Let us make in-depth study of the important parts of a flower.
In the evolution of the land plants one finds a progressive specialization of the reproductive structure until, in the flowering plants or Phanerogams, these reproductive structures or sporophylls are found to be borne in a specialized shoot called the flower.
The flower is the most conspicuous structure in the Angiosperms and the evolution of the Angiosperms means, mainly, the evolution of this flower structure. The importance of the flower lies in its connection with reproduction which is a very manifest property of life.
Parts of a Flower:
The different parts of a flowering shoot are best understood by referring to a typical flower.
Like any shoot, there is an axis of the flower called the thalamus. On this thalamus the floral members are arranged as the leaves are arranged on any stem. Just like the phyllotaxy of leaves, the arrangement of the floral leaves on the thalamus may also be spiral or cyclic.
A typical flower shows the cyclic or whorled type of phyllotaxy. Most common flowers show this cyclic arrangement in which the members of the consecutive whorls alternate with one another.
Lotus, Magnolia, etc., show the spiral arrangement and these families are sometimes considered as more primitive. In some flowers like the rose, the .floral phyllotaxy is partly spiral and partly cyclic when it is termed spirocyclic or hemicyclic.
A typical flower shows four sets of members arranged in four successive whorls on the thalamus:
(1) A calyx composed of sepals,
(2) A corolla of petals,
(3) An androecium of stamens and
(4) A gynoecium or pistil composed of carpels.
The two outermost whorls, calyx and corolla, serve for secondary purposes like protection of the flower and making it attractive. So, these two whorls are called the accessory whorls.
On the other hand, the stamens are the microsporophylls and the carpels are the mega- sporophylls which are the male and the female sex organs essential for reproduction. Hence these two whorls, i.e., the androecium and the gynoecium, are the essential whorls.
In some flowers the calyx and the corolla cannot be distinguished, there being only one set of these accessory members which can neither be termed calyx nor corolla. Then this accessory outermost whorl is termed perianth.
While the above is the general plan of a flower there is a large number of flowers which do not conform to this scheme.
When all the four sets of members are present in a flower it is termed complete; if not, incomplete. When both the essential whorls, i.e., stamens and carpels, are present in a flower it is called perfect which is also bisexual or hermaphrodite.
A flower missing either stamens or carpels is imperfect and also unisexual or diclinous.
Unisexual flowers are either staminate (male) or pistillate (female). If male and female flowers develop on the same plant as in Cucurbita maxima or maize, the plant is called monoecious. Some Compositae bear hermaphrodite and female flowers on the same plant and these are called gynomonoecious while Veratrum of Liliaceae shows the andromonoecious condition bearing both hermaphrodite and male flowers on the same plant.
If the male and the female plants are quite separate from one another, it is termed dioecious (also spelt dioicous) as seen in papaw (Carica papaya), fan palm (Borassus flabellifer), palwal (Trichosanthes dioica), etc. In thyme (Thymus of Labiatae) it is found that some plants are completely female while others bear hermaphrodite flowers. This condition is known as gynodioecism. Similarly, there are androdioecious species showing hermaphrodite plants and male plants.
If staminate, pistillate and hermaphrodite flowers develop on the same plant (as in mango or litchi) it is called polygamous. On the other hand, plants like Silene (Caryophyllaceae) show separate plants of three kinds —male, female and hermaphrodite, so that they are trioecious.
There is a number of families with flowers showing neither sepals nor petals. Flowers are formed only by stamens and carpels—either both or only one (unisexual). Such flowers are called naked or achlamydeous. A flower having only one accessory whorl (calyx or corolla or perianth) is called monochlamydeous or haplochlamydeous. Ordinary flowers with both calyx and corolla are dichlamydeous. Pea and rose are dichlamydeous.
Tuberose is monochlamydeous while plants belonging to the families Pipfraceae, Salicaceae, etc., are achlamydeous. A flower having no functional stamen or carpel is neuter or sterile as are some ray florets of many Compositae flowers.
In the typical flower illustrated, the floral members are proportionally arranged round the thalamus so that the flower divides into two equal and symmetrical halves if one cuts the flower into two -through any vertical plane passing through the axis.
Such a flower as seen in rose, china-rose, etc., is termed regular or actinomorphic. On the other hand, there are some flowers where members of the different whorls are not arranged so uniformly. Here, the flower can be cut into two equal halves only through one particular vertical plane. Such flowers, as those of pea, bean, Cassia, Ocimum, etc., are irregular and zygomorphic.
Some flowers are such that they cannot be cut into two symmetrical halves along any plane. Such a flower, as found in Canna, is termed irregular and asymmetrical.
In flowers, specially in irregular ones, it is seen that the flower is oriented on the stem in a definite way. The side of the flower which always faces the stem or the rachis is called the posterior or back whiles the side opposite, i.e., the side facing the subtending bract is the anterior or front.
The plane passing through the axis of the stem, the back and the front of the flower and the bract is called median or the antero-posterior plane while the plane at right angles to it is the lateral or transverse plane. Similarly, there may be two diagonal planes.
When the number of sepals, petals, stamens and carpels is uniform, i.e., the same or multiple of the same, the flower is termed isomerous. According to the number of floral members a flower may be bimerous, trimerous, tetramerous or pentamerous according as the number is a multiple of 2, 3, 4 or 5.
When the number of members of different whorls is irregular, then the flower is termed heteromerous. Trimery is common among monocotyledons and pentamery among dicotyledons. It should be noted, however, that these numbers often vary. This variation is discussed later.
A typical flower has one whorl (cycle) of each of calyx, corolla, androecium and gynoecium. It is, therefore, tetracyclic. But, some flowers have two whorls of androecium. It is then called pentacyclic. Some dicotyledons have two whorls of sepals and are also pentacyclic. If the number of whorls increases further, then it becomes polycyclic. Thus wild rose and Potentilla with several whorls of androecium and gynoecium are polycyclic.