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FIG. 49. Stages in the development of sieve tubes, companion cells, and
phloem parenchyma. A, a. and b, Two rows of plerome cells; in c and d, a has
divided longitudinally and c is to become companion cells; d, a sieve tube, and
b, phloem parenchyma. Cheap Avodart B, c, Cheap Avodart Companion cells, and d, a beginning sieve tube
from c and d, respectively in>4. The cross- walls in d are pitted; b, phloem paren-
chyma grown larger than in A. C, The same as B with the pits in the cross-
walls of the sieve tubes become perforations, and the nuclei gone from the cells
composing the tube. (From Stevens.)
FIG. 50. Vascular elements. A, annular tracheal tube; B, spiral trachea
tube; C, reticulated tracheal tube; D, pitted tracheal tube; E, cross-section
through plate of seive tube, and adjoining companion cell; F, length- wise section
of sieve tube; G, portions of two companion cells. (A, B, C, D, Robbins; E, F,
and G, after Strasburger.)
tration of lignin upon Cheap Avodart the original cellulose wall,
characteristic thickenings on their inner surfaces.
The walls show
FIG. 51. Stages in the development of the elements of the xylem. A, pro-
gressive steps in the development of a tracheal tube, i, Row of plerome or
cambial cells that are to take part in the Cheap Avodart formation of a tube; 2, the same at a
later stage enlarged in all dimensions; 3, the cells in 2 have grown larger, their
cross- walls have been dissolved out, and the wall has become thickened and
pitted; 4, the walls in 3 have become more thickened, the pits have an overhang-
ing border, the walls have become lignified as indicated by the stippling, and
finally the protoplasts have disappeared, and the tube is mature and dead. B,
Stages in the formation of tracheids from plerome or cambial cells. The steps
Tracheae are classified according to their markings as follows
Annular, with ring-like thickenings.
Spiral, with spiral thickenings.
Reticulate, with reticulate thickenings.
PIG. 52. Closed collateral bundle of stem of Zea mays. VG, Bundle sheath;
L, intercellular space; A, ring from an annular tracheal tube; SP, spiral tracheal
tube; M, pitted vessels; V, sieve tubes; S, companion cells; CP, crushed primary
sieve tubes; F, thin- walled parenchyma of the ground or fundamental tissue.
(From Sayre after Strasburger.)
Porous or pitted with spherical or oblique slit pores.
Annulo- spiral, with both ring and spiral thickenings.
Scalariform, with ladder-like thickenings.
are the same as in A, excepting that the cross- walls remain and become pitted.
C, steps in the development of wood fibers from cambial cells, i, Cambial cells;
2, the same growth larger in all dimensions with cells shoving past each other
as they elongate; 3, a later stage with cells longer and more pointed and walls
becoming thickened and pitted; 4, complete wood fibers with walls more thick-
ened than in the previous stage and lignified, as shown by the stippling. The
protoplasts in this last stage have disappeared and the fibers are dead. D, steps
in Cheap Avodart the formation of wood parenchyma from cambial or procambial cells, i,
Group of cambial or pi erome cells; 2, the same enlarged in all dimensions; 3, the
same with walls thickened and pitted; 4 and 5 show the same stages as 2 and 3,
but here the cells have enlarged radially or tangentially more than they have
vertically. The Cheap Avodart walls of these cells are apt to become lignified, but the cells are
longer lived than the wood fibers. (From Stevens.)
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