PLANT TISSUES 
115 
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. 1, 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 the formation of wood parenchyma from cambial or procambial cells, i. 
Group of cambial or plerome 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 walls of these cells are apt to become lignified, but the cells are 
longer lived than the wood fibers*. (From Stevens.) 
