262 FIRST YEAR COURSE IN GENERAL SCIENCE 



289. The Cell. After the invention of the microscope 

 about the year 1600, people began to make use of its magni- 

 fying power to examine all kinds of plant and animal sub- 

 stances. It was not until 1838, however, that the fact was 

 established that all organisms are made up of definitely 

 formed parts or units, somewhat in the manner in which the 

 walls of brick buildings are made up of separate bricks. 



These small units had been seen two hundred years earlier, 

 and at that time were called cells, because it was thought that 

 they were practically empty spaces. In 1838 it was demon- 

 strated that the cells 

 were in reality separate 

 masses of protoplasm, 

 generally surrounded by 

 an envelope which is 

 called the cell wall. The 

 size of cells varies from 



Ts^oiy f an mc h m 

 diameter to two inches, 



FIG. 138. Two KINDS OF CELLS. 

 (magnified about 100 times) 



Figure A represents two animal cells, 

 many of which make what we call a muscle. 

 Figure B represents several cells from the 

 thin covering on the surface of the body. 

 There is one point of resemblance between 

 these two kinds of cells. What is it? 



the average diameter 

 being about -jniW f an 

 inch. The shape of 

 cells also varies greatly; 



some are globular, some flattened, some thread-like, some like 



a pillar. 



290. Tissues. Cells more or less alike are often grouped 

 in a plant or animal. Such a collection of like cells is called 

 a tissue. Examples of tissues are the cells of the skin covering 

 the human body, wood cells, pith cells, and muscle cells. 



291. Organs. Different kinds of tissue are often com- 

 bined to form a definite part of an organism, called an organ, 

 such as a leaf, a hand, an eye, or a root. A leaf consists of a 

 thin covering, green pulp, and veins. There are more than 

 three kinds of tissue in the hand. An organ is a part of an 

 organism which has a special kind of work to do. 



