Flower: Important Parts and its Anatomy

Flower: Important Parts and its Anatomy

Introduction to a Flower:

The flower consists of an axis, also known as receptacle and lateral appendages. The appendages are known as floral parts or floral organs. They are sterile and reproductive. The sepals and petals which constitute the calyx and corolla respectively are the sterile parts. The stamens and the carpels are the reproductive parts. The stamens compose the androecium, whereas the free or united carpels compose the gynoecium.

The vegetative shoot shows unlimited growth, whereas the flower shows the limited growth. In flower, the apical meristem ceases to be active after the formation of floral parts. In more specialized flowers there is a shorter growth period and they produce a small and more definite number of floral parts than the more primitive flowers.

In still more advance flowers there are specialized characters, such as, whorled arrangement of parts instead of spiral, adnation of parts of two or more different whorls, cohesion of parts within a whorl, zygomorphic instead of actinomorphic condition, and epigynous condition instead of hypogynous condition.

Important Parts of a Flower:

Sepals:

The sepals resemble leaves in their anatomy. Each sepal consists of ground parenchyma, a branched vascular system and an epidermis. The chloroplasts are found in the green sepals but usually there is no differentiation in the palisade and spongy parenchyma. They may contain crystal—containing cells, laticifers, tannin cells and other idioblasts. The epidermis of sepals may possess stomata and trichomes.

The traces are similar in origin and number. From the evidence of vascular system, sepals are clearly, in nearly every case morphologically bracts—that is, and they have been derived directly from leaves and are not sterile sporophylls.

Petals:

The petals also resemble leaves in their internal structure. They contain ground parenchyma, a more or less branched vascular system, and an epidermis. They may also contain crystal containing cells, tannin cells, laticifers and certain other idioblasts. They contain pigments—containing chromoplasts.

Very often, the epidermal cells of the petals contain volatile oils which emit the characteristic fragrance of the flowers. In certain flowers the anticlinal epidermal walls of the petals are wavy or internally ridged, whereas the outer walls may be convex or papillate. The epidermis may also possess stomata and trichomes.

Stamen:

Commonly the stamen consists of a two-lobed four-loculed anther. The anther is found to be situated on a slender filament which bears single vascular bundle. In certain primitive dicotyledonous families the stamens are leaf-like and possess three veins, whereas in advance types they are single-veined.

The structure of filament is simple. The vascular bundle is amphicribral and remains surrounded by parenchyma. The epidermis is cutinized and bears trichomes. The stomata may also be present on the epidermis of both anther and filament. The vascular bundle is found throughout the filament and culminates blindly in the connective tissue situated in between the two anther- lobes.

The outermost wall of the anther is the epidermis. Just beneath the epidermis there is endothecium which usually possesses strips or ridges of secondary wall material mainly on those walls which do not remain in contact with the epidermis. The innermost layer is composed of multinucleate cells; this is nutritive in function and known as tapetum.

The wall layers which are located in between the endothecium and tapetum are often destroyed during the development of the pollen sacs. On the maturation of the pollen the tapetum disintegrates and the outer wall of the pollen sac now consists of only the epidermis and endothecium. At the time of dehiscence of the anthers the pollen are released out through stomium.

Gynoecium:

The unit of gynoecium is called the carpel. A flower may possess one carpel or more than one. If two or more carpels are present they may be united or free from one another. When the carpels are united the gynoecium is known as syncarpous; when they are free the gynoecium is said to be apocarpous. A gynoecium with single carpel is also classified as apocarpous.

The apocarpous gynoecium is termed simple pistil, whereas the syncarpous gynoecium is termed compound pistil. The carpel is commonly interpreted as foliar structure. The carpel of an apocarpous or syncarpous gynoecium is being differentiated into the ovary and the style. The upper part of the style is differentiated as a stigma. The stigma is sessile.

The ovary consists of the ovary wall, the locule or locules and in a multilocular ovary, the partitions. The ovules are found to be situated on the inner or adaxial (ventral) side of the ovary wall. The ovule-bearing region forms the placenta. According to Puri (1952) the position of the placentae is related to the method of union of carpels.

In a carpel the placenta occurs close to the margin. Since there are two margins, the placenta is double in nature. The two halves may be united or separate. The number of double placenta in compound ovaries is equal to number of carpels. When the carpels are folded, the ovary is multilocular and the placentae occur in the centre of the ovary where the margins of the carpels meet.

This is axile placentation. When the partitions of the ovary disappear, it becomes free-central placentation. When the carpels are joined margin to margin and the placentae are found to be situated on the ovary placentation is parietal.

Most commonly the carpels has three veins, one dorsal or median and two ventral or lateral, and the vascular supply of the ovules has been derived from the ventral bundles. The vascular bundles of the ovary, possessing axile placentation appear in the center of the ovary, with the phloem turned inward and the xylem wall, the outward.

The ovary and style are composed of epidermis, ground tissue of parenchyma, and vascular bundles. The outer epidermis is cuticularized and may have stomata. The ovule consists of a nucellus which encircles the sporogenous tissue. There are two integuments of epidermal origin, and a stalk, funiculus. The ovule consists of parenchyma and contains a more or less dominant vascular system.

Vascular Anatomy of Floral Parts of Flower:

The study of the vascular anatomy has helped in solving many intricate problems of floral morphology. It has shown that many structures are not what they appear to be or what they are commonly taken to be. The fundamental vascular plan remains more or less unaltered and can always be of some help (Puri, 1952).

Morphologically the flower is a determined shoot with appendages, and these appendages are homologous with leaves. This commonly accepted view is sustained by the anatomy of the flowers. Flowers, in their vascular skeletons, differ in no essential way from leaf stems.

They are often more complex than most stems. Taxonomy and comparative morphology have in large measure determined the structural nature of the flower. Anatomy of the flower has aided in the solution of certain puzzling conditions.

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.

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.

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.

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.

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 (Fig. 44.6 B), or they may grow together laterally in a closed folded condition (Fig. 44.6 C), or they may become laterally united in an open folded condition (Fig. 44.6. A).

The junction of carpels in an open condition may result in a unilocular ovary showing parietal placentation as shown in fig. 44.6 A. 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 (Fig. 44.6 B, C).

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.