Angiosperms: Introduction, Characteristics and Structures

What is angiosperm?

• Within the kingdom Plantae, Angiosperms are the flowering plants and they are the most diverse group with largest number.
• Angiosperms make up nearly 80% of all recognized green plants that now live.
• Angiosperms dominate the surface and vegetation of the Earth in more ecosystems than any other group of plants, especially terrestrial habitats.
• Birds and mammals including humans rely on Angiosperms, as it is the ultimate source for the food.
• Furthermore, the most economically significant group of green plants are flowering plants, which serve as a source of pharmaceuticals, fiber products, timber, ornaments and other commercial products.
• Although the angiosperm taxonomy is still not well understood, a broad body of comparative data derived from DNA sequence studies is included in the new classification scheme.
• It is identified as the Angiosperm Phylogeny Group IV (APG IV) botanical classification system.
• Angiosperms is known to be a category called Anthophyta at the level of division (comparable to the phylum level in animal classification systems), however the APG system only identifies informal groups above the level of order.
• The number of forms found among angiosperms is larger than that of any other group of plants.
• The size ranges from the smallest individual flowering plant, possibly the watermeal (Wolffia) which is less than 2 millimeters (0.08 inch), to one of the largest angiosperms, Australia’s mountain ash tree (Eucalyptus regnans) at around 100 meters.
• Angiosperms of nearly every size and form lie between these two extremes.
• Succulent cacti (Cactaceae), delicate orchids (Orchidaceae), baobabs (Adansonia species; Malvaceae), vines, rosette plants (Asteraceae) and carnivorous plants such as sundews (Drosera; Droseraceae) and Venus flytrap(Dionaea muscipula) are examples of this variability.
• It is important to consider the basic structural plan of the angiosperms to understand this vast array of types.
• The fundamental forms of angiosperms are woody or herbaceous.
• Woody forms(generally trees and shrubs) are abundant in secondary tissues, while herbaceous forms (herbs) seldom have any.
• The herbs that complete their growing cycle within the same season are annuals.
• Cultivated garden plants, like maize (Zea mays; Poaceae), beans (Phaseolus and other genera; Fabaceae) and squashes (Cucurbita; Cucurbitaceae), along with wildflowers such as some butterflies (Ranunculus; Ranunculaceae) and poppies, are the examples of annuals.
• Biennials are also herbs, but their growing period, unlike annuals, lasts two years: during the first year, vegetative (non-reproductive) plant growth occurs from seed, and during the second, development of the flowers and fruit takes place.
• Well-known biennials include the beet (Beta vulgaris; Amaranthaceae) and the carrot (Daucus carota; Apiaceae).
• A perennial grows for several years and mostly flowers yearly.
• At the end of each growing season, the aerial parts of a perennial die back to the ground in temperate regions and new shoots from such subterranean parts as bulbs, rhizomes, corms, tubers, and stolons are produced in the following season.

Characteristics of Angiosperm:

• The sporophyte can be differentiated into stems, roots and leaves.
• At some point in Angiosperm’s life, all plants have flowers.
• The flowers are the plant’s reproductive organs, which provide them with a way to share genetic material.
• Angiosperms are vascular seed plants and the ovule is fertilized and develops into a seed within an enclosed hollow ovary.
• In Angiosperms, the ovary lies within the flower.
• Flower is the part of the angiosperms that comprises the male or female reproductive organs or ie. both the stamens (microsporophyll) and the carpels (megasporophyll) are arranged.
• Four microsporangia are present in each microsporophyll.
• The ovules are enclosed at the base of the megasporophyll in the ovary.
• Fruits are formed from the angiosperm plant’s maturing floral organs, and are thus typical of angiosperms.
• In contrast to non-vascular plants such as bryophytes, where each cells of the body is responsible for the functions essential for the support, nourishment and to expand plant body, angiosperms have developed specialized cells and tissues that perform these functions.
• In the xylem and companion cells in the phloem, the vascular system has real vessels.
• It comprises of extensive root systems that support the plant and is responsible for the absorption of water and minerals from the soil, stem that aids the growing plant, and leaves that are the major sites for the process of photosynthesis for majority of the angiosperms.
• The existence of localized plant growth regions called meristems and cambia, which extend the length and width of the plant body, respectively, is another important evolutionary development over the nonvascular and the more primitive vascular plants.
• These regions are the only places where mitotic cell division occurs in the plant body, except under some circumstances, although cell differentiation tends to occur throughout the life of the plant.
• Angiosperms are heterosporous, producing two kinds of spores, microspores (grains of pollen) and megaspores.
• Within the nucellus, a single functional megaspore is permanently retained.
• Transfer of the pollen grains from the anther to the stigma and reproduction takes place by pollination.
• They account for the transmission of genetic information from one flower to the other.
• The fertilization process in angiosperm is faster.
• Because of the smaller female reproductive parts, the seeds are also produced quickly.
• The pollen grains carrying the inherited information are produced by them.
• The developing seeds are enclosed by carpel, that may turn into a fruit.
• One of the main benefits of angiosperm is the production of endosperm.
• After fertilization, the endosperm is formed and is a source of food for seed and seedling growth.
• Angiosperms in a number of environments, including marine habitats, may survive.

