FIBROUS TISSUE 33 
Except in old cells the thickening rarely extends out 
upon the wall lying between the angles. The cells 
remain alive, for a long while, and usually ¢ontain chloro- 
plasts. They remain capable of growth longitudinally. 
Accordingly collenchyma is found to be the chief mechan- 
ical tissue in growing parts of plants, such as stems, 
leaf-stalks, ete. The thickened parts of the walls are 
composed of cellulose and transmit the light with a pecu- 
liar pearly luster when viewed in cross-section, the lumen 
of the cell under these conditions appearing darker than 
the cell walls. 
44, Fibrous tissue consists of elongated cells, thick-- 
ened on all sides, usually overlapping at their more or 
less tapering, often pointed, ends. The walls show 
minute, usually obliquely placed, slit- r 
like pits. After they reach full develop- 
ment, the cell contents die, so that the 
cells are incapable of further growth or 
development. The thickened walls are | ‘ 
usually strongly lignified. In cross-sec- 
tion the cells are round or by mutual Fic. 13—Wood and 
3 bast cells. 
pressure, angled. Fibrous tissue is 
found as the chief mechanical tissue in parts of sinats 
which have completed their longitudinal growth. Two 
types can be distinguished, viz., bast and wood fibers. 
The former are located in the outer part of the stem 
(in the cortex in the Dicotyledoneae), the latter in the 
true wood. Bast fibers are usually longer than wood 
fibers, and more slender, with often thicker but less com- 
pletely lignified and hence more elastict walls. Their 
usual length is from 1 to 2 mm. but in Boehmeria nivea, 
the ramie plant (according to Haberlandt) they reach a 
length of 220 mm., the longest plant cells known. Wood 
fibers are usually shorter (mostly 0.3 to 3.1 mm.) often 
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