DEVELOPMENT OF LIVING THINGS 



229 



some of the scum which has formed in the jar. Examine 

 it under a microscope. Look for single-celled animals swim- 

 ming about. By observing them for some time find out how 

 they reproduce. 



Activity 141. How do flowering plants reproduce their 

 kind? 



Read the text on pages 230-231 before performing this exer- 

 cise. Examine carefully the parts of a typical flower. The 



of muscle, bone, nerve, or any other part of your body, 

 you would find them also made up of cells (Fig. 363). 

 All animals and plants are either single cells or com- 



Musclt 



Fat xne-forming 



. 

 (EfithelialJ 



m 



Onion 



FIG. 361 



flower of a gladiolus or a tulip makes a good specimen to 

 study. Figure 361 is a diagram of a typical flower. Label the 

 parts of the flower on a similar diagram. How is reproduction 

 carried on by the flower? 



Activity 142. How do frogs 

 reproduce? 



Female frogs lay their eggs 

 in the spring of the year. Visit 

 a pond and look for frogs' eggs 

 floating in the water. Collect 

 a mass of eggs and keep them 

 in an aquarium in the class- 

 room. Watch them develop. 

 Read in the text the descrip- 

 tion of how frogs reproduce. 



In your notebook complete 

 the following statements. 



When frogs reproduce, a 



cell called the unites with 



a cell called the This 



process is called 



Courtesy General Biological Sup- 

 ply House 



FIG. 362. COLLECTING FROGS' 

 EGGS 



READINGS WHICH WILL HELP ANSWER THE 

 PROBLEM QUESTIONS 



What are living things made of? When you look 

 at a brick building from a distance, you cannot see the 

 bricks of which it is composed. As you come nearer 

 to the building, however, you observe that it is made 

 of smaller units of matter or bricks. When you look 

 at the leaf of a plant or the bone of an animal with the 

 naked eye, each appears to be a solid mass. If, how- 

 ever, you were to examine under a compound micro- 

 scope a thin section of a plant or scrapings from the 

 inside of your cheek, you would see that they are 

 composed of very small units. These units are called 

 cells. If you were to examine, in a similar manner, bits 



FIG. 363. TYPES OF CELLS 



binations of cells, and the life of a living organism is 

 the combined life of its individual cells. 



There is great variation 

 in the cell structure of liv- 

 ing things. Some plants and 

 animals are so tiny that 

 their bodies consist of but 

 a single cell. In a pond of 

 water millions of tiny one- 

 celled animals and plants 

 may be living. Bacteria and 

 yeasts are simple one-celled 

 plants, but most of the 

 plants and animals we 

 know consist of many cells. 

 Our own bodies are made 

 up of countless millions of 

 cells. 



What is the structure of 

 cells, tissues, and organs? 

 There is an almost endless 

 variety in the shape, size, and structure of cells. Most 

 cells, however, have certain things in common (see 

 Fig. 365). They are generally surrounded by a cell 

 tea//, a thin layer that is somewhat harder than the 

 contents of the cell. The most important part of the 

 cell is the living matter inside it, a jelly-like substance 

 called protoplasm. A part of the protoplasm is a tiny 

 structure, usually of spherical shape, called the nu- 

 cleus. A cell is, then, a bit of protoplasm containing a 

 nucleus, and it is the simplest structure known to 

 show the characteristics of life. Cells grow to a certain 

 size and then divide. By means of this division a large 



WRAMECIUM VORTICELLA 



FIG. 364. ONE-CELLED ORGAN- 

 ISMS THAT LIVE IN 

 WATER 