Structure of Angiosperm:

• There are three parts to the basic angiosperm body: roots, stems and leaves.
• The vegetative (nonreproductive) plant body is constituted by these primary organs.
• The stem and its attached leaves, together, constitute the shoot.
• Jointly, the roots of an individual plant constitute the root system and the shoots the shoot system.

1. Root systems of angiosperm:

• The roots anchor a plant, accumulate minerals and water, and provide a food storage area.
• A primary root system and an adventitious root system are the two basic forms of root systems.
• Primary root system: The most popular form, the primary system, comprises of a taproot (primary root) that grows vertically downwards.
• Smaller lateral roots (secondary roots) that develop horizontally or diagonally are formed from the taproot.
• Such secondary roots also produce their own smaller lateral roots
• Thus, from a single prominent root, the taproot, many orders of roots of descending size are produced.
• Many dicotyledons generate taproots, such as the dandelion (Taraxacum officinale), for example.
• The taproot system is in some cases, modified into a fibrous or diffuse system in which the initial secondary roots are soon equal to or larger than the primary root.
• The outcome is many broad, positively geotropic roots that generate higher-order roots that can expand to the same size as well.
• Thus no well-defined single taproot exists in fibrous root systems.
• Fibrous root systems are typically shallower than taproot systems.
• Adventitious root system: The second root system type, the adventitious root system, varies from the primary variety in that the primary root is almost always short-lived and many roots that form from the stem substitutes them.
• There are adventitious roots in most monocotyledons; examples include orchids (Orchidaceae), bromeliads (Bromeliaceae), and many other tropical epiphytic plants.
• Grasses (Poaceae) and many other monocotyledons, with the development of adventitious roots, generate fibrous root systems.
• Adventitious roots, as in maize or some figs, are named prop roots when modified for aerial support.
• Wide woody prop roots grow from adventitious roots on horizontal branches in many tropical rainforest trees and provide additional anchorage and support.
• There are contractile adventitious roots in many bulbous plants that draw the bulb deeper into the soil as it grows.
• With specialized adventitious roots, climbing plants usually grasp their supports.
• Some lateral mangrove roots are specialized in saline mud flats as pneumatophores; pneumatophores are lateral roots that extend upwards (negative geotropism) for varying distances and act as the oxygen intake site for the submerged primary root system.
• For special functions, many primary root and adventitious root systems have been modified, the most common being the development of tuberous (fleshy) roots for food storage.
• For instance, carrots and beets are tuberous roots modified from taproots, and a tuberous root modified from an adventitious root is cassava (manioc).

