ASSIMILATION OF CARBON BY PLANTS 175 



alkaline reaction. Evidently, this may be explained by the fact 

 that under these conditions, the iron salts are insoluble and 

 cannot be absorbed by the roots. Some plants, e.g., lupines and 

 tobacco, are very much subject to chlorosis on such soils. Chlor- 

 osis may be induced also by other causes that check the normal 

 supply and translocation of mineral salts in the plant. In 

 some southern crop plants (tea, pomegranate), chlorosis is 

 observed in the spring. It is induced by the low temperature 

 of the soil, which inhibits the activity of roots. 



Chlorosis is produced also in cases of infections due to diseases. 

 Sometimes it is localized, when the leaves show green as well as 

 colorless portions, as in mosaic diseases of tobacco, potato, etc. 



The phenomenon of chlorosis must not be confused with 

 albinism. The latter is observed when a plant, owing to heredi- 

 tary causes, is not able to produce chlorophyll even under the 

 most favorable external conditions. Such albino plants may 

 show no chloroplasts on microscopic examination. Frequently 

 among the seedlings of corn and of other cereals, pale yellow 

 specimens are observed, which of course cannot assimilate 

 carbon dioxide and therefore die from starvation after the 

 development of two to three leaflets. Other plants may show 

 partial albinism, when some of the leaves, or separate portions 

 of the leaf, may be whitish. Such plants with variegated leaves 

 are much valued in decorative horticulture. In the colorless 

 portions of such leaves, an increased amount of oxidizing enzymes 

 will usually be found. But whether this increase in oxidation is 

 the direct cause of the inability to accumulate green pigment, or 

 whether it is but an accompanying phenomenon, is not known. 



41. Importance of the Energy of Light for Carbon Assimilation. 

 The Role of Chlorophyll in the Absorption of Light. Optical 

 Properties of Chlorophyll. — The mere presence- of green plastids 

 does not assure carbon assimilation. Light is an essential con- 

 dition for this process. As early as 1779, Ingen-Housz definitely 

 established that only in the presence of light do plants ''improve 

 the air." Light is the only source of energy that can be used to 

 break the strong linkage between oxygen and carbon that is 

 necessary for the production of carbohydrates from carbon 

 dioxide. To be available in the process of photosynthesis, the 

 energy of light in the first place must be absorbed by the chloro- 

 phyll and transformed into chemical energy. Chlorophyll 



