BIOLOGY Volume 1 - A Guide to General Biology - 2004

2. THE DIVERSITY OF LIFE ON EARTH

2.7. Kingdom Plantae

2.7.5. Division Angiospermophytae (Flowering Plants or Angiosperms)

The Main characteristics of angiosperms are listed in Table 2.10.

Angiosperms are better adapted to terrestrial life than any other plant group. Emerging in the Cretaceous period approximately 135 million years ago, they rapidly outcompeted conifers, becoming the dominant vegetation across the planet and successfully colonizing a vast array of habitats. A few angiosperms have subsequently returned to a freshwater lifestyle, and a handful of species to brackish waters.

One of the most defining features of angiosperms, aside from the enclosed seeds mentioned previously, is the appearance of flowers instead of cones. The evolution of flowers allowed these plants to enlist insects, and occasionally birds and bats, as agents of pollination. Bright floral coloration, scent, edible pollen, and nectar are all adaptations designed to attract animal pollinators. In some cases, pollinators and flowering plants coevolved, establishing highly specialized and mutually beneficial relationships. Floral adaptations generally evolved to maximize the chances of pollen transfer by insects, making this method much more reliable than wind pollination. Consequently, insect-pollinated plants do not require the massive pollen production characteristic of wind-pollinated species. Nevertheless, many flowering plants have secondarily adapted to wind pollination.

Monocotyledons and Dicotyledons

Angiosperms are divided into two major groups, which are granted Class status. These two groups are commonly referred to as monocotyledons (monocots) and dicotyledons (dicots). Table 2.11 outlines their key distinguishing features (see also Fig. 2.40). According to modern views, monocots are believed to have evolved from dicot ancestors.

Table 2.11. Main differences between dicotyledons and monocotyledons

Fig. 2.40. Leaf Structure of monocots (A) and dicots (B).

Angiosperms can be either herbaceous (non-woody) or woody. Woody plants include shrubs and trees, which produce substantial amounts of secondary xylem (wood) that provides structural support and Functions in Water transport. This xylem is generated by The activity of the vascular cambium, a layer of meristematic Cells situated between the xylem and phloem in stems and roots. Cambial cells retain The ability to divide. The new xylem produced through this process is referred to as secondary xylem or wood.

Herbaceous plants, or herbs, rely entirely on cellular turgor pressure and mechanical Tissues such as collenchyma, sclerenchyma, and xylem for structural support; consequently, it is not surprising that they are generally small in stature. Herbaceous plants either lack a cambium entirely or possess one with very limited activity.

Many herbaceous plants are annuals, meaning they complete their entire life cycle from seed to seed within a single year. Other herbaceous species develop perennating Organs such as bulbs, corms, or tubers, which allow the plant to survive freezing winters or unfavorable periods such as droughts (Ch. 20). Such herbaceous plants may be biennials or perennials. Biennials set seed and die during their second year, whereas perennials live for many years. Shrubs and trees are perennial plants that can be either evergreen or deciduous. Evergreens produce and shed foliage year-round, maintaining a canopy of leaves at all times, whereas deciduous plants drop all their leaves during cold or dry seasons.

The Diversity of angiosperms is illustrated in Figs. 2.41–2.44 using typical representatives of this plant group.

Fig. 2.41. STRUCTURE OF THE flower and Vegetative organs of the herbaceous monocot meadow fescue (Festuca pratensis). Second leaves are shown in grey. Leaves are typically arranged in two alternating rows on opposite sides of the stem. A. Structure of vegetative organs. B. Inflorescence structure. C. Detailed structure of a single open flower, or floret; the lodicules (two small petal-like structures covering the Ovary) are not shown.

Fig. 2.42. Structure of the flower and vegetative Organs of the herbaceous dicot creeping buttercup (Ranunculus repens). This common perennial is found in wet meadows, damp woodlands, gardens, and abandoned pastures throughout the UK.

Fig. 2.43. Structure of the flower and vegetative organs of the pontic rhododendron (Rhododendron ponticum), an evergreen dicotyledonous shrub. Originally introduced, this plant has successfully naturalized and thrives on acidic soils (such as sand and peat), on moorlands, and in woodlands.

Fig. 2.44. Structure of the flower and vegetative organs of the horse chestnut (Aesculus hippocastanum), a deciduous dicotyledonous tree that can reach heights of 30 m or more.