THE CELI-WALL. 149 
If through a perfectly uniform deposition of new particles of 
cellulose the mass of the cell-wall becomes equally enriched all 
around, but not actually thickened, it is compelled to assume a 
spherical form, and the cells become isodiametric,’ as in many 
young tissues. Their mathematical regularity, however, is. 
altered, as soon as the reception of constructive material takes. 
place more energetically in certain places. The outline of the 
Fic. 64.—Schematie representation of the development of the wall of a wood-cell. a, 
youngest state; f, the finished condition. It is only in the first three stages that the 
nucleus of the cell is preserved (Hartig). A film of intercellular substance closes the 
pits of the cells. 
cells is also very essentially controlled by the fact that they 
mutually oppose their free expansion. In such cases the form 
of a sphere becomes flattened to that of a dodecahedron, which 
is the most uniform of those forms of cells of so frequent 
occurrence which we designate as spherically-polyhedral, since 
Iéos equal, and é:a@uyrep diameter. 
