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 contain 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, etc. 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- 

 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 FIG. is. Wood and 



, , T-,., .. . bast cells. 



pressure, angled. Fibrous tissue is 

 found as the chief mechanical tissue in parts of plants 

 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 



