250 PLANT PHYSIOLOGY 



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of iron in the life of the plant, however, as it is equally necessary 

 for organisms without chlorophyll. As has been seen in Art. 24, 

 Warburg attributes to iron a central role in the process of respira- 

 tion and supposes likewise that it participates in photosynthesis. 



The significance for the plant of those sUght traces of zinc, 

 boron, silicon, and perhaps some other elements, the necessity 

 of which for the successful growth of plants in water cultures 

 has been recently revealed, remains unknown so far. Evidently 

 these ions have a role as catalysts necessary for certain processes 

 of metabolism. 



The significance of separate cations, such as potassium, as 

 well as of anions, especially phosphoric acid, for blooming and 

 fruiting will be discussed later in Chap. XII, devoted to the 

 problems of production. 



58. Absorption of Mineral Elements by the Roots. — For a 

 long time, the absorption of mineral elements by the roots of 

 plants was regarded as a simple process of their diffusion from 

 the soil. Especially great significance was attributed to the 

 so-called ''transpiration stream," i.e., the movement of water 

 from the soil into the roots and upward along the stem to the 

 leaves, caused by the continuous evaporation of water from the 

 leaves in transpiration (see Art. 68). According to this concept, 

 the highly dilute soil solution penetrates into the roots in an 

 almost unaltered state, ascends the stem, and is then concentrated 

 in the leaves as a result of the evaporation of water. The process 

 of transpiration was regarded as one of the essential conditions 

 of mineral nutrition of plants. 



Further investigations have shown, however, that the process 

 is considerably more complicated and that the amount of mineral 

 substances penetrating into and accumulated within the plant 

 is not proportional to the amount of water that has passed 

 through the plant. The experiments carried out by Hasselbring 

 on the Cuban tobacco plantations are especially interesting in 

 this respect. In the growing of cigar tobacco, large areas are 

 shaded by means of cheesecloth. This greatly decreases the 

 general amount of water used by the plants, but the accumulation 

 of mineral substances proves to be not less but even greater in 

 the shaded plants, as compared with those grown in full sunlight. 



It is convenient to compare the absorption of water and 

 mineral salts by plants grown in water cultures. Numerous 



