THE METABOLISM OF PLANTS. 169 



are well illustrated by experiments in which layers of tissue 

 are removed. Thus Knight observed that when a ring of 

 cortical tissue down to the wood was removed from the stem 

 of a dicotyledonous tree below the leaves, the part of the stem 

 below the incision scarcely grew at all, whereas the part of the 

 stem above the incision grew normally; again Hanstein found 

 that if a ring of cortical tissue be removed from a -detached 

 branch of a tree of this kind, and the branch be placed in 

 water, roots are formed abundantly above the incision but 

 scarcely at all below it; if, however, the experiment be tried 

 upon branches of plants which contain scattered fibrovascular 

 bundles in their medulla, the removal of the ring of cortical 

 tissue has no appreciable effect upon the development of the 

 roots: Faivre too has obtained similar results by the same 

 method. The significance of these facts is this, that the 

 removal of the ring of cortical tissue, including the soft-bast, 

 from a branch or stem of a normal dicotyledonous plant 

 almost completely cuts off the supply of organic substance to 

 the parts below the incision; if the branch or stem contains 

 fibrovascular bundles in its medulla, the supply of organic 

 substance is diminished by the operation, but enough is still 

 conveyed to enable the parts below the incision to maintain 

 their growth. 



With regard to the laticiferous tissue, it appears, from the 

 researches of Faivre, that it contains organic substances which 

 are derived from the leaves and which are used up in building 

 up the tissues of the growing parts of the plant. 



It has been suggested that a formation of proteid takes place in the 

 sieve-tubes ; Strasburger is however of opinion that, since no nuclei have 

 been found in them, this is not the case (see p. 151). 



In order to obtain a connected idea of the changes which 

 these nitrogenous and non-nitrogenous organic substances 

 undergo in connexion with their distribution throughout the 

 plant, we will trace them to some organ in which they are 

 to be stored up for a time as reserve-materials, and then from 

 this organ when, after a period of quiescence, it resumes 

 active life. For this purpose we will take the seed as our 



