Angiosperm are better adapted to life on land than any other plants. After their appearance during the cretaceous period, 135 million years ago, they rapidly took over from conifers as the dominant land vegetation on a world scale and spread as different habitats were successfully exploited. Some angiosperm even return to fresh water, and a few to salt water.
One the most characteristic features of angiosperms apart from the enclosed seeds already mentioned is the presence of flowers instead of cones. This has enabled many of them to utilise insects, and occasionally birds or even bats, as agents of pollination.
In order to attract these animals, flowers are usually brightly coloured, scented and offer pollen or nectar as food. In some cases the flower have become indispensable to insects. The result is that, in some cases, the evolution of insects and flowering plants has become closely linked and there are many highly specialised, mutually dependent, relationships.
The flower generally becomes adapted to maximise the chances of pollen transfer by the insect and the process is therefore more reliable than wind pollination. Insect pollinated plants need not, therefore, produce as much pollen as wind pollinated plants. Nevertheless, many flowers are specialised for wind pollination.
Classification of angiosperms: Dicotyledons and monocotyledons
The angiosperms are divided into two major groups that are given the status of classes. The most commonly used names for the two groups are the monocotyledons and Dicotyledons, usually abbreviated as monocots 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 is produced 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 on turgidity 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, if present, it shows restricted activity. Many herbaceous plants are annuals, completing their life cycles from germination to seed production in one year.
Some produce organs of perennation such as bulbs, corns and tubers by means of which they overwinter or survive periods of adverse conditions such as drought. They may then be biennial, in which case they produce their seed and die in their second year, or perennial, in which case they survive from year to year. Shrubs and trees are perennial, and may be evergreen, producing and shedding leaves all year round so that leaves are always present, or deciduous, shedding leaves in seasons of cold or drought.
Characteristics of angiosperms
- Their seeds are covered inside the ovary of fruit
- Reduced gametophytic phase in its life cycle
- Ovules develop to form seeds
- Well-developed vascular tissues
- They are heterosporous forming two spores of different sizes and sexes; megaspore (female spore) and microspore (female spore)
- Consist of all flowering plants
- They are able to grow in a variety of habitats as trees, shrubs as well as herbs
- They are completely adapted to life on land, only few thrive in aquatic environment
- Their leaves are more or less large and generally have an expanded lamina and reticulated or parallel venation
- Presence of root hairs that help in better absorption of water and minerals from the soil
- Have flower, the organisation of microsporophyll (stamen) and megasporophyll (carpel)
- Various pollinating agents; wind, water insects
- Occurrence of double fertilisation which leads to the formation of zygote (2n; develop into sporophyte) and triploid endosperm cell (3n; produce the endosperm)
Growth forms in angiosperms
Angiosperms consist of a number of distinct growth forms ranging from large to discrete to minute halves. These growth forms 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:
- Phanerophytes: plants with persistent shoots and the buds are carried far above the soil surface e.g. tree plants such as orange, mango, Gmelina.
- Chamaephytes: have their resting buds closer to the soil surface e.g. small bushes such as shrubs and herbs
- Hemicryptophytes: have their resting buds on the soil surface e.g. creepy plants such as grasses
- Cryptophytes: these are also called geophytes, they have their resting buds below the soil surface e.g. Imperata cylindrical
- Therophytes: they are plants that have their resting buds in the form of embryo enclosed in seeds.
Based on the length of life of the whole plant
- Annuals: germinate, grow and reproduce within a growing season, some live for shorter duration (ephemerals). Ephemerals are plants that grow for short season, usually 3 to 4 weeks e.g Phylantus amarus.
- Biannuals: these are plants that complete their life cycle in two season i.e equivalent to 2 growing seasons. In the first year, they produce flowers and seeds e.g. onion plant, cabbage
- Perennials: grow for 3 or more years before they 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 differences between 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|