40 
PLANT MORPHOLOGY. 
III. THICKENING OR MARKING OF WALLS. 
In the formation of the cell wall each cell appears to 
work in unison with its neighbors for the building up 
of the plant. The thickening of the walls of the cell is 
primarily for the purpose of strengthening the walls, 
but if the walls were uniformly thickened, osmosis, or 
the transferral of cell sap from one cell to another, 
would be hindered. Thus, we find that the contiguous 
walls of the cells are thickened correspondingly at defi- 
nite places opposite each other, leaving pores or canals 
which permit rapid osmosis between the thickened 
portions. The pores thus formed are known as simple 
pores, and in surface view are somewhat elliptical or 
circular in outline, and may be mistaken for some of 
the cell-contents. These thickenings assume a number 
of forms, which are quite characteristic for the plants in 
which they are found. They may have the form of 
transverse or oblique rings, longitudinal spirals, or 
they may be ladder-like or reticulated in appearance. 
In other instances the thickening of the wall is quite 
complex, as in the wood of the pines and other Conif- 
erse. The thickening, or sculpturing, as it is some- 
times called, may not only occur on the inner surface 
of the wall, when it is spoken of as centripetal, but may 
also take place on the outer surface, when it is known 
as centrifugal ; as examples of the latter, may be men- 
tioned the spores of lycopodium and the pollen grains 
of the Composite. 
E. FORMS OF CELLS. 
Upon examining sections of various portions of the 
plant, it is observed that not only the cell-contents 
and cell wall vary in composition, but that the cells 
themselves are of different forms, depending more or 
less upon their function; they may be classified, for 
