Flowers

Flowers are the sites of reproduction in angiosperms.

Flowers are composed of 4 sets of specialized appendages (which are grouped in whorls):

A. sepals - the outermost whorl. Leafy structures that protect and cover the unopened flower bud. They are usually green and photosynthetic. The collective term for all the sepals of a flower is calyx.

B. petals - the whorl of flower parts just inside the sepals. Petals are often brightly colored and conspicuous. Their primary function is to attract animal pollinators. The collective term for all the petals of a flower is the corolla.

Together, the calyx and corolla comprise the perianth.

C. androecium - the whorl of male structures usually just inside the corolla. The androecium (male house) is composed of the stamens, each of which consists of the pollen-bearing anthers, supported by a relatively long filament (stalk).

Monadelphous stamens – filaments are fused together forming a tube, but the anthers at top are free. Typical of the mallow family: cotton, hibiscus, etc.

Diadelphous – 9 stamens fused together, 1 stamen free. Typical of legumes.

Didynamous – having one pair of stmens longer than the other pair; e.g. minty family

Tetradynamous – having 4 long stamens and two shorter one; typical of the mustard family (Brassicaceae).

Synantherous – anthers are fused together, filaments are free. Typical of Asteraceae

(sunflower family)

D. gynoecium - the collective term for the female reproductive structures, and located in the middle of the flower (female house), comprised of ovary (basal, enlarged part), style (elongated), and stigma.

Flowers can have 1-many carpels (the ovule-bearing portion of a flower).

A simple pistil is a gynoecium with 1 carpel.

Many carpels fused together in a flower is referred to as a compound pistil.

The first pistil was a leaf that was somehow modified to a pistil through evolution. The margin of the leaf may have in-rolled, fusing two margins forming a cylindrical figure. The first seeds borne on the pistil were formed near the margins. When rolled, it looks as if they are borne in a single row. This is called a carpel.

Ovary - enlarged, basal portion of the carpels or pistil.

Ovules - structures that become seeds at maturity, are housed within the ovary.

Style - a slender column that rises from the top of the ovary, bearing a stigma.

Stigma - the expanded tip of the style that is functionally receptive to the pollen. Often

. times sticky.

Position of the ovary

1. Superior ovary (hypogynous) – floral parts are below the level of the ovary.

2. Inferior ovary (epigynous) – floral parts are fused onto ovary and above the ovary. Pistil can’t be lifted out without tearing the hypanthial tissue.

3. Perigynous condition – is where the floral parts are around, but not fused to, the ovary The ovary is still considered superior because the other floral parts have their derivation below the ovary. The pistil here may be lifted out without tissue damage.

Bracts - found outside the calyx. They may be leaflike or petal-like. The red in poinsettias or Indian paintbrush or bougainvilleas are really bracts, not petals.

Complete/Incomplete and Perfect/Imperfect flowers

1. Complete flowers - Flowers that have all 4 appendages - sepals, petals, androecium, and

gynoecium. (E.g. geranium, evening-primrose).

2. Incomplete flowers - Flowers lacking any appendages are said to be incomplete. (E.g. grasses,

pine).

3. Perfect flowers - Flowers bearing both androecium and gynoecium are said to be perfect. (E.g.

rose, evening primrose)

4. Imperfect flowers - Flowers lacking either reproductive structure are said to be imperfect.

(E.g. saltbush, Cannabis/hemp).

Flowers can, therefore, be incomplete but still perfect (if they are only lacking petals).

However, flowers that are imperfect must also be incomplete. (Completion is the more inclusive term).

1. Monoecious flowers – imperfect and incomplete flowers that have both male and female flowers on the same plant, but separated in different flowers. Flowers are unisexual. E.g. pines (Pinus), cucumbers (Curcubita), corn (Zea mays).

2. Dioecious flowers – imperfect and incomplete flowers that have only male (staminate) or female (pistillate) flowers on a plant. The flowers and plant are said to be unisexual. E.g. saltbush (Atriplex), Mormon tea (Ephedra).

Monocots - generally have their floral parts in 3s, or multiples of 3s. Lilies have 3 sepals, 3 petals, 6 stamens, and a 3-part ovary (fusion of 3 carpels).

Dicots - generally have a numerical plan of 4s or 5s, or multiples thereof. Geraniums have 5 sepals, 5 petals, 10 stamens, and a compound ovary of 5 carpels with 5 separate stigmas.

Position of ovary

The position of the ovary, in relation to the other floral parts of a flower, may be important in the classification of flowers. It is certainly a good diagnostic feature of certain plants.

1. Superior ovary - Where the sepals, petals, and stamens are inserted beneath the ovary, the arrangement is referred to as a superior ovary (hypogynous condition) E.g. geraniums, petunias, yucca.

2. Inferior ovary - Where the sepals, petals, and stamens are inserted above the ovary, the ovary is said to be inferior (epigynous ). E.g. evening-primrose.

3. Perigynous ovary - Where the bases of the floral parts are fused into a cuplike structure surrounding the ovary, the condition is referred to as perigynous]. E.g. some roses and members of the rose family.

Floral patterns of symmetry

1. Regular flowers (actinomorphic, radial) display a radial symmetry; i.e. they can be dissected into mirror images of themselves along any line. E.g. tulips, daffodils,evening-primroses.

2. Irregular flowers (zygomorphic, bilateral) can only be dissected into identical halves along one line. E.g. orchids, pea flowers, snapdragons, rosemary.

