Elementary botany : theoretical and practical / by Henry Edmonds.

  • Edmonds, Henry.
Date:
1910
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Licence: In copyright

Credit: Elementary botany : theoretical and practical / by Henry Edmonds. Source: Wellcome Collection.

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    central cylinder, or stele. The central jiart of the stele is occupied by parenchymatous cells forming the pith, or medulla (m). In the young root, the protoxylem, or first xylem formed, is always towards the outer part of the cylinder, fresh xylem being develojred towards the centre until, in many cases, the centre of the cylinder is filled up with xylem cells. ^ *G* 5X.—Longitudinal section of main root of f icia Faba, r, r, cortex of main root ; yi fibro-vascular bundles : h, «, lateral rootlets developing from pericambium, and breaking through cortical tis.sue; /<. pilcorhiza of side rootlets. (After Prantl.’) Fig. 52.—Kpidermis of root with hair'i. Suoh a stele, where the xylem and phloem portions are placed side by side, is spoken of as radial. Roots increase in thickness by the formation of a layer of nieristein j tissue known as oambinm. This is first produced just within the phloem ; groups by a division of the cells of the ground tissue between the xylem 1 strands. i These cambium masses soon meet in the pericycle, just outside the xylem 1 groups. The cells of the cambium undergo division, pnxlucing xylem on ] the inner and phloem on the outer side. Thus, whilst the primary xylem is j developed from without inwards, the secondary xylem is developed from ! within outwards. | Many of the epidermis cells near the apex of the main root and of its j branches are produced into tlelicate prolongations, the root hairs. These j are always developed close behind the growing point, and extend for a short distance up the root. They are the true absorbtive parts of the roots. 1 They bring the roots into very close and intimate relation with the soil. If a young plant be carefully removed from the ground it is found that a con- siderable quantity of soil clings to the lower part of the root. This is held in )X)sition by the hairs. In the older parts of the root, the epidermis disappears, amt is replaceel by a cuticularised cortical layer, or exodermis, from the ground tissue.
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