. Plant anatomy from the standpoint of the development and functions of the tissues, and handbook of micro-technic. solid rod. This difference of the wood skeletonin the stem and the root is relatedto the difference in the direction ofthe stresses which they have toovercome. In the stem it is theweight of the crown and the alter-nate stretching and compressingwhen swaying in the wind; while inroots it is the pulling force whichthe swaying stem exerts on the roots,and the compressing force with which the soil resists growth inthickness of the roots, and both of these stresses the solid woodcyli
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. Plant anatomy from the standpoint of the development and functions of the tissues, and handbook of micro-technic. solid rod. This difference of the wood skeletonin the stem and the root is relatedto the difference in the direction ofthe stresses which they have toovercome. In the stem it is theweight of the crown and the alter-nate stretching and compressingwhen swaying in the wind; while inroots it is the pulling force whichthe swaying stem exerts on the roots, and the compressing force with which the soil resists growth inthickness of the roots, and both of these stresses the solid woodcylinder of roots is well adapted to withstand. In most monocotyledonous stems the problem of locatingthe skeletal tissues is simplified because no allowance needs tobe made for secondary increase in the vascular bundles, andseldom for secondary increase in the stem as a whole, and thisfact is taken advantage of by encasing, and so bracing and FIG. 39.—Camera-lucida outlineof portion of cross section of corn-stalk, showing at g bast fiber zonebeneath the epidermis and surround-ing the outermost vascular bundles.. f « d FIG. 40.—Cross section of a portion of palm stem, e, xylem; /, phloem portions ofvascular bundle; g, sclerenchyma tissue about vascular bundle; d, fundamental or groundtissue; c, larger tracheal tubes in vascular bundle. (After Engler and Prantl.) strengthening each vascular bundle in a sheath of sclerenchymaor bast fibers developed from the ground parenchyma tissues(Figs. 28 and 40). In grasses and similar Monocotyledons many vascular bun-dles are massed close to the epidermis, each with its protecting 86 THE PLANT SKELETON and strengthening sclerenchyma cylinder, and all are boundtogether by thick-walled ground parenchyma, and bundles ofsubepidermal bast fibers flank the most exterior bundles (Fig.39). These facts account for the hardness and strength of the exterior part of many grassstems, such as corn and bam-boo. In palm stems the skeletonconsists of numer