ELASTICITY OF LIGNIFIED TISSUES. 



219 



degree of rigidity. We find particularly exquisite examples of this in the 

 haulms of Grasses, and in the extraordinarily long scapes of many Rushes and 

 Sedges, &c. {/uncus, Sdrpus, Cyperus). These organs form columns, which, though 

 one, two, or even three metres high, only possess a diameter of a few millimetres. 

 In spite of their exceedingly slender form, they are very rigid : under the 

 pressure of the wind they may be bent down into a semicircle, and in spite of 

 being weighted at the apex with fruits and leaves, nevertheless spring up again like 

 elastic steel wire. Examination shows, however, that these slender columns are 

 either hollow, as the haulms of the Grasses, where the wall of the hollow cylinder is 

 often only of the thickness of ordinary paper ; or the interior of the columnar organ 

 consists principally of very loose pith. At the circumference of the cylinder, however, 

 either quite close beneath the epidermis, or on the outer and inner sides of the slender 

 vascular bundles, which are arranged in a circle on the transverse section, run thin 

 cords of strongly lignified elastic 

 fibres, upon which alone the 

 rigidity of such organs depends. 

 These lignified strands of fibre are 

 scarcely at all extensible, and their 

 elasticity may be not inaptly com- 

 pared to that of wrought iron. As 

 in the artificial construction of a 

 thin hollow column, so Nature distri- 

 butes these firm layers and strands 

 chiefly at the circumference of the 

 organs, because there their essen- 

 tial properties are at the greatest 

 advantage. If the haulm of a 

 Grass or the scape of a Rush, 

 or any other similarly constructed 

 cylindrical organ is bent, the elastic 

 layers and strands on the convex side 

 must be thereby slightly elongated, 

 and those of the concave side slightly 



compressed ; since they possess a high degree of elasticity, however, they forcibly 

 assume their natural length again when the external pressure ceases, and straighten 

 the cylindrical organ. Schwendener\ who has closely investigated the relations 



* The significance of the strings and layers of lignified sclerenchymatous cells which accompany 

 the vascular bundles or which run without these beneath the epidermis or in the parenchymatous 

 tissue, as conducing to the rigidity of stems and leaves, especially in the Monocotyledons, 

 was first correctly recognised and established by Schwendener in his book 'Das tncclianischc 

 Princip in anatomischen Bau der Monocoty/en' (Leipsic, 1874). That Schwendener sought to 

 introduce these forms of tissue into science as ' bast ' was however a mistake. The supporting ring in 

 the flowering scape of the species of Allium for instance consists of parenchymatous fundamental 

 tissue, the cells of which are only narrower, thicker-walled, and strongly lignified. I also find that 

 Schwendener laid too little stress upon the fact that the rigidity of stems actively growing in 

 length must depend on quite another principle, since in these the strings and layers in question are 

 not yet lignified and consequently are very extensible. Schwendener's researches essentially account 

 only for the rigidity of fully developed intemodes and leaves. 



Fig. 190.— Transverse section of the flowering scape of Allium 

 loenoprasum. sr the cyUnder of supporting tissue, consisting of 

 lified fundamental tissue. 



