FIBRES 33 



(Fig. 14, C) irregularly scattered through the thin-walled par- 

 ench3-ma forming the flesh. The cavity of each stone-cell is 

 very much reduced and inconspicuous owing to the strong 

 thickening of the wah (Fig. 14, C and D,s.c.). This shows distinct 

 stratification, and is traversed by a number of dark hues {p.) 

 radiating from the centre and branching as they approach the 

 surface. These are the pits (spoken of as pit-ca)ials when the\- 

 exhibit this narrow elongated form) which serve for the trans- 

 ference of nourishment to the protoplast during the process of 

 thickening, on the completion of which the hving matter cUes. 

 T)i& fibre is a narrow, very much elongated, cell with tapering 

 pointed ends, and in the adult condition its protoplast is fre- 

 quently dead. The walls are in general strongly thickened and 

 hgnified (Fig. 16, B-D), and usually bear a number of oblique 

 slit-shaped pits. In the mature plant fibres are generally the 

 most important form of mechanical element, and compose a 

 large proportion of the wood of thicker branches (cf. p. 121) - 

 these wood-fibres sometimes bear a modified t^-pe of bordered pit 

 (see below). The corte.x in the young stems of manv woody 

 plants exhibits a continuous mechanical ring composed of alter- 

 nating groups of fibres and stone-cells. 



Plant-fibres can undergo considerable elongation and can 

 bear very heavy strains without losing the power of again con- 

 tracting to their original length ; as a matter of fact, loads 

 varying from 14 to 25 kilogrammes are required to produce 

 permanent lengthening. The powers of fibres in this respect 

 may be compared with those of metal rods {e.g. of wrought iron 

 and steel) which, whilst they become permanently stretched 

 under similar strains, exhibit far less extension before the limit 

 of their elasticitj- is reached. Fibres wiU thus permit bending 

 on the part of a plant-organ (under the influence of the wind, 

 for example), and will not give way, even under considerable 

 strain ; moreover, their elasticitj' wiU bring about a return to 

 the normal position as soon as the strain is relieved. WTiilst 

 the breaking strain (10-12 kilogrammes) for collenchyma is not 

 much less than that for many fibres, it exhibits permanent 

 elongation under quite low tension, so that it is especially suited 

 to the mechanical needs of a growing organ where constant 

 extension is taking place. 

 3 



