STONE-ELEMENTS. 



127 



Sect. 29. The term Short sclerenchymatous elements may be applied to 

 all forms which have not pointed tapering ends; these are sometimes iso-diametric, 

 sometimes moderately elongated. To this group belong — 



{a) The sione-elements (' stone-cells ' of the Pharmacologists), so called after the 

 stony bodies in the flesh and stalk of many pears, which are composed of them, 

 are almost iso-diametric, rarely rod-like elongated derivatives of cells (' rod-cells '), 

 with stratified, very strongly thickened membrane, lignified to a stony hardness : 

 this wall is perforated frequently by numerous, usually branched pit-canals, of circular 

 appearance in transverse section (Fig. 52). The narrow internal cavity, v/hich usually 

 disappears, is occupied by a watery fluid with a few granules, 

 or often by a reddish brown, apparently formless mass. 

 Stone-elements of this sort are widely spread among the 

 Dicotyledons, especially in sappy, fleshy parts ; in the suc- 

 culent parenchyma they are sometimes isolated, but usually 

 in uninterrupted connection with one another, forming cir- 

 cumscribed groups, or masses, of which the elements bor- 

 dering on the thin-walled tissue may graduate into the latter 

 by the thickening of their walls at this limit being one-sided 

 and weaker. In the so-called stout succulent plants, how- 

 ever, such as the Crassulaceae, Cactacese, &c., stony formations 

 are generally wanting. Exquisite examples are supplied by 

 the fleshy body of Helosidese, Lophophytum, Langsdorffia^ 

 fleshy tuberous roots, e.g. Paeonia, Dahlia (Sachs) ; Rhizomes, e. g. Dentaria pinnata, 

 the pith of Hoya carnosa ^ Medinilla spec.,^ and especially the cortex of ligneous 

 Dicotyledons, in w-hich they are mainly derived from secondary sclerosis of parenchy- 

 matous cells, as will be more closely described in Chap. XV. 



Transitional forms to the sclerenchymatous fibres are supplied by the rod- 

 shaped stone-elements of many cortical layers, the short pointed fibres of the 

 Cinchonese, the short and pointed branched stone-elements of the cortex of Firs 

 and Larches, &c. 



In the Monocotyledons the elements of this category are rare ; but we must 

 include under this head the multiseriate dense layers beneath the epidermis of stems 

 of Palms ■*, and elements with large cavity, and large pits, which form in the cortex 

 of the root of many Aroids (e. g. Tornelia fragrans) 3-4 layers of cells outside the 

 endodermal sheath of the vascular bundle, and in Raphidophora angustifolia ^ also in 

 the inner cortex of the stem a ring of 1-2 layers in thickness. 



Typical stone-elements are wanting in the Cryptogams. 



(<5) A second form of short sclerenchyma is represented by the peculiar covering 

 plates which Mettenius'' first distinguished in species of Trichomanes under the name 



Fig. 52. — Transverse section of 

 a short sclerenchymatous (stone) 

 element from the root tuber of 

 Dahlia variabilis. / lumen, .^ pit- 

 canals; J/ split, by which an inner 

 system of layers is separated ("-oo). 

 From Sachs' Textbook. 



' Hooker, Trans. Linn. Soc. vol. XXII. — Graf Solms-Laubach, in Pringsheim's Jahrb. VI. p. 

 530. — Eichler, Balanophorese Brasilienses, Tab. II. 



^ Mohl, Ranken- und Schling-pflanzen, p. 89. — Ibid. Poren d. P^lanzenzellgewebes, p. 32. 



^ A. Gris, Ann. Sci. Nat. 5 ser. XIV. p. 50. 



* Mohl, Palmarum structura, pag. VI. Tab. A. C. Verm. .Schriften, p. 136. — Botan. Zeitg. 

 1871, Taf. II. 



'' Van Tieghem, Struct, des Aroidees, /. c. 



Hymenophylleen, I.e. p. 418. 



