172 METABOLISM 



be transformed, when a direct supply from the leaf ceases. Such transitory 

 formation of starch always accompanies a translocation of sugar, whether it takes 

 place for long distances in sieve-tubes or for short distances in parenchyma. 

 According to statements already made this formation of starch is easily under- 

 stood, since it serves the purpose of maintaining the degree of concentration 

 necessary for diffusion. 



If the circulation of carbohydrates in sieve-tubes be interrupted, these 

 elements are found to be in general the conducting organs of migratory organic 

 substances. The sieve-tubes have been for long claimed to subserve the carriage 

 of proteid, and the open passage from segment to segment has been referred to as 

 especially of importance, rendering possible the rapid movement of a substance 

 in itself diffusible with difficulty. We need not enter further into a consideration 

 of the translocation of proteid and its decomposition products ; what little is 

 known proves that conditions similar to those governing the translocation of 

 carbohydrates prevail here also. It must be noted, however, that materials of 

 the ash also, sometimes as such, at other times in an organic form, must be 

 transported by the same path as proteid and sugar, after they have ascended 

 in the transpiration current from the root and been partly altered into some 

 other form. It is more than possible that sieve-tubes are aided in the per- 

 formance of their functions by laticiferous tubes (compare HABERLANDT, 1883, 

 SCHIMPER, 1885, GAUCHER, 1900). [According to KNIEP (1905), this does not 

 as yet rest on sufficiently secure evidence.] 



There remains for us to inquire into a phenomena which is especially 

 exhibited by trees. When in springtime starch is dissolved, the sugar, in order 

 to reach the seat of metabolism has often to travel from a few to more 

 than 100 metres. Hence it may be concluded that it travels by another path 

 and not, or not entirely, by the sieve-tubes, but follows the water-stream in the 

 vessels, just as the salts of the soil do after being absorbed by the root. This 

 conclusion is arrived at from experiments on ringing, as TH. HARTIG has already 

 shown (1858). While, as above noted, such a cortical ringing prevents the ac- 

 cumulation of starch in the basal part of the stem, if the operation be performed 

 after the storage of starch in autumn, the whole of it disappears in the following 

 spring out of the wood and cortex of the stem base. After A. FISCHER'S (1890) 

 researches there can be no doubt that the glucose is transferred to the opening 

 leaf -buds by the wood and especially by the vessels. And since the transpiration 

 current is effected for many metres while diffusion perhaps is active for only 

 millimetres or microns, one can comprehend the advantage which a plant obtains 

 by this arrangement. Again, since in the sap excreted by a tree in the process of 

 bleeding both amides and proteids have been found, one may well assume that 

 nitrogenous materials travel the same way as do carbohydrates. 



TH. HARTIG and also A. FISCHER (1890) and STRASBURGER (1891) have gone 

 further in this respect. They affirm that in trees the upward movement of car- 

 bohydrates in spring takes place exclusively in the wood, and that only a down- 

 ward movement can take place in the cortex. The reasons advanced in support 

 of this view do not appear to us to be quite sound, and it may be that fresh 

 experiments may show that the phloem also is capable of transporting mobile 

 reserves. This would appear all the more probable since in herbaceous and 

 shrubby plants the vessels are said never to be called into service for the upward 

 transport of reserves, and such a fundamental difference between woody and 

 herbaceous parts would scarcely be intelligible. 



The destinations of the migratory materials are always those regions of the 

 plant where materials are being actively used up. The more rapidly the altera- 

 tion of these materials takes place in the regions of activity the greater the 

 diffusion between the two termini of the movement and the more rapid the move- 

 ment itself. The dissolution of the reserves will also be accelerated when the 

 migration of the dissolved substances takes place rapidly. In nature there exist 



