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By van Jaarsfeld E.J., de Villiers P.U.

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Absorption of soil water is facilitated by root hairs that develop just above the zone of root elongation. As the short-lived root hairs die in days to weeks, new ones form. Specialized and modified roots include adventitious roots, aerial roots, and mycorrhizae (symbiotic associations between nonpathogenic or weakly pathogenic fungi and living root cells). In mycorrhizae the host plant supplies the fungus with carbohydrates and other metabolites while the fungus increases absorption of soil water and mineral nutrients.

THE WOODY PLANT BODY Resin duct Transfusion tissue Endodermis Phloem Xylem Guard cell Stoma Chlorenchyma Cuticle Epidermis Hypodermal sclerenchyma FIGURE 2,11. Transection of a secondary needle of eastern white pine. When grown in the shade, gymnosperm needles usually show similar responses to those of angiosperm leaves, being thinner, having higher chlorophyll content, and showing reduced stomatal frequency when compared with sun-grown needles. Needles of western hemlock that developed in the shade were thinner and had a higher ratio of width to thickness, thinner palisade mesophyll, and a higher ratio of surface area to weight than those developed in full sun (Fig.

In gymnosperms, the earlywood tracheids are wider toward the stem base than near the top of the stem within the same xylem increment. The transition between the last earlywood tracheids and first-formed latewood tracheids of the annual increment also is sharper in the lower stem than in the upper stem. Some tracheids fit the usual definition of latewood because of a decrease in the radial diameter, without appreciable change in wall thickness. Other tracheids, however, become latewood because of an increase in wall thickness without a change in diameter.

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Aisoaceae by van Jaarsfeld E.J., de Villiers P.U.

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