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Anatomy of Plants

Angiosperms (Oak, Maple, Basil, ...)

Angiosperms are the flowering plants with seeds enclosed by fruit. All hardwood trees (broad-leaved trees, e.g., oak, ...), including all the deciduous trees (trees that shed their leaves in the fall, e.g., maple, ...) of the temperate zone and the broad-
Angiosperms 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
[view large image]

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

Table 02 Monocots, Dicots, and Gymnosperms Comparison

In angiosperms, the reproductive structures are located in the flower. The flower attracts insects and birds that aid in pollination, and it produces seeds enclosed by fruit. There are many different types of fruits, some of which are fleshy (e.g.,
Flowering Plant Life Cycle 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]

Seed and Fruit 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
[view large image]

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.

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