268 BOTANY PART I 



abundant glucose. In the other direction a stream of mobilised 

 reserve material can pass downwards from the foliage leaves by way 

 of the sieve-tubes ( 42 ). While, however, the mechanical causes of the 

 transpiration stream are at least partially understood, so far as they 

 depend upon the evaporation of water, we do not know the forces 

 concerned in movements in mass in the sieve-tubes. 



Another example of translocation is afforded by leaves shortly 

 before they are shed. In many but not all cases the useful materials 

 in the leaf are transferred to the stem and thus are not lost to the 

 plant. Phosphoric acid, potassium, and nitrogenous substances are thus 

 transferred to the stem, but the cell walls, a protoplasmic layer, and 

 osmotically-active substances in the vacuole remain so that the leaf 

 falls in a turgescent condition ( 43 ). 



C. FURTHER METAMORPHOSES OF SUBSTANCE 



Regeneration of Reserve Materials. Sooner or later the reserve 

 materials mobilised by the help of enzymes are again converted into 

 substances with large molecules. This occurs at any rate at the end 

 of their transport, whether they are again deposited as reserve 

 materials or are employed as constructive substances. Thus, for 

 example, glucose formed in a leaf may pass to a seed or a tuber and 

 be there transformed into starch or cell wall. When the transport 

 is for a considerable distance the formation of reserve material may 

 go on by the way and not only at the end of the journey. This is 

 specially well seen in the case of starch. Along the routes of sugar 

 transport so-called transitory starch may be formed in every cell. 

 This starch formation diminishes the concentration of the solution, 

 and thus helps to maintain the continued motion of the diffusion 

 current. 



Other Products of Metabolism (* 4 ). Only a small proportion of 

 the substances met with in plants have been enumerated above. It 

 will be sufficient to mention here the organic acids, ta.nnins, glucosides, 

 alkaloids, colouring matters, ethereal oils, resins, gum-resins, caoutchouc 

 and gutta-percha among the legion of substances which are derived 

 from the products of assimilation. The organic acids will be referred 

 to later (p. 271); the origin and physiological significance of the 

 others are too little known for them to be dealt with. It is known 

 that as a rule they are not further utilised after their formation. They 

 are probably, therefore, by-products of the metabolism of the plant. 

 They need not, however, for this reason be useless, and it is believed 

 that some bitter or poisonous substances protect the plant from being 

 eaten by animals ; some pigments are of use in the attraction of 

 animals which distribute pollen, seeds, and fruits, or frighten away 

 injurious animals (warning colours). Resin and latex when they 

 exude and harden may assist in the closing of wounds. 



