. Plant life, considered with special references to form and function. Plant physiology. 244 PLANT LIFE. among gymnosperms (fig. 247), or the carpels* may form a covering, as in angiosperms. In these plants the ovule may terminate the axis, as in sunflower and buckwheat families (fig. 257); or the ovules may be lateral iiijon the surface of an enlargement of the axis within the ovulary, as in pinks and prinn'oses (fig. 25S). It is usual, however, for the ovules to arise upon a carpel, either singly or in clusters which occupy definite i.]Ortions of Its surface. The cushion or ridge from which
. Plant life, considered with special references to form and function. Plant physiology. 244 PLANT LIFE. among gymnosperms (fig. 247), or the carpels* may form a covering, as in angiosperms. In these plants the ovule may terminate the axis, as in sunflower and buckwheat families (fig. 257); or the ovules may be lateral iiijon the surface of an enlargement of the axis within the ovulary, as in pinks and prinn'oses (fig. 25S). It is usual, however, for the ovules to arise upon a carpel, either singly or in clusters which occupy definite i.]Ortions of Its surface. The cushion or ridge from which the ovules arise is called the placenta. In the pines the placenta is a. Fig. 256.—Diagrams of median longitudinal sections of three sorts of ovules to show curvatures due to unsymmetric growth. A, a straight. A, an inverted, C, a bent ovule. In all : /. the stalk ; /,•. the sporangium ; /?', the inner integument; ai. the outer in- tegument; ?;/, the micropyle ; c, the base of the sporangium where the integuments arise (called the chalazal; ?-, the ridge (rhaphe) formed by the union of stalk and outer integument; tv//, the megaspore. As C develops further ent may become sharply bent on itself.—After Prantl. scale-like outgrowtli from the upper surface of the carpel, bearing two ovules ( fig. 246 ), and as the cones mature these gradually outgrow the carpels and constitute the main por- tion of the ripened cone. To such placentas the o\ ules are attached by one side ; they are therefore entirely sessile. The * Althougli the cncU).sin^ leaves in do not bear tlic sporangia, and are, therefore, riot strictly sporophylls, their similarity in form renders it convenient to retain tlie name carpel even for those pistils in which they are a mere roof over a convex or hollowed axis liearing the ovules, (.See fif;, ). Please note that these images are extracted from scanned page images that may have been digitally enhanced for readability - coloration and appearance
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