CHAPTER IX 



TRANSLOCATION OF SUBSTANCES IN THE PLANT 



73. General Concept of the Ascent and Descent of Water in 

 the Plant. — The plant consists of diverse organs, situated in 

 physically widely differing conditions and having distinct func- 

 tions. The leaves elaborate organic substances and are in need 

 of water supply and mineral salts; the roots absorb water and 

 mineral substances but require for their respiration a supply of 

 organic substances requisite for absorption and for their further 

 growth; flowers, fruit, and growing tips rapidly consume organic 

 substances, and so there must be a continuous supply from the 

 leaves. A considerable part of the photosynthate is stored by 

 the plant; for its synthetic activity is interrupted at night and 

 during cold and dry periods of the year. For these periods of 

 checked activity as well as for the regrowth of its deciduous 

 organs, it requires a store of all the organic substances necessary 

 for the maintenance of its vital processes and for the formation 

 of the cells. Lacking this, some of the older organs, especially 

 leaves, are destroyed to release their ''food" material. 



This translocation of substances is elaborately developed in 

 woody plants, w^hich often reach very large dimensions. Trees 

 serve as the first objects for the study of the mechanism of 

 translocation and of its basic laws. 



The first exact information concerning the translocation of 

 substances was obtained by the Italian scientist Malpighi in 

 1670 by means of ringing of the bark. Soon after this, the 

 knowledge of the transport mechanism was considerably extended 

 by the classical investigations of the English scientist Hales 

 (1727). 



These first physiological experiments were later repeated by 

 other investigators with more exact methods. Experiments 

 have shown that the translocation of substances in the tree trunk 

 proceeds in two directions. One stream ascends from the roots 

 to the leaves and carries water and mineral elements. As the 

 general direction of this movement is upward, it is termed the 



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