Nature and development of plants . spores, also called pollen spores orgrains, arc minute cells, provided with a cell wall often variousljsculptured and contain a nucleus and protoplasm like an ordinarycell ( Fig. 7 11. \t maturity the anthers break open in a mannerthat varies in differenl plants exposing the microspores to theair (Fig. 75. B). The pistil or carpel is quite differenl incharacter. At or near th< top is a more or less modified part, NATURE OF PLANTS 103 the stigma, below which is an elongated stalk, the style, thatbroadens out into a base or ovary. By longitudinal and trans-v
Nature and development of plants . spores, also called pollen spores orgrains, arc minute cells, provided with a cell wall often variousljsculptured and contain a nucleus and protoplasm like an ordinarycell ( Fig. 7 11. \t maturity the anthers break open in a mannerthat varies in differenl plants exposing the microspores to theair (Fig. 75. B). The pistil or carpel is quite differenl incharacter. At or near th< top is a more or less modified part, NATURE OF PLANTS 103 the stigma, below which is an elongated stalk, the style, thatbroadens out into a base or ovary. By longitudinal and trans-verse sections of a pistil we gain a better idea of its real character(Fig. 74, D-F). The ovary is now seen to contain a cavityfrom the walls of which one or more rather spherical bodies,the ovules, are developed. The structure of the ovule is verycomplicated. If a thin section is made through the center ofone of them it will be seen to contain usually but one large spore,therefore called the megaspore (Fig. j6, nig). This spore is. Fig. 75. Structure of the anther: A, diagram of the anther cut across,showing the four cavities, sporangia (sing, sporangium), filled with , cross-section of a mature anther. The tissue about the spores hasbroken down, thus forming two cavities and at the left the breaking openof the wall of the anther is shown. developed in a mass of tissue, the nucellus, and the whole issurrounded by one or two coats, the integument, which growsup from the base of the ovule but never quite surrounds it,thus leaving a small opening or micropyle (Fig. 76, mi). Themegaspore, unlike the microspore, is never set free but is re-tained permanently in the ovule. It is not known how the sporophylls have originated but thereare indications that they may have arisen through the modifica-tion of leaves for the purpose of forming spores instead of con-structing foods. It is for this reason that these organs arecalled sporophylls, or spore-forming leaves, from phylon, a
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