660 



SECONDARY GROWTH IN THICKNESS 



Typical wood-fibres (libriform fibres or cells) are prosenchymatous 

 cells, with thick walls which bear narrow oblique (sinistrorse) pits (Fig. 

 274, If); they contain air and occasionally also shrivelled remains of the 



?7V 



HWJllSs&ssS^ l 



Fig. 274. 



Part of a T.S. through the secondary xylem and phloem of a twelve-year-old branch 

 of Cytism Laburnum (at the end of October), p, conducting parenchyma ; 6, plate of 

 fibrous tissue ; I, leptome (the larger cells are the sieve-tubes) ; c, cambium ; If, wood- 

 fibres ; m, mestome (including intermediate cells, xylem-parenchyma, traeheides 

 and narrow vessels) ; g, boundary between autumn and spring wood ; in, medullary 

 ray. 



former protoplasmic contents. Branched wood-fibres are of rare occur- 

 rence, but occur, for example, in Tectona grandis (Teak). Lignification is 

 much more general and also usually more intense in the case of 

 wood-fibres than in that of bast-fibres. Sanio has described a number 

 of cases (Cytism Laburnum, Caragana arborescens, Gleditschia triacanthos, 



