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 cotyledous; viz., 
the soluble and structureless proximate principles are met- 
amorphosed into the insoluble and organized oncs of the 
same chemical composition. Thus, dextrin may })ass 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 
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. 
