308 LECTURE XVIII. 



of entire plants, in the most various stages, that the starch and sugar etc. which 

 jiass from place to place and are used up in the petioles, stems, growing buds, etc., 

 were derived from the green assimilating leaves ; and, consequently, when assimi- 

 lation ceases in these, the starch must also disappear from the chlorophyll-corpuscles 

 themselves. This conclusion also proved correct. This is not the place to repeat 

 ■in detail the numerous proofs, in part direct, in part indirect, which I brought 

 •forward during the years 1862-1865, to establish the fact that the starch-grains 

 observed in the chlorophyll-corpuscles of normally vegetating plants are products 

 of assimilation, and that, after their origin under the influence of light, ' they are 

 dissolved and conveyed from the leaves, through their petioles, into the shoot- 

 axes, and thence into the buds and apices of the roots, to provide the material 

 for the growth of the organs; and that a portion of this original product of 

 assimilation is made use of in metabolism for the formation of proteid substances, 

 while, on the other hand, f^ts may arise by relatively slight changes from the carbo- 

 hydrates, and therefore, ultimately, from the assimilated st9,rch. The thought arose 

 that, in the nutrition of plants, it is only necessary in the first place to decompose 

 carbon dioxide under the influence of light in the cells containing chlorophyll, 

 with the co-operation of certain mineral matters absorbed by the roots, and to 

 produce at the cost of its carbon an organic substance-^starch — which then repre- 

 sents the starting-point, so to speak, from which all the other organic substances 

 of the plant proceed by progressive chemical changes. This conclusion has in the 

 course of twenty years become more and more established as the correct one. 



