30 



COMPOSITION OF LIVING THINGS 



time a cell splits to form two new cells, each chromosome splits length- 

 wise and the parts go in coual numbers into the nucleus of each of the two 

 new cells thus formed. These chromosomes are supposed to be the bearers 

 of the qualities which we beheve can be handed down from plant to plant 

 and from animal to animal ; in other words, the inheritable qualities which 

 make the offspring like its parents. 



The bulk of the nucleus is filled with a fluid, and in some nuclei a body 

 known as a nucleolus is found ; it does not, however, seem to be a constant 

 structure. The protoplasm surrounding the nucleus is called cytoplasm. 



The protoplasm in some cells collects into little bodies called plastids. 

 In plant cells the plastids are frequently colored green. This green color- 

 ing matter, which is found only in plant cells, is called chlorophyll, and 

 green plastids are called chlorophyll bodies. The cytoplasm of a cell con- 

 tain spaces, which are usually filled with a fluid known as cell sap. These 

 spaces in the cytoplasm are given the name of vacuoles. Frequently non- 

 living materials are found within the cytoplasm of the cell. 



The cell is surrounded by a very delicate living structure called the 

 cell membrane. Outside this membrane a wall is formed by the activity 

 of the protoplasm in the cells of plants. These cell walls form wood. 



Z] 



How Cells form Others. 



|)lit into two new cells. 



Stages in the division of one cell to form two cells. 

 Note the separation of the chromosomes in the 

 nucleus. Which part of the cell divides first ? 



— Cells grow to a certain size and then 

 In this process, which is of very great 

 importance in the 

 growth of both plants 

 and animals, the nucleus 

 divides first. The 

 chromosomes also di- 

 vide, each splitting 

 lengthwise and the 

 parts going in equal 

 numbers to each of the 

 two cells formed from 

 the old cell. Lastly, 

 the cytoplasm sepa- 

 rates, and two new cells 

 are formed. This pro- 

 It is the usual method of growth found 



cess is kno"\vn as fission. 



in the tissues of plants and animals 



^>A^^Cells of Various Sizes and Shapes. — Plant cells and animal cells are 



^of very diverse shapes and sizes. There are cells so large that they can 



easily be seen with the unaided eye ; for example, the root hairs of plants 



