Phylum angiospermophyta – the flowering plants

Angiosperm are better adapted to life onland than any other plants. After their appearance during the cretaceousperiod, 135 million years ago, they rapidly took over from conifers as thedominant land vegetation on a world scale and spread as different habitats weresuccessfully exploited. Some angiosperm even return to fresh water, and a fewto salt water.

One the most characteristic features ofangiosperms apart from the enclosed seeds already mentioned is the presence offlowers instead of cones. This has enabled many of them to utilise insects, andoccasionally birds or even bats, as agents of pollination.

In order to attract these animals, flowersare usually brightly coloured, scented and offer pollen or nectar as food. Insome cases the flower have become indispensable to insects. The result is that,in some cases, the evolution of insects and flowering plants has become closelylinked and there are many highly specialised, mutually dependent, relationships.

The flower generally becomes adapted tomaximise the chances of pollen transfer by the insect and the process istherefore more reliable than wind pollination. Insect pollinated plants neednot, therefore, produce as much pollen as wind pollinated plants. Nevertheless,many flowers are specialised for wind pollination.

Classification of angiosperms: Dicotyledons andmonocotyledons

The angiosperms are divided into two majorgroups that are given the status of classes. The most commonly used names forthe two groups are the monocotyledons and Dicotyledons, usually abbreviated asmonocots and dicots.

Angiosperms may be herbaceous (non-woody) or woody.Woody plants become shrubs or trees. They grow large amounts of secondary xylem(wood) that offers support, as well as being a conducting tissue, and isproduced as a result of the cambium.This is a layer of cells found between the xylem and phloem in stems and roots.The cells retain the ability to divide. The new xylem produced is called secondary xylem or wood.

Herbaceous plants, or herbs, rely onturgidity and smaller quantities of mechanical tissues such as collenchyma,sclerenchyma and xylem for support, and they are consequently smaller plants.

They either lack a vascular cambium or, ifpresent, it shows restricted activity. Many herbaceous plants are annuals, completing their life cyclesfrom germination to seed production in one year.

Some produce organs of perennation such asbulbs, corns and tubers by means of which they overwinter or survive periods ofadverse conditions such as drought. They may then be biennial, in which case they produce their seed and die in theirsecond year, or perennial, in whichcase they survive from year to year. Shrubs and trees are perennial, and may beevergreen, producing and sheddingleaves all year round so that leaves are always present, or deciduous, shedding leaves in seasonsof cold or drought.

Characteristics of angiosperms

  • Their seeds are covered insidethe ovary of fruit
  • Reduced gametophytic phase inits life cycle
  • Ovules develop to form seeds
  • Well-developed vascular tissues
  • They are heterosporous formingtwo spores of different sizes and sexes; megaspore (female spore) andmicrospore (female spore)
  • Consist of all flowering plants
  • They are able to grow in avariety of habitats as trees, shrubs as well as herbs
  • They are completely adapted tolife on land, only few thrive in aquatic environment
  • Their leaves are more or lesslarge and generally have an expanded lamina and reticulated or parallelvenation
  • Presence of root hairs thathelp in better absorption of water and minerals from the soil
  • Have flower, the organisationof microsporophyll (stamen) and megasporophyll (carpel)
  • Various pollinating agents;wind, water insects
  • Occurrence of doublefertilisation which leads to the formation of  zygote (2n; develop into sporophyte) andtriploid endosperm cell (3n; produce the endosperm)

Growth forms in angiosperms

Angiosperms consist of a number of distinctgrowth forms ranging from large to discrete to minute halves. These growthforms is classified on the basis of; the length of life of the whole plant,and, the position & protection of the resting bud.

Based on the position and protection of the resting bud:

  1. Phanerophytes: plants with persistentshoots and the buds are carried far above the soil surface e.g. tree plantssuch as orange, mango, Gmelina.
  2. Chamaephytes: have their resting budscloser to the soil surface e.g. small bushes such as shrubs and herbs
  3. Hemicryptophytes: have their restingbuds on the soil surface e.g. creepy plants such as grasses
  4. Cryptophytes: these are also calledgeophytes, they have their resting buds below the soil surface e.g. Imperatacylindrical
  5. Therophytes: they are plantsthat have their resting buds in the form of embryo enclosed in seeds.

Based on the length of life of the whole plant

  1. Annuals: germinate, grow and reproducewithin a growing season, some live for shorter duration (ephemerals). Ephemerals are plants that grow forshort season, usually 3 to 4 weeks e.g Phylantusamarus.
  2. Biannuals: these are plants thatcomplete their life cycle in two season i.e equivalent to 2 growing seasons. Inthe first year, they produce flowers and seeds e.g. onion plant, cabbage
  3. Perennials: grow for 3 or more years beforethey mature, produce seeds. Subsequently, they flower on yearly basis,producing seed and can live for several years e.g. the fruit trees

Major Differences between Dicotyledons and Monocotyledons

The table below outlines the differencesbetween monocots and dicots.

  Class dicotyledoneae Class monocotyledoneae
Examples Pea, rose, buttercup, dandelion Grasses, iris, orchids, lilies
Leaf morphology Net-like pattern of veins (reticulate venation), Lamina (blade) and petiole (leaf stalk), Dorsal and ventral surfaces differ Veins are parallel (parallel venation), typically long and thin (grass-like), identical dorsal and ventral surfaces
Stem anatomy Ring of vascular bundles, vascular cambium usually present, giving to rise to secondary growth Vascular bundles scattered, vascular cambium usually absent,  so no secondary growth (exceptions occur, e.g. palms)
Root morphology Primary root (first root from seed) persists as tap root that develops lateral roots (secondary growth) Adventitious roots from the base of the stem take over from the primary root, giving rise to a fibrous root system
Seed morphology Embryo has two cotyledons (seed leaves) Embryo has one cotyledon
Root anatomy Few groups of xylem (2-8), vascular cambium often present, giving rise to secondary growth Many groups o xylem (commonly up to 30), vascular cambium usually absent, so no secondary growth
Flowers Parts mainly in fours and fives, usually distinct petals and sepals, often insect pollinated Parts usually in threes, no distinct petals and sepals. These structures are combined to form perianth segments, often wind pollinated

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