. Botany for high schools. Botany. Fig. 375- Nearly mature pollen grain of tril- lium. The smaller cell is the genera- tive cell. which floats in the tube cell but at first is s:parated by a thin arched cell wall (fig. 375). This is the generative cell or body cell, and was formed on germination of the young microspore by the division of the single nucleus of the microspore before the pollen grain was ripe. After the pollen grain has reached the stigma of the pistil (see Pollination, Chapter XVIII) it germi- nates, producing a tube 'which travels down the style (or in the stylar canal in some
. Botany for high schools. Botany. Fig. 375- Nearly mature pollen grain of tril- lium. The smaller cell is the genera- tive cell. which floats in the tube cell but at first is s:parated by a thin arched cell wall (fig. 375). This is the generative cell or body cell, and was formed on germination of the young microspore by the division of the single nucleus of the microspore before the pollen grain was ripe. After the pollen grain has reached the stigma of the pistil (see Pollination, Chapter XVIII) it germi- nates, producing a tube 'which travels down the style (or in the stylar canal in some plants), enters the ovary, and passing through the micropyle * bores its way through the nucellus until it reaches the embryo sac. The tube nucleus has followed along with the advance of the tube. Soon after the pollen grain germinates the generative cell divides into two cells, the sperm cells, and these move into the tube, if the generative cell was not already there before division. The stigma of the pistil is covered by a moist somewhat viscous secretion w^hich holds the pollen grains. The moisture and certain saccharine or other secretions favor or stimulate the germination of the pollen grain. 546. The female gametophyte.—The female gametophyte is developed from a special cell in the nucellus of the ovule The o\iile is an outgrowth usually from some part of the ovary and enclosed by it (the main part of the carpel or macrosporophyll which in development folds in such a way as to form a case). It is the macrosporangium (or megasporan- Fig. 376. gium). It consists of the nucellus fp^iLTgrSTp^^ surrounded by two coats which X^^^^, do not quite cover up the nucellus torm^'the two^Jperm at one end, thus leaving a minute deus7adi nelr ^ opening (the micropyle). In the ^^'"'^ lily a subepidermal cell in the nucellus, near the micro- pyle, becomes larger than its neighbors, with a large n * In a few plants, as in some of the ament-bearing plants, the pollen tube enters the
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Keywords: ., bookcentury1900, bookdecade1910, booksubjectbotany, bookyear1910
