PLANT TISSUES 
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walls of these tubes are of varying thickness, usually, however, 
thinner than those of woody fibers. The thickness is due to an infil- 
Fig. 49. 
Fig. 50. 
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. B, c, Companion cells, and d, a beginning sieve tube 
from c and d, respectively in A. 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 the original cellulose wall. The walls show 
characteristic thickenings on their inner surfaces. 
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