364 HOW CROPS GROW. 



which set up and maintain the transfer of the materials 

 of the seed into the infant plant. 



3. Assimilation is the conversion of the transferred 

 nutriment into the substance of the plant" itself. This 

 process involves two stages, the first being a chemical, 

 the second, a structural transformation. 



The chemical changes in the embryo^ are, in part, 

 simply the reverse of those which occur in the cotyle- 

 dons; viz., the soluble and structureless proximate prin- 

 ciples are metamt)rphosed into the insoluble and. organ- 

 ized oues of the same or similar chemical composition. 

 Thus, dextrin may pass into cellulose, and the soluble 

 albuminoids may revert in part to the insoluble condi- 

 tion in which they existed in the ripe seed. 



But many other and more intricate ehanges proceed in 

 the act of assimilation. With regard to a few of these 

 we have some imperfect knowledge. 



Dr. Sachs informs us that when the embryo begins to 

 grow, its expansion at first consists in the enlargement 

 of the ready-formed cells. As a part elongates, the 

 starch which it contains (or which is formed in the early 

 stages of this extension) disappears, and sugar is found 

 in its stead, dissolved in the juices of the cells. When 

 the organ has attained its full size, sugar can no longer 

 be detected ; while the walls of the cells are found to 

 have grown both in circumference and thickness, thus 

 indicating the accumulatiTO of cellulose. 



Oxygen Gas needful to Assimilation. — Traube 

 has made some experiments, which prove conclusively 

 that the process of- assimilation requires free oxygen to 

 surround and to be absorbed by the growing parts of the 

 germ. This observer found that newly-sprouted pea-, 

 seedlings continued to develop in a normal manner when 

 the cotyledons, radicles, and lower part of the stem 

 were withdrawn from the influence of oxygen by coat- 

 ing with varnish or oil. On the other hand, when the 



