154 MECHANICAL SYSTEM 



and the lumen is very often greatly diminished or even partially 

 obliterated (as in srJp. of Corchorus). The middle lamella is generally 

 very thin, though in a few cases (e.g. Bwcus) it may be somewhat 

 thickened. The secondary thickening layers are usually very massive 

 and often exhibit distinct stratification. Striation depending upon 

 inequalities of water-content occurs in the bast-fibres of the Apocyna- 

 ceae and Asclei'Iadaceae. Very characteristic of typical bast-fibres 

 is the presence of narrow slit-shaped pits, the greatest (transverse) 

 diameter of which is placed longitudinally or obliquely (Fig. 47 A 1? D). 

 The oblique arrangement is the most frequent, and in this case tlie 

 pits are almost always disposed in left-handed spiral series. The 

 interest of this spiral arrangement of the pits consists in the fact that 

 it indicates a corresponding arrangement of micellar rows. Various 

 circumstances support the view that in prosenchymatous cells the 

 micellae i.e. the ultimate particles of the cell- wall are united in rows 

 which form ultramicroscopic fibrils. 1 ' An obliquely pitted bast-fibre 

 may therefore be regarded as an aggregate of exceedingly numerous 

 and delicate fibrillae twisted together into a spiral coil of many turns 

 which surrounds a longitudinal canal consisting of the cell-cavity. It 

 is in fact comparable to a rope, in which the several fibres also follow 

 a spiral course ; and, just as a rope owes some of its tenacity to the 

 twisted condition of its strands, so it is probable that the spiral course 

 of the micellar rows in the wall of a bast-fibre is advantageous from a 

 mechanical point of view. 



With regard to the chemical nature of the wall it may be stated 

 that the thickening layers generally consist of practically unmodified 

 cellulose. Lignified walls are, indeed, not uncommon, -and every 

 degree of lignification can be illustrated by various types of bast- 

 libres. There appears, however, to be no direct connection between 

 the chemical character of bast-fibre walls and their mechanical pro- 

 perties. 



It is self-evident that bast-fibres must retain their living contents 

 as long as they are growing and thickening their walls. Treub has 

 shown that the fibres of Asclepiadaceae, Apocynaceae and Ukticageae 

 are typically multinucleate ; the same condition prevails, according 

 to the author's own observations, in Lin inn usitatissimum and in certain 

 Lecjuminosae. The presence of several nuclei appears advantageous 

 when the very considerable length of many bast-cells and their active 

 growth in length and thickness are taken into account. The septation 

 which frequently takes place should very possibly be regarded in a 

 similar light ; it consists in the appearance of several thin transverse 

 septa which divide the fibre into separate chambers, each corresponding 

 ontogenetically to a cell. These delicate partitions are of no mechanical 



