230 



LIFE ON THE EARTH 



v Cell Memlrant 

 Nucleus 

 Cyioplasm. 

 Vacuole 



FIG. 365. DIAGRAM OF A CELL 



ORGAN 



CELL 



number of cells may be formed in the body of a plant 

 or an animal. 



If you were to examine a bit of muscle under a 

 compound microscope you would find the cells more 



or less alike in size and 

 shape. Masses of cells that 

 are somewhat similar and 

 that do the same work are 

 called tissues. We have 

 bone tissue, all made of 

 bone cells ; we have blood 

 tissue, brain tissue, nerve 



tissue, and so on. When a number of tissues are com- 

 bined to perform a definite work in a plant or animal, 

 the structure so formed is called an organ. The heart is 



an organ made of muscle, 

 blood, and nerve tissue. Its 

 work is to pump the blood 

 1 \ to all parts of the body. The 

 . \ organs of plants and ani- 

 T1SSUE] mals are their most con- 

 spicuous parts. Is it not the 

 root, the stem, the leaf, the 

 seed, or the flower of a 

 plant that attracts our at- 

 tention? These structures 

 are organs. Tissues and 

 cells must be magnified be- 

 fore we are able to study 

 them. 



^Does life always come 

 from life? It is customary 

 in science to divide ob- 

 jects about us into two 

 classes : living and non- 

 living things. We are able 



FIG. 366. CELLS, TISSUES AND ... . 



ORGANS to distinguish living from 



lifeless matter by differ- 

 ences that are easily recognized. All living things have 

 the power to grow. They grow by taking in food, di- 

 gesting it, and adding it to themselves. Lifeless mat- 

 ter is unable to grow in this way. Living -things are 

 active and are capable of independent movement while 

 non-living things move only when acted upon by 

 some external force. Plants and animals are more 

 complex in structure than non-living matter. Their 

 bodies are composed of cells, tissues, and organs, all 

 highly organized and working together for the benefit 

 of the individual. 



A unique characteristic of living things is their 

 power to reproduce their own kind. The ability to re- 

 produce is found only in the living world. Until about 

 the seventeenth century most people believed that 

 some forms of life developed from non-living matter. 



It was thought that decaying meat produced flies, that 

 fish developed from water, and that frogs came from 

 slime. In 1680 Redi, an Italian, performed a simple 

 experiment. He covered some meat in order that flies 

 could not get to it and found that if flies were kept 

 away from decaying meat, maggots would never de- 

 velop in it. He concluded that life comes only from 

 living things. His experiments started a dispute which 

 in turn led many scientists to devise experiments to 

 study the question of the origin of life. The dispute 

 lasted for nearly two centuries until Pasteur and Tyn- 

 dall, two famous scientists, by careful experiments 

 proved that lifeless matter does not generate living 

 matter. Today it is firmly established that all life 

 comes from life. 



Higher plants and animals are made of two kinds of 

 cells. First, there are the ordinary body cells that make 

 up the bones, muscles, nerves, and blood of animals, 

 and the cells which form the roots 



... i" 1 .!'."-"-; :, 



stems, and leaves of plants. The .^p 



others cells, called germ cells or sex jj~- .1^' 



cells, are set apart from the body i^v-V. 



cells early in the life of the individ- %>. 



ual. So much alike are the germ 



cells in both plants and animals that -,_ 



scientists use the same names in SPERM 



describing them. Female sex cells 



are called eggs (ova) and male sex cells are called 



sperms. Reproduction takes place through the uniting 



of a sperm cell with an egg cell. Since a new individual 



is formed by a union of cells from the parents, should 



we not expect the individual to resemble its parents? 



What is the structure and function of a flower ? The 

 flower of a plant contains the reproductive organs. 

 These organs produce the sperms and eggs that unite 

 to produce more plants. The great variety and beauty 

 of flowers attract our attention. We observe their dif- 

 ferent colors; sizes, and shape. Yet all complete flow- 

 ers are much alike, in that they have the same kinds 

 of parts. Figure 368 shows the structure of a typical 

 flower. Study it carefully as you read. Also examine 

 some live flowers and compare them with the diagram. 



The buds of flowers are usually protected by a 

 green, leaf-like covering. When a bud opens, these 

 coverings are pushed apart and are called sepals. Next 

 inside the sepals are the petals, the colored, attractive 

 parts of the flower. Inside the group of petals is a cir- 

 cle of parts called stamens. In the anther, located at the 

 top of the stamen, yellow pollen is produced. Inside the 

 circle of stamens is the pistil. The enlarged base of the 

 pistil is the ovary, in which the seeds develop. At the 

 top of the pistil is a flat, sticky structure called the 

 stigma. The stigma is connected with the ovary by the 

 style. 



