WOOD-FIBRES 155 



value, so that the mere formation of walls cannot be the ultimate 

 object of the septation : it is possible, however, as has already been 

 suggested, that the development of septa is determined by the same 

 causes as the above-mentioned multiplication of nuclei. In the adult 

 condition of a bast-fibre the living protoplast is superfluous, and hence 

 usually undergoes degeneration; the cavity then becomes filled with 

 a watery liquid or with air. 



Stereides agreeing in every particular with typical bast-fibres are 

 found even among Mosses. There the prosencbymatous attenuation 

 of the ends is sometimes very marked indeed {e.g. midrib of the leaf 

 of Atrichum undulatum), more so even than in most Monocotyledons : 

 these stereides also sometimes possess slit-shaped pits (C/imacitnn 

 dendroides) of longitudinal or oblique (sinistrally inclined) cross-section. 

 Bast-fibres, however, undoubtedly attain to their highest development 

 among Monocotyledons, especially in the Gramineae and Cvperaceae. 



2. 1 Toad -Jib res or libriform alls. 



The distinction between bast-fibres and wood-fibres (or libriform 

 cells) is not based to any great extent upon tangible morphological 

 differences ; so far as it has any justification at all, it depends upon 

 topographical considerations. It has been a very widespread custom, 

 since Sanio first introduced the conception of libriform or " bast-fibre- 

 like " cells, to apply this term to the mechanical components of 

 Dicotyledonous wood. More generally, and perhaps more correctly, 

 one may designate all intracambial mechanical cells libriform cells or 

 wood-fibres in contradistinction to the extracambial or genuine bast- 

 fibres. Obviously, however, it is quite an arbitrary proceeding to 

 employ location with reference to the cambium as a criterion for 

 distinguishing between different types of cells. Any such distinction 

 rests upon a purely topographical basis ; from the point of view of 

 the physiological value in the present instance the mechanical signifi- 

 cance of a given type of cell, it is a matter of complete indifference 

 whether the elements that conform to this type are situated within 

 the cambial cylinder or outside it. Nevertheless for reasons of 

 convenience the detailed discussion of wood-fibres will be deferred 

 until the final chapter. 



Collenchyma. 91 



o. 



While bast-fibres and wood-fibres perform the task of strengthening 

 fully grown organs in a perfectly satisfactory manner, neither of these 

 types of mechanical element is suited to the needs of young organs 

 which are still growing in length. For it must be borne in mind 



