Io8 THE PLANT CELL 



and like the nucleus are never produced de novo. The chloro- 

 plasts, sometimes known as chlorophyll-granuks, are green, their 

 substance being saturated with a green-colouring matter named 

 chlorophyll. All green parts of plants owe their colour to the 

 chloroplasts in their cells, and the very important ' assimilation ' 

 process (chapter xvi.) is due to their activity. 



The chromoplasts, which are frequent in the cells of flowers 

 and fruits, are yellow or red, instead of green, the parts of the 

 plants in which they occur being rendered conspicuous by them 

 and attractive to birds and insects. 



The term leucoplast is applied to all colourless plastids : 

 examples are met with in roots, tubers and other underground 

 parts of plants. They possess the power of forming starch-grains 

 from sugar. The three kinds of plastids are convertible into one 

 another ; the chloroplasts of green unripe fruits usually become 

 chromoplasts when the fruit is ripe, and the leucoplasts of a 

 potato tuber become green when the latter is exposed to light. 



(iii) The cell-sap filling the vacuole of the cell consists of 

 water in which a number of substances are dissolved. In the 

 cells of beetroot, as well as in many fruits, flowers, and leaves, 

 the cell-sap contains a purple or reddish colouring-matter ; most 

 frequently, however, it is colourless. It is generally acid, but the 

 nature and amount of the compounds present in it often varies 

 from cell to cell in different parts of the same plant Various 

 products of the activity of the protoplasm, such as sugars, soluble 

 proteid, acids, and organic salts, are commonly present, as 

 well as nitrates, sulphates, phosphates, and other inorganic com- 

 pounds, absorbed from the soil. 



Most of the peculiar taste of the fruits and vegetables we eat 

 is due to the substance dissolved in their cell-sap, the protoplasm 

 and cell-wall being tasteless. 



4. The cells of the body of a plant at the time of their forma- 

 tion at the growing-points of the root and stem, are all about the 

 same size and cubical or polyhedral in form. They soon 



