. Adventures with animals and plants. Biology. PROBLEM 3. Hom^ Cojnplex Flams Stigma Pollen tube Micropyle zed egg eel Fertilized ble nucleus. Fig. 392 Section through pistil after fertiliza- tion has occtirred. Only one ovule and one complete pollen tube are shown. Compare with diagra?7is of ovule and pollen (Figs. ^89 and 391). do not enter dry up and disappear. When the embryo sac is reached by the pollen tube, the cell wall at the end of the tube dissolves. The sperm nuclei pass into the embryo sac and one of them fuses with the t^^ nucleus, producing the fertilized tg^ cell. The second sp
. Adventures with animals and plants. Biology. PROBLEM 3. Hom^ Cojnplex Flams Stigma Pollen tube Micropyle zed egg eel Fertilized ble nucleus. Fig. 392 Section through pistil after fertiliza- tion has occtirred. Only one ovule and one complete pollen tube are shown. Compare with diagra?7is of ovule and pollen (Figs. ^89 and 391). do not enter dry up and disappear. When the embryo sac is reached by the pollen tube, the cell wall at the end of the tube dissolves. The sperm nuclei pass into the embryo sac and one of them fuses with the t^^ nucleus, producing the fertilized tg^ cell. The second sperm nucleus sometimes unites with the double nucleus lying in the center of the embryo sac. Since the double nucleus is not an egg nucleus, this union does not result in a fertilized Q^^ cell or zygote. The final result in some plants is a large group of food-stor- ing cells called endosperm. Sometimes the second sperm nucleus does not fer- tilize the double nucleus. Almost always, no doubt, more grains settle on the stigma than there are ovules; then the first tube to reach the ovule is Reproduce 44:5 the one to enter. Its nucleus fertilizes the Qgg cell in the ovule. If some ovule is not reached by a pollen tube its egg cell will not be fertilized. The embryo plant is formed. The fer- tilized tg^ (zygote) becomes surrounded by a cell wall, and a series of divisions begins. Division occurs again and again until a mass of cells is formed; the proc- ess is somewhat like that of cleavage in an animal. However, these cells do not later arrange themselves in the form of a hollow ball or cup; such embr\"onic stages are found in animals only. Differ- entiation does occur; some groups of cells become pith, some become phloem, some xylem, some epidermis. Soon the tiny root, stem, and leaf become appar- ent. Although embryos are small and often curled up, the new plant parts can usually be recognized in the seed. The leaves are still without color since the embryo is located within
Size: 1338px × 1867px
Photo credit: © Library Book Collection / Alamy / Afripics
License: Licensed
Model Released: No
Keywords: ., bookcentury1900, bookpublisherbostondcheath, booksubjectbiology
