16 ELEMENTS OF 
PLANT ANATOMY. 
Centripetal growth is by far the more frequent and includes 
all growth in thickness of walls common to two cells. It is 
B 
ry 
ies 
FIG. 9. 
A piece of rhizoid of Marchantia poly- 
morpha. B epithel cells of petal of 
Pelargonium zonale. v projections into 
cell lumen. X 300. —(Wiesner.) 
I. Small circular spots of surface thick- 
ened, producing cone or rod-like projections 
which extend into the lumen. 
ExAMPLEs: Rhizoids of Marchantia and 
certain ducts whose walls are strengthened by 
(See Fig. 9.) 
conical inward projections. 
II. The thickening is not limited to 
spots but runs in continuous spirals with 
varying closeness of coil. 
EXAMPLES: Spiral ducts. 
III. Wall thickened in separate rings. 
ExampLes: Ring ducts. 
IV. The thickened ridges run parallel 
(Fig. 10.) 
found in cells of both classes, 
those where the thickening is 
uniform, and those where it is ir- 
regular. Of the former class, the 
ordinary bast and libriform cells 
furnish the best examples, where 
the primary wall is increased in 
thickness centripetally until only 
a small cavity remains. 
Of the latter class there are 
various kinds. Beginning with 
those where only a small portion 
is thickened, the larger part re- 
maining thin, we have: 
ae 
—— 
— 
"© 
Fia. 10. 
Fragments of ducts from 
to the long axis of growth and are con- “ stem of Rye in long sec- 
nected by short cross-bars. 
tion. <Aspiral. Bring 
duct. 7 ring loosened 
and separated from duct. 
EXAMPLES: Ladder, or scalariform ducts. x 250. — ( Wiesner.) 
e¢ 
