. Botany; principles and problems. Botany. 350 BOTANY: PRINCIPLES AND PROBLEMS microsporophyll (Fig. 222) bears many sporangia or anthers and produces a large amount of pollen. The ovules are generally large and thick-walled, and the megaspore, arising as one of four potentially spore-producing cells in the middle of the nucellus, develops into a large embryo-sac. At the tip of the nucellus. just under the micropyle, a large, liquid-filled pollen chamber arises. The pollen grain enters this chamber through the micro-. FiG. 219.—Cycas revohita, one of the Cycadales. Male plant with cone. (Photo
. Botany; principles and problems. Botany. 350 BOTANY: PRINCIPLES AND PROBLEMS microsporophyll (Fig. 222) bears many sporangia or anthers and produces a large amount of pollen. The ovules are generally large and thick-walled, and the megaspore, arising as one of four potentially spore-producing cells in the middle of the nucellus, develops into a large embryo-sac. At the tip of the nucellus. just under the micropyle, a large, liquid-filled pollen chamber arises. The pollen grain enters this chamber through the micro-. FiG. 219.—Cycas revohita, one of the Cycadales. Male plant with cone. (Photo hyG. S. Torrey). pyle and there germinates. Its two male cells are each provided with a spiral band of cilia and swim about in the liquid, a remark- able persistence of the habit of swimming sperms which harks back through pteridophytes and bryophytes to their remote algal ancestry. A pollen-tube is formed and penetrates the adjacent nucellar tissue, but it seems rather to absorb food than to assist in the transference of the male cells, for the pollen- chamber gradually enlarges itself until it reaches the embryo- sac, where one of the sperms enters an archegonium and effects fertilization. Cycads are confined to the warmer regions of the globe. They are an inconspicuous group today but in earlier ages, notably in the Mesozoic Era, were abundant and diversified. Their close relatives, the Bennettitales (now extinct), were for a time. Please note that these images are extracted from scanned page images that may have been digitally enhanced for readability - coloration and appearance of these illustrations may not perfectly resemble the original Sinnott, Edmund Ware, 1888-. New York, McGraw-Hill
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Keywords: ., bookcentury1900, bookdecade1920, booksubjectbotany, bookyear1923
