BAST-FIBRES 



153 



Bast-fibres so belong to the class of prosenchymatous cells : they are 

 elongated and spindle-shaped, with sharp awl-shaped ends. Their 

 characteristic form is largely produced by independent apical growth on 

 the part of the individual cells ; each fibre consequently wedges itself 

 firmly between its neighbours. It is obvious 

 that the cells of the bast can become very firmly 

 attached to one another owing to the large 

 surface of contact which is produced by their 

 peculiar mode of growth ; the firm consolidation 

 of the separate elements of a bundle of bast- 

 fibres, in fact, constitutes the physiological ex- 

 planation of their prosenchymatous form. 



A morphological peculiarity of obscure sig- 

 nificance is the local dilatation which is 

 characteristic of the bast-fibres of certain plants 



(ASCLEPIADACEAE, APOCYXACEAE, Linum usitn- 



tissim urn, etc.) ; as Krabbe has shown, the 

 swellings are often occupied by the protoplasts, 

 which are encased in secondary cellulose layers 

 (Fig. 48.) 



The stereides of the floral scapes of most 

 species of Allium are among the few exceptions 

 to the usual prosenchymatous shape ; here 

 the mechanical cells, which are about 4 to 

 5 mm. long- have obtuse ends because their 

 cross-walls are not oblique but strictly trans- 



Fio. 48. 



A. Local dilatation of a bast- 

 fibre of Linum perenne. B. Bast- 

 fibre of Nerium Oleander, show- 

 ing a swelling in which the 

 protoplast has become encysted 



verse, as in the case of parenchymatous cells through the deposition of se- 



x ^ condarv layers of cellulose. 



(Fig. 47 d.) 



The length of bast-fibres varies within wide limits; in general, 

 however, it approximates to a definite value in each species. Most 

 frequently it lies between 1 and 2 mm. The linear dimensions of 

 some typical bast-fibres are given in the following table : 



Tilia sp., - 



Cor chorus sp., 



Li mini usitatissimum, 



Cannabis satire, 



Urtica dioica, 



Boehmeria nivea, 



Length. 



99-2-65 mm. 



8-4' 1 mm. 



20-40 mm. 

 10 mm. and over. 

 up to 77 mm. 

 up to 220 luni. 



Greatest width. 



015 mm. (on an average). 

 016 mm. 

 15--17 mm. 

 15-"28 mm. 



04- -08 mm. 



It will be seen that certain bast-fibres represent the longest known 

 vegetable cells. 



The walls of bast-fibres are always more or less thickened : apart 

 from a few exceptional cases the thickening takes place uniformly, 



