the Teacher. Any simple plant or animal tissue can be used to demon- 

 strate the cell. Epidermal cells may be stripped from the body of the frog 

 or obtained by scraping the inside of one's mouth. The thin skin from 



