30 Elementary Principles of Agriculture 



plant, take an apple or potato and peel off the epidermis and place 

 in an exposed place beside an impeded specimen. Note how quickly 

 the peeled specimen will shrivel and dry, while the other retains its 

 form. 



48. Carbon Assimilation. The soft tissue between the 

 upper and the lower epidermis is the real food factory 

 of the plant. It is composed of several layers of cells, 

 all arranged sponge-like, so that the carbon dioxid 

 of the air can reach every cell. All these cells contain 

 minute green bodies, called chloroplastids (chlo-ro- 

 plast-ids). The green coloring matter in these bodies 

 is formed only in the light. It does not form in leaves 

 growing in the dark. The yellowish stems of potatoes 

 growing in dark cellars is a familiar example. The green 

 color will disappear if plants are kept from the light. 

 Advantage is taken of this property in " blanching" 

 celery. When the light shines on the leaves, the chloro- 

 phyll absorbs the energy of the sun's rays and forms 

 the starches, sugars, etc., from the water and carbon 

 dioxid. This process goes on through all daylight hours. 

 (1) Light, (2) living cell with (3) chlorophyll, (4) water 

 and (5) carbon dioxid must all be present. This explains 

 why plants do not grow unless they get plenty of sun- 

 light. This process of making plant substance under 

 the influence of sunlight is called "carbon assimilation." 

 It is not confined to the leaves, but takes place in any 

 green cell when the other conditions exist. (See Figs. 

 20 and 21.) 



49. How Green Plants Purify the Air. When carbon 

 dioxid combines with water, the excess of free oxygen 

 of the carbon compound escapes into the air. By this 

 means, growing green plants purify the air. They take 

 up the carbon dioxid given off from the lungs, or that 



