18 ELEMENTS OF PLANT ANATOMY. 
Centrifugal. { 
Coarser 
structure ( 
of wall. 
Centripetal. 
A B 
l 
a fd 
L-- Z 
yd 
a cf 
Fia. 12. 
Fragments of isolated bast 
cells. J lumen of cells, A 
from Linum usitatissimum, 
B from Corchorus capsula- 
ris irregularly thickened, C 
from Sponia Wightii, lu- 
men in some places entirely 
filled with thickening mate- 
rial. x 300. —( Wiesner.) 
a. Pollen grains. 
b. Growth of wall into intercellular spaces. 
I. Cones or beam-like 
projections. 
Spirals. 
Rings. 
Ladder thickenings. 
Reticulated thicken- 
LE 
; raat 
IV 
i 
R ings. 
- 
a. Projections from 
surface into 
cell. 
b. Whole surface 
thickened with 
few exceptions. 
ec. Additional forms. { 
Simple and bordered 
pores. 
Collenchyma. 
Bast. 
Of these various forms, those with 
pores require special notice. The simple 
pore results from a small portion of the 
primary wall being left unthickened on 
both sides. In this way a small channel 
is formed with a membrane across its 
central portion, the pore not being a com- 
plete opening between the two cells as 
its name implies. 
The bordered pore is a complex little 
piece of mechanism, varying somewhat 
in different cells and also in different 
plants. The portion of primary wall 
which is left unthickened is here usually 
larger than in case of the simple pore. 
Instead of the material being added so as 
to form a single channel, the thickening 
portion gradually extends itself over that 
part of the primary membrane left free, 
in the form of an arching roof, but never 
entirely covers it. An aperture is always 
left, and the new material is so deposited 