2. Stem of angiosperm:

• The stem is an aerial axis of the plant bearing leaves and flowers.
•  It transports water and minerals from the roots and food from the site of synthesis to areas where it is to be used.
• Via a transition region called the hypocotyl, the main stem of a plant is continuous with the root system.
• The embryonic axis that bears the seedling leaves is the hypocotyl in the developing embryo.
• In a maturing stem, a node is called the area where a leaf connects to the stem, and an internode is called the region between successive nodes.
• At the nodes, stems bear leafy shoots (branches) that grow from buds.
• Lateral branches arise from buds located in the region between the leaf and the stem, either axillary or lateral, or from terminal buds at the end of the shoot.
• These buds have extended periods of dormancy in temperate-climate plants, while the duration of dormancy is either very short or non-existent in tropical plants.
• In order to understand the diversity of the shoot system in angiosperms, the precise positional relationship of the stem, leaf, and axillary bud is significant.
• Branching can be dichotomous or axillary in angiosperms.
• As a result of an equal division of a terminal bud (i.e. a bud developed at the apex of a stem), the branches develop in dichotomous branching .
• It is divided into two equal branches that are not derived from axillary buds, even if axillary buds are available elsewhere on the plant body.
• Some cacti, palms (Arecaceae), and bird-of-paradise plants are the few examples of dichotomous branching between angiosperms.
• The two angiosperm axillary branching modes are monopodial and sympodial.
• As the terminal bud begins to develop as a central leader shoot, monopodial branching occurs and the lateral buds remain subordinate, such as beech trees (Fagus, Fagaceae).
• Sympodial branching takes place when the terminal bud stops to develop (usually because a terminal flower has formed) and when an axillary bud or buds become new leader shoots, called renewal shoots, such as the Joshua tree (Yucca brevifolia, Asparagaceae).
• In general, plants with monopodial growth are pyramidal in shape, whereas those with sympodial growth mostly resemble a candelabra.
• By integrating monopodial and sympodial branching in one plant, several different tree configurations have developed.
• Example: In dogwoods (Cornus), the main axis is monopodial and the lateral branches are sympodial.
• By simply adjusting the length of the internodes, several different plant forms are formed.
• Excessive shortening of the internodes tends to result in rosette plants, such as lettuce (Lactuca sativa; Asteraceae), in which the leaves grow but the internodes between them do not elongate till the plant “bolts” while flowering.
• Excessive internode lengthening also leads to twining vines, as in the yam, (Dioscorea esculenta).

3. Leaves of angiosperm:

• A leaf base, two stipules, a petiole, and a blade compose the basic angiosperm leaf.
• The slightly enlarged area where the leaf attaches to the stem is the leaf base.
• When present, the paired stipules are situated on either side of the base of the leaf and may mimic scales, spines, glands, or structures that are leaflike.
• A stalk which connects the blade with the base of the leaf is the petiole.
• The main photosynthetic surface of the plant is the blade and it appears to be  green and flattened in a plane perpendicular to the stem.
• The leaf is considered simple when only a single blade is inserted directly onto the petiole.
• Simple leaves can be lobed along their margins in different ways.
• Simple leaves’ margins may be entire and smooth or they may be lobed in different ways.
• The rough teeth of dentate margins project at 90^o, while those of serrate margins point toward the leaf apex.
• Crenulate margins possess rounded teeth or scalloped margins.
• In one of two patterns, pinnate or palmate, the leaf margins of simple leaves can be lobed.
• The lamina i.e.the leaf blade is indented equally deep along each side of the midrib in the pinnately lobed margins (as in the white oak, Quercus alba; Fagaceae).
• The lamina is indented along many major veins in the palmately lobed margins(as in the red maple).
• A wide range of base and apex shapes are also found.
• A blade has two or more subunits called leaflets in compound leaves.
• The leaflets ramifies from the single point at the distal end of the petiole in palmately compound leaves.
• A row of leaflets are formed on the either side of the extension of the petiole termed as rachis in pinnately compound leaves.
• Some pinnately compound branch again forming a second set of pinnately compound leaflet.
• In bipinnately or tripinnately compound leaves, the high compoundness makes them appear to be shoot systems.
• However, they can still be differentiated since axillary buds are located at the angle between the stem and the petiole (axil) of pinnate or palmate compound leaves, but not in the leaflet axils.
• Alternate, opposite (paired), and whorled are the three patterns of leaf arrangement on stems in angiosperm.
• The leaves are present as single at each node in alternate-leaved plants, and are found along the stem alternately in an ascending spiral.
• The leaves are paired at a node and placed opposite to each other in opposite-leaved plants.
• When there are three or more evenly spaced leaves at a node, a plant has whorled leaves.