. Un libro di botanica per scuole e università ... La botanica. Steli 703. Fig. 1030. - Una sezione trasversale schematica del rizoma di J uncus balticus lUtoralis; nota la simmetricamente ar- variava camere d'aria (o) nella corteccia (c); e, endodermis; v, tratto vascolare, composto da una matrice di celle di meccanica (m) che circonda i gruppi di con- ductive navi (0- Resistenza alla trazione. - Resistenza alla tensione non dipende dalla posizione ma sulla dimensione del filamento meccanica. Tuttavia, dal momento che la tensione al- la maggior parte sempre è ineguale e quindi coinvolgere la flessione, un massing di trefoli in una centrale, cilindro solido

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. A textbook of botany for colleges and universities ... Botany. STEMS 703. Fig. 1030. — A diagrammatic cross section of the rhizome of J uncus balticus lUtoralis; note the symmetrically ar- ranged air chambers (o) in the cortex (c); e, endodermis; v, vascular tract, composed of a matrix of mechanical cells (m) surrounding groups of con- ductive vessels (0- Tensile strength. — Resistance to tension depends not on the position but on the size of the mechanical strand. However, since tension al- most always is unequal, hence involving flexion, a massing of strands into a central, solid cylinder is most advantageous, since the danger of rupture through differential strain is reduced to a minimum. Roots are subject to considerable tension, espe- cially when the stems sway in the wind, and they differ from erect stems somewhat generally in that the cen- tral portion is occupied by thick- walled wood cells of considerable strength instead of by a core of pith. Rhizomes (as in Lathyrus a.nd Juncus, fig. 1030) are subject to the same tensions as are roots, and like them differ from erect stems in the absence or weak development of central pith. The relatively central vascular tract usually has thick-walled wood elements, and there may be a mechanical cylinder or scattered mechanical strands outside the wood. Lianas are subject to rather extraordinary strains, the combination of tension, flexion, and pressure in a woody twiner, owing to the growth of the supporting tree, being especially severe; in such stems there is a large development of bast and of thick-walled wood. In tendril climbers much of the strain is borne by the relatively slender tendrils, which often have central mechanical tissues that may become strengthened on attachment. In flattened and spirally twisted tendrils (as in Echinocysiis) the strains are similar to those sustained by twining stems. Submersed stems, especially in running streams, may be subject to considerable tension, but the conditions are

Parole chiave associate all'immagine

• bookcentury1900
• bookdecade1910
• booksubjectbotany
• bookyear1910