424 UTILIZATION OF MINERAL SALTS 



doubtful if iron toxicity occurs in natural soils except possibly under condi- 

 tions of extreme acidity, or serious lack of aeration, or both. 



The proportionate amount of iron in plant tissues is very low; much of 

 that present is a constituent of organic compounds. Iron is one of the most 

 immobile of all elements within plants, no appreciable redistribution ever 

 occurring from one tissue to another. If plants which have been supplied 

 with iron are transferred to a solution culture lacking this element the 

 subsequently developing leaves exhibit a marked iron chlorosis, while the 

 older leaves retain their normal green color. This is a graphic demonstra- 

 tion of the fact that no appreciable transfer of iron occurs from the older 

 to the younger leaves. 



Boron. — Recent investigations (Sommer and Lipman 1926, Brenchley and 

 Warington 1927, MclVIurtrey 1929, Johnston and Fisher 1930, McHargue 

 and Calfee 1933, and others) have demonstrated convincingly that boron is 

 essential for a number of species of plants although it is required only in 

 minute quantities. Species for which this element has been shown to be 

 necessary include beans, barley, buckwheat, melons, mustard, flax, castor bean, 

 cotton, tomato, tobacco, sunflower, peas, sugar beet, sugar cane, citrus fruits, 

 and lettuce. In all probability boron is an essential element for all species 

 of green plants. The exact role of this element in plants is unknown, but 

 it is noteworthy that its absence usually affects the meristematic tissues, caus- 

 ing blackening and death or growth abnormalities. In any appreciable con- 

 centration boron is toxic to plants. 



Alanganese. — A number of recent investigations (McHargue and Calfee 

 1932, Haas 1932, Clark 1933, Hopkins 1934) as well as some earlier ones, 

 indicate conclusively that this element is necessary for a number of species 

 of plants, and manganese is now generally considered a necessary element for 

 all green plants. IVIanganese is, as a rule, most abundant in the physiolog- 

 ically active parts of plants. It is supposed to play a part in oxidation and 

 reduction phenomena, perhaps largely or entirely through its influence on the 

 activity of oxidase enzymes (Chap. XXX). In fact some investigators think 

 that oxidases may be manganese compounds. Others hold that manganese 

 compounds serve as co-enzymes of the oxidases. Manganese is also related 

 in some way to chlorophyll synthesis, as a deficiency of this element usually 

 results in a development of a chlorotic condition in plants. Chlorosis due to 

 manganese deficiency is different from chlorosis resulting from lack of iron 

 or magnesium (Table 41). Except in very low concentrations manganese 

 compounds are distinctly toxic to plants. 



Copper. — This element is widely distributed in plants although it never 

 constitutes a large proportion of the ash. In general copper is highly toxic 



