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leave evergreen trees (e.g., boxwood, ...) of the tropical zone, are angiosperms, although sometimes the flowers are inconspicuous. All herbaceous (nonwoody, nonpersistent) plants (e.g., basil, ...) common to our everyday experience, such as grasses and most garden plants are flowering plants (Figure 22). Angiosperms are adapted to every type of habitat, including water (e.g., water lilies, ...). Angiosperms have well-developed vascular and supporting tissues. Their xylem tissue contains vessel elements beside the tracheids. Thus the woody angiosperms are considered as hardwood trees, whereas the gymnosperms are softwood trees. | |
Figure 22 Angiosperms |
The angiosperms are divided into two classes: the monocots (e.g., rice, ...) and the dicots (e.g., potato, ...). The distinction between these two groups is not always clear, some of the general characteristics (including the gymnosperms) are outlined in Table 02 below. |
Characteristic | Dicots | Monocots | Gymnosperms |
---|---|---|---|
Embryo | Two cotyledons (seed leaves) | One cotyledon (seed leaf) | One to many |
Flowers | Parts in 4 / 5 | Parts in 3x | No true flower |
Vascular Bundles | Ring | Scattered | Ring |
Habit | Herbaceous or Woody | Herbaceous | Herbaceous or Woody |
Roots | Taproot | Fibrous | Taproot |
Leaf Venation | Net | Parallel | Needle-like |
Pollen | Tricoplate (3 furrows or pores) | Monocoplate (1 furrow or pore) | Tow lobular wings |
Growth | Primary and Secondary | Primary | Primary and Secondary |
apple, tomato, peach, ...) and some of which are dry (e.g., pea enclosed by pod, nut, grain, ...). They all provide protection for the seeds. The life cycle of the flowering plant is shown in Figure 23. Within a flower, there is a diploid megaspore mother cell in each ovule of the ovary. The mother cell undergoes meiosis, producing one functional megaspore, whose nucleus divides mitotically until there are eight haploid nuclei. This is the female gemetophyte, which sometimes is called the embryo sac. At one end of the embryo sac there the three cells, one of which is the egg cell. Male gametophytes are produced in the stamens. An anther contains four pollen sacs with many microspore mother cells, each of which undergoes meiosis to four microspores. After a mitotic division, each misrospore has two cells, one of which later divides again to give two sperm. Pollination, which is simply the transfer of pollen from the anther to the stigma, is brought about by wind or with the assistance of a particular pollinator. The plant uses the pollinator to ensure cross-pollination, and the pollinator uses the plant as a source of food in the form of nectar. When a pollen grain lands on a stigma of the same species, it germinates, forming a pollen tube. The pollen tube grows as it passes between the cells of the stigma and the style to reach the female gemetophyte. | |
Figure 23 Flowering Plant Life Cycle [view large image] |
Double fertilization takes place to produce seeds and fruits as shown in Figures 22 and 23. One sperm nucleus from the pollen tube unites with the egg nucleus, forming a zygote, and the other sperm nucleus unites with the polar nuclei, forming a triploid (3N) endosperm nucleus. The endosperm nucleus divides, forming the endosperm, which is a nutrient material for the developing embryo and sometimes for the young seedling as well. The zygote develops into an embryo. The outer layers (integuments) of the ovule harden and become the seed coat. A seed is a structure formed by the maturation of the ovule; it | |
Figure 24 Seed and Fruit |
contains a sporophyte embryo plus stored food. The ovary and sometimes other floral parts develop into the fruit. A fruit is a mature ovary that usually contains seeds. Therefore, angiosperms are said to have covered seeds. |