. A textbook of botany for colleges and universities ... Botany. 702 ECOLOGY times is considered to be a sort of " water storage " tissue. The latter tiieory may be dismissed summarily, and the former theory is at least doubtful, in view of the permeability of the walls. Flexile strength. — The flexion of a stem induces tension on one side and compression on the other, each strain decreasing to zero at the center. Consequently a stem with a strong cylinder of peripheral mechanical tis- sue may have a central region of pith or may even be hollow (as in Equisetum and in the grasses) an
. A textbook of botany for colleges and universities ... Botany. 702 ECOLOGY times is considered to be a sort of " water storage " tissue. The latter tiieory may be dismissed summarily, and the former theory is at least doubtful, in view of the permeability of the walls. Flexile strength. — The flexion of a stem induces tension on one side and compression on the other, each strain decreasing to zero at the center. Consequently a stem with a strong cylinder of peripheral mechanical tis- sue may have a central region of pith or may even be hollow (as in Equisetum and in the grasses) and yet have considerable rigidity. It has been computed that if the mechanical periphery forms one seventh of the diameter, the strength will be sufficient to meet all usual strains. Further development of mechanical tissue would be not only useless but actually disadvantageous, because it would re- quire a considerable amount of struc- tural energy and material, and also be- cause space would be used that might be taken by other tissues. The aver- age erect herbaceous stem illustrates admirably these mechanical principles. Sometimes (as in Dianthus, fig. 1027) there is a mechanical cylinder sur- rounding the leptome, while in other cases strands of bast form an interrupted cylinder. External to the bast is the collenchyma cylinder, and internal to the bast is the cylinder of secondary wood which is of great mechanical im- portance. Although the conductive and mechanical bundles in mono- cotyl stems are scattered, the decrease of bast elements toward the center results essentially in a broken peripheral cylinder of mechan- ical tissue (fig. 1028). In angled stems (notably in the mints, fig. 1029) strains are accentuated at the angles, where there is consider- able collenchyma just inside the epidermis, and often a bast crescent just outside the leptome. The advantage of flexile strength is well illustrated on mountain slopes that are subject to snowslides and avalanches, flexi
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Keywords: ., bookcentury1900, bookdecade1910, booksubjectbotany, bookyear1910
