GERMINATION. 325 



the intermediate cambial tissue, which is destitute of air- 

 passages, and are present in largest relative quantity at 

 the extreme ends of the rootlets and of the plumule. 



In another chapter we shall notice at length the phenom- 

 ena and physical laws which govern the diffusion of liq- 

 uids into each other and through membranes similar to 

 those which constitute the walls of the cells of plants, 

 and there shall be able to gather some idea of the causes 

 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 nutri- 

 ment 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 cotyledons ; viz., 

 the soluble and structureless proximate principles are met- 

 amorphosed into the insoluble and organized ones of the 

 same chemical composition. Thus, dextrin may pass into 

 cellulose, and the soluble albuminoids may revert in part 

 to the insoluble condition in which they existed in the 

 ripe seed. 



But many other and more intricate changes proceed in 

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



O 



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 accumu- 

 lation of cellulose. 



