Flowers come in many different shapes, sizes and colours, from huge flowers like hibiscus to the tiny flowersof gentiana. Many are scented, otrhers have no scent. Some are a brightly colored whilst others are duller. Whatever their size or form, all flowers are for one purpose – sex- and their anatomy is geared towards achieving this.
Any flower, big or small, colored or dull, is a mass of sterile and fertile tissues. The sterile parts include the sepals and the petals. The sepals create the calyx and the petals the corolla. In many flowers these are not separate and in these cases the segments are called tepals.
The fertile parts include the stamens and carpels. The stamens together (there are usually more than one) create the androecium and the carpels the gynoecium. Flowers can be regarded as modified shoots but while vegetative shoots have limitless growth in theory, flowers grow to a set size, shape and form according to species. Each flower can be ‘perfect’ with male and female sex parts of ‘imperfect’ with just female or male parts, in which case they are unisexual.
The petals and sepals are silimar to leaves internally. They also posess specialised cells which may contain crystals and tannins.The colour of petals comes from pigments in cells called chromoplasts and from the cell sap. The pigments in the flower parts have evolved in response to the need to attract pollinators. In some flowers, the petals contain cells which contain flavinol glucosides that absorb UV light. These are visible to insects and form patterns which make a ‘nectar guide’ to lead the insect to the nectaries. In some flowers, cells in the epidermis contain specialised oils which give scent to the flowers.
The male stamens which produce male gametes are born on filaments and a stamen can could be described as an anther which is divided into several pollen sacs. Each pollen sac has a cavity in which pollen grains are produced. The filament is simple in structure with a single central vascular bundle (xyleman dphloem) stopping just under the anther.
The carpel or female reproductive part usually consists of the ovary, style and stigma. A flower may have just one or several and they may be separate or joined. The top of the style, which is a stalk, has special cells where pollen germinates and this is the stigma. In each ovary there is the ovary wall, cavity or cavities and, if there is more than one cavity, the partition. The ovules themselves develop from parts of the ovary wall called placenta. When a flower is open the ovary will not be developed fully but starts to develop and is fully developed in the fruit.
The style is solid and has a vein containing canal in the middle. the stigma itself may have chemicals on its surface or it may not, depending on the type of flower. These chemicals provide the best environment for germination of pollen grains from flowers of the same species , and may inhibit germination of grains from different species. It is the receptive surface for pollen grains. There may or may not be hairs or papillae on the epidermis of the stigma. The stigma tissue is connected to the ovary by a specialised tissue called transmitting tissue which acts as a path for the pollen tube after pollination.
Flowers are colorful, attractive, scented and beautiful but their anatomy, though attractive to us, has evolved with one purpose in mind- get pollinated, reproduce and continue the species.