22 Science of Plant Life 



grouped together. Such groups of cells with like functions 

 are called tissues. The epidermis of a leaf, for example, is a 

 tissue covering the mesophyll and veins. 



In order to do its work a tissue needs a source of supplies 

 and a means of disposing of its products. Hence the group- 

 ing of the tissues may be mutually advantageous. When 

 several kinds of tissues are arranged together so that by their 

 cooperation they can carry on some general function of the 

 plant, they form an organ. The leaf, for example, is an organ 

 especially concerned with the manufacture of food. It is 

 made up, as we have seen, of five different tissues, each com- 

 posed of thousands of cells. 



The chloroplasts. Of the several structures found within 

 the mesophyll cells, the most important in the primary process 

 of food manufacture are the chloroplasts. These are round 

 or lens-shaped bodies which contain a green coloring matter 

 called chlorophyll. They are composed of living material 

 and belong to a group of structures called plastids, that are 

 found in the cytoplasm of all plant cells. Cells may contain 

 many or only a few chloroplasts, and these may be located 

 deep within the leaf or near its surface (Fig. 17). Since 

 the chloroplasts are the special apparatus for the manu- 

 facture of food, the amount of food produced by a plant 

 under any given conditions is roughly proportional to their 

 number. 



The chlorophyll. Chlorophyll is held in the chloroplasts 

 in much the same way that water is held in a sponge. It 

 stains the chloroplasts green, and it may be removed from 

 them by putting the leaf in alcohol, in which the chlorophyll 

 is soluble. After the chlorophyll is dissolved, the chloroplasts 

 remain in the cell, but they are colorless and the leaf is white 