Inflorescences

Flowers are often borne in clusters, either at the terminal end of branches, or in the axils of branches. These clusters are referred to as inflorescences [lit: to begin to blossom]. The arrangement of flowers within the cluster determines the type of inflorescence

spike - an unbranched, elongated inflorescence of sessile or subsessile flowers, maturing from the bottom upwards.

raceme - an unbranched, elongated inflorescence of pedicilate flowers, maturing from the bottom upwards.

panicle - a branched inflorescence with flowers maturing from the bottom upwards.

umbel - a flat-topped inflorescence with the pedicels arising from a more or less common point, like the struts of an umbrella. Carrot, parsnip, St. Anne’s lace.

head - a dense cluster of sessile flowers, borne on a receptacle, like those of the sunflower family.

Pollination - refers to the transfer of pollen from the anther to the stigma.

Self-pollination - from the anther to the stigma of the same flower or a flower on the same plant.

Outcrossing (cross-pollination) - transfer of pollen from one plant to another.

Pollination can be accomplished in a number of ways: in all ways, however, pollination is carried out inadvertently by the pollinator, when the insect, bird, or bat is dusted with the pollen of one plant and then visits a flower of another plant.

entomophilous - insect pollinated; flowers generally yellows and blues (color), and often fragrant

Insects see best in that range of visible light, and have highly developed sense of smell..

ornithophilous - bird pollinated – flowers in reds and oranges mostly; little fragrance.

anemophilous - wind pollinated – flowers generally small, inconspicuous (grasses), incomplete, with exserted (protruding from the corolla) stamens and feathery stigmas, with inflorescences of spikes or catkins. (It is the anemophilous plants that are most often the offending culprit to those with hay fever and allergies).

Co-evolution – mutual adaptation in which two different organisms interact so closely that they become increasingly adapted to one another.

Flowers have also, thru evolving morphological changes, facilitated pollination by attracting very specific potential pollinators by the shape of the corolla. You’ve seen the long, trumpet-like flowers that hummingbirds feed on, or how the irregular flowers, as in the legume (pea) family, provide a landing platform for insects that cannot hover, to work their way into the flower. In fact, orchids, which comprise the largest family of vascular plants, has evolved an enormous array of floral shapes by their continual association with specific pollinators (mutualistic = relationship between two organisms of different species that benefits both organisms). A mutualism that periodically adjusts itself, in a sequential manner, over time, is referred to as co-evolution - reciprocal adaptations - (although co-evolution needn’t necessarily be mutually advantageous).

So, in reference to entomophily and ornithophily, the shape and fragrance of a flower is a plant’s way of luring a visitor to inadvertently carry away its pollen.

Pollination is a prerequisite for fertilization, without which seeds cannot be produced in most species. However, pollination is a random process that frequently results in pollen of the wrong species landing on a stigma. Instead of this resulting in a cross or mating between a maple and an elm, e.g., there are a variety of mechanisms preventing the development of the pollen grain in this foreign environment.

Altho many plants are self-fertile, a major contributing factor for the introduction of variation and subsequent adaptability of plants to changing environmental conditions is cross pollination. It may be advantageous for a species to have mechanisms that enhance the chances of cross pollination (and diminishes the chances of self-pollination).

Self-incompatability - prevents self-pollination by the inhibition of pollen tube formation if pollen from an individual lands on the stigma of flowers of the same individual or even of different but genetically similar individuals. This happens in a number of angiosperms: apples, pears, plums, e.g.

Protandry ( protandrous) - when anthers of one plant releases its pollen before the stigma of the same flower is receptive.

Protogyny (protogynous) - when the stigma of one flower is receptive before the anthers of the same flower releases its pollen.

Dioecious condition – imperfect flowers; staminate and pistillate flowers are separated from each other, on separate plants.

The pollen grain is a highly efficient structure for the long-distance transfer of the male gametes (sex cell). Wind, insects, or other animals.

The pollen grain provides a mechanism to transport the sperm directly to the egg by means of the pollen tube without any need for water. These features have enabled the seed plants - angiosperms and gymnosperms - to attain a much wider geographic distribution than all other vascular plants.

Fertilization

Once a pollen grain lands on a receptive and compatible stigma, the pollen grain can germinate (start developing). A pollen grain contains two cells. One generates two male gametes (sperm cells), and the other produces a pollen tube through which the sperm cells will reach the ovule.

Once inside the ovule, the pollen tube delivers two sperm cells. One enters the ovule’s egg cell, where the nuclei of the two cells fuse (fertilize), eventually producing a zygote, and then, embryo.

The second sperm cell delivers its nucleus to a central cell, comprised of two polar nuclei. The result of this union is unique to flowering plants: the endosperm (embryonic nutritive material), triploid.

1. endosperm is rich in fats, proteins, and carbohydrates.

2. it supplies nutrients to the embryo and prepares it for germination and early survival.

3. it allows seeds to weather inclement climatic conditions in a state of dormancy, before germinating.

4. for humans, by eating the seeds of corn, rice, wheat before they have had the chance to germinate, we are feeding on the nutrients stored in the endosperm of the seeds.

This double fertilization is a distinctive feature of angiosperm reproduction.

Vegetative reproduction – gives rise to new individuals that are genetically identical to the parent, and perpetuates gene combinations that are well-adapted to the local environment. Individuals with favorable genetic combinations quickly spread throughout the immediate area where the parent plant is growing.

Sexual reproduction – a more complex sequence of events including the production and growth of a group of structures making up the flower; production and fertilization of gametes; development of embryo, seed, and fruit.

Two main advantages of sexual reproduction:

1. new genetic combinations in each generation

2. production of seeds, which can disperse over a wide area and which are protected against adverse environmental conditions that might kill the parent plant.