Floral anatomy: general account
Floral anatomy:-
Introduction:- Flower is the reproductive part of the plant in which fruits and seeds develop. From morphological point of view flower is a highly condensed and modified shoot meant for reproduction.
In a typical flower there are four types of floral leaves. These are arranged in four whorls. Floral leaves are arranged on the swollen upper parts of flower stalk. The flower stalk is called pedicel and the swollen upper part is called Thalamus (=torus). In the thalamus are present nodes and highly reduced internodes. Floral leaves are borne on these nodes in whorls.
Floral Parts:- The four floral whorls taken from periphery to centre are as follows:
1. Calyx:- It is the outermost whorl and floral leaves of this whorl are called sepals. Their function is that of protection of inner whorls and also that of photosynthesis when green.
2. Corolla:- It is the second whorl of floral leaves which are called petals. These protect the inner two whorls and also function for attraction of insects for pollination.
Note:- Calyx and corolla are called accessory whorls of the flower. In some plants although two whorls are present but they are not differentiated from one another. In such a case both whorls are called perianth and each part of the perianth is called tepal.
3. Androecium:- This is third whorl of floral leaves and is the male reproductive organ of the flower. Each modified floral leaf is called microsporophyll or stamen.
4. Gynoecium (=Pistil):- This is the last (fourth) central whorl of floral leaves and is the female reproductive organ of the flower. Each modified floral leaf is called megasporophyll or carpel. Gynoecium is made up of one or more megasporophylls (carpels). Androecium and gynoecium are called essential whorls of the flower.
Anatomy of Floral Parts of Flower:-
1. Pedicel:-
> The Pedicel and the receptacle have typical structure, with a normal vascular cylinder. The cylinder may be unbroken or it may contain a ring of vascular bundles. In the region where floral organs are borne, the pedicel expands into the receptacle.
> The vascular cylinder also expands and the vascular bundles increase somewhat in number, and finally traces begin to diverge. In the simplest cases vascular traces for different organs and whorls of organs arise quite independently (e.g., in Aquilegia). In other cases various degrees of fusion may take place between bundles situated more or less in the same sectors.
> The appendage traces are derived from the receptacular stele exactly as leaf traces are derived in typical stems. When the floral organs are numerous and closely placed the gap of traces break the receptacular stele into a meshwork.
2. Sepals:-
> The sepals are with very few exceptions, anatomically like the leaves of the plant in question. A sepal usually receives three traces derived from the same or different sources. As regards the morphological nature of the sepals, they have often been considered as equivalent to bracts and foliage leaves.
> Such a view is born out by a study of vascular anatomy which reveals practically the same vascular pattern as exists in foliage leaves and bracts of the same plant.
3. Petals:-
> In their vascular supply the petals are sometimes leaf like, but much more often they are like stamens. The petals may have one, three or several traces. Very commonly there is but one trace. The petals appear to be sometimes modified leaves, like the sepals, but in the great majority of families they are sterile stamens.
> However, since stamens are the homologous of the leaves, it is not always possible to determine from anatomical evidence along whether one trace petals in certain families are modified stamens or whether they have come more directly from leaf-like structures.
4. Stamens:-
> A stamen generally receives a single trace which remains almost un-branched throughout its course in the filament. In the anther region it may undergo some branching. In a few Ranalian families and rarely elsewhere as in some members of the Lauraceae and Musaceae, three traces are present in each stamen. In Ravenala (Musaceae) each filament is traversed by 25 to 28 small vascular bundles.
> Most of these disappear as the anther is approached, and the system of central bundles consisting of three or four bundles, continues into the connective. From other evidence the above mentioned families appear to be fairly primitive, it seems highly probable that the single trace condition is one of reduction from three.
> In the simple flower of Aquilegia the stamens traces pass off, one to each organ in several whorls. Above the supermost whorl of stamens the vascular cylinder becomes complete again.
5. Carpels:-
> The carpel is commonly looked upon as a leaf-like organ folded upward, i.e., ventrally with its margins more or less completely fused and bearing the ovules. This conception has been supported by the anatomy. The details of origin, number and course of the bundles forming the vascular supply are exactly like those of leaves the carpel has one, three, five or several traces.
> The three trace carpel is most common. The five-trace carpel is nearly as common as the three traces, and carpels with seven, nine and more traces are increasingly less and less common. The evidence that the one-trace carpel (nearly always an achene) has been derived by reduction from the three-trace type.
> The median trace which leaves the stele below the other carpel traces is known as the dorsal trace because it becomes the dorsal (midrib) bundle of the folded organ. The outermost traces are known as ventral or marginal traces because they become the bundles that run along the ventral edge of the carpel, i.e., along or near the margins of the organ if it were unfolded.
> The upward and inward folding of the sides of the carpel brings about the inversion of these ventral bundles. The phloem remains on the ventral side in the carpel, whereas it is on the dorsal side in the midrib (dorsal) bundle. This important condition may be easily understood when it is remembered that the carpel is leaf-like, with its margins folded upward. The ovule traces are derived from the ventral bundles.
> When floral parts are fused, the vascular bundles of these parts may also be fused. If carpels are united, the lateral bundles, either those of the same carpel or those of two adjacent carpel, may be fused in pairs.
> The fusion in the vascular tissue of a carpel may be present in the ventral bundles from an origin as one trace throughout their length, or may exist only in part of the carpel; where the ventral bundles arise as separate traces, they may unite at any point in their course.
> In syncarpy there are fusion changes similar to those in free carpels. The lines that separate the carpels and their margins have been disorganised. The inverted ventral bundles form a ring of bundles in the centre. These bundles usually lie in pairs. Here each pair consists of the ventral bundles, of the same carpel, or more often of bundles from each of two adjacent carpels.
> In the centre of a three carpellary syncarpous ovary there may be a ring of six or three ventral bundles. If the ring consists of three bundles, each bundle is morphologically double and represents either the two ventral traces of one carpel or one from one carpel and one from the adjacent carpel.
> Several workers proposed that the evolutionary changes in the structure of the gynoecium of the flower of angiosperms involve various manners of union of carpels of the same flower.
> In such angiospermous flower the carpel may become joined by their margins to the receptacle, or they may grow together laterally in a closed folded condition, or they may become laterally united in an open folded condition.
> The junction of carpels in an open condition may result in a unilocular ovary showing parietal placentation. Folding combined with union of carpels with each other may form an ovary with as many locules as there are carpels. In such cases the ovules are borne on the central column of tissue where the carpels come together showing, axile placentation.
> The Inferior Ovary:-
- The inferior ovary is formed by the adnation of the sepals, petals and stamens to the carpels or by the sinking of the gynoecium in a hollowed receptacle with fusion of the receptacle walls about the carpels. The vascular system is thought to show this structure in that the bundles found in the appendages of different whorls are variously fused but all show the usual orientation of xylem and phloem.
- In certain flowers with inferior ovary (e.g., Calycanthaceae, Santalaceae and Juglandaceae) there is evidence that the ovary is partially enclosed in hollowed receptacle. Here the vascular bundles are prolonged from the axis to the level below the insertion of floral parts, other than the carpels, where traces to the parts diverge.
- The main bundles continue farther from the periphery in a downward direction with a corresponding inversely oriented position of the xylem and the phloem. These bundles at lower levels give branches to the carpels. This type of orientation of the vascular system is thought to be the result of the invagination of the receptacular axis.