24 TEXTBOOK OF PLANT PHYSIOLOGY 



When thus treated, parts of the plant drawing directly on these 

 roots will turn green. 



The necessity of iron for the formation of chlorophyll was for 

 a long time regarded as a strong proof of the fact that iron is one 

 of the constituents of chlorophyll. The exact analyses by Will- 

 statter have shown that this view is not correct. The iron ions 

 must be regarded, therefore, as specific catalyzers without which 

 some of the preliminary stages of the production of green pigment 

 cannot go on, as the formation of leucophyll, for instance. 



Chlorosis is sometimes observed under natural conditions, most 

 frequently on soils rich in lime and giving a somewhat 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, for instance, lupins and 

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

 Chlorosis may be induced also by other causes which check 

 the normal supply and translocation of mineral salts in the plant. 

 Thus, 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 some infectious 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 albin- 

 ism. The latter is observed in cases where a plant, due to some 

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

 most favorable external conditions. Frequently among the seed- 

 lings of corn and of other cereals, entirely white specimens are 

 observed, which, of course, cannot assimilate carbon dioxide and 

 therefore perish 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 white. 

 Such plants with variegated leaves are much valued in decorative 

 horticulture. In the colorless portions of such leaves usually an 

 increased amount of oxidizing enzymes will 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 yet clear. Variegation and albinism are 

 hereditary characters. In crossing plants possessing these charac- 

 ters with normal ones, mendelian segregation is observed. 



