Biology of Higher Plants — Anatomy and Physiology 233 



plants can manufacture their own foods from raw materials (water and 

 carbon dioxide) while animals cannot. 



Algae absorb carbon dioxide through their surface from their sur- 

 roundings, while higher plants usually take it through the regulating 

 stomata of the leaf epidermis. The water and its contained salts are 

 osmosed into the roots by means of numerous root hairs. From the 

 roots these materials are conducted by means of the xylem tissues of the 

 roots and stems to the veins of the leaves. The carbon dioxide of the 

 air is being constantly used by the many green plants, but the supply 

 is replenished by such sources as animal metabolism, the combustion of 

 luels, industrial combustions, volcanic eruptions, etc. 



Willstatter (1912) showed that chlorophyll actually consists of a mix- 

 ture of two substances which he called chlorophyll A and chlorophyll B. 

 Both these chlorophylls may form green crystals when extracted with 

 ethyl alcohol. They may be separated from each other by their differ- 

 ent solubilities in organic solvents. Chlorophyll A is blue-green in trans- 

 mitted light and blood red in reflected light. Chlorophyll B is yellowish- 

 green in transmitted light and brownish-red in reflected light. Chemi- 

 cally, chlorophyll is an ester (a combination of an acid and an alcohol). 

 In both chlorophylls about 2.7 per cent of magnesium is the center. 

 Iron is necessary for the plant to manufacture chlorophyll, but no iron 

 enters into the composition of the chlorophyll. It is evident that the 

 two chlorophylls are quite similar in most respects, differing only in the 

 amounts of hydrogen and oxygen. 



Biophysical Aspects of Photosynthesis.^ — Chlorophyll has the physical 

 property of selectively absorbing certain wave lengths of light while 

 other wave lengths are transmitted. When a green leaf, or a solution of 

 chlorophyll, is placed between a source x)f light and a prism, dark bands 

 appear in the spectrum (Fig. 368), showing that some of the light is 

 absorbed by the chlorophyll, while the rest of the light is passed through 

 the chlorophyll or reflected from the leaf surface. The color of a leaf 

 is green because those wave lengths are not absorbed by the leaf but 

 are reflected from its surface to the eye. In strong sunlight absorption 

 is greatest at the red end of the spectrum where the wave lengths are 

 longer (0.00076 mm. long). In diff"use light more absorption occurs at 

 the violet end with its shorter wave lengths (0.00039 mm. long). The 

 red wave lengths arc more efficient because of the presence of more 

 energy. It is well known that "the photosynthetic work accomplished 



