THE CELL 17 



The nucleus and nucleolus, to which we have also referred. 

 (3) The plastids, which are variable in size and form and are 

 divided into three groups dependent on color but are in reality 

 the same thing. They are as follows: (a) The leucoplasts, which 

 are colorless, (l>) The chloroplasts, which are developed from 

 the leucoplasts and carry the -green coloring matter or chloro- 

 phyll, (c) The chromoplasts, which carry other coloring ma- 

 terials and are developed from the chloroplasts. In the parts of 

 the plant which are not exposed to the sunlight we find leucoplasts 

 only. But when these parts are exposed to the sunlight the leuco- 

 plasts are transformed into chloroplasts and the parts become 

 green. This is well illustrated when potatoes are exposed to the 

 sun. The color in flowers and ripe fruits is due to the fact that 

 chloroplasts have become chromoplasts and contain the coloring 

 materials. (d) The mitochondria or chondriosom.es found 

 in cells are very small elongated bodies in the cytoplasm; we 

 know very little concerning their character or importance. 



The most important non-protoplasmic bodies and compounds 

 found in the cell are (1) chlorophyll, which is the green color- 

 ing material. It is confined to the chloroplasts and absorbs 

 energy from the sun, which it utilizes in the formation of sugar 

 and starch (see page 195). (2) The other coloring materials, 

 which are very generally found in the flowers, fruits and other 

 parts of most plants. (3) Cell sap, sugars, starch, gum, fats and 

 oils, proteins, aleurone, resins, acids, alkaloids, tannins, crystals 

 of various kinds and many other compounds. These compounds 

 exist in various amounts in different parts of the plant and 

 are the factors which determine the value of the plant for food 

 and other uses. This question will be discussed more fully in 

 Chapter XI on plant products. 

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