The below mentioned article provides an overview on the gynoecium of a flower.
Gynoecium:
The gynoecium (also spelt gynaeceum) or pistil is the central or the topmost whorl of the flower usually terminating the thalamus. It is composed of one or more carpels or megasporophylls.
When there is a single carpel the pistil is called simple or monocarpellary which is not very common although it is a characteristic of the large families of Leguminosae and Gramaneae .
A typical carpel has three parts—ovary, style and stigma. The lowermost swollen part is the ovary containing one or more swollen bodies called ovules which are the rudiments of seeds.
Above the ovary the carpel is protruded into a long or short style which ends in a somewhat rounded and usually sticky stigma on which the pollens are deposited during pollinatior. A sterile pistil devoid of fertile ovules is called a pistillode.
Compound or polycarpellary gynoeciums are much more common than the simple type. In such a gynoecium, the different carpels may remain completely free from one another when it is termed apocarpous (apocarpous multiple, as opposed to simple, as there are multiple carpels) or the carpels may unite with each other, wholly or partially, forming syncarpous gynoeciums.
Apocarpous gynoeciums may be seen in the familie Annonaceae, Magnoliaceae, Ranunculaceae, Nymphaeaceae, Rosaceae, etc.
In Annonaceae and Magnoliaceae the carpels are arranged on a more or less elongated thalamus, in Nymphaeaceae they are sunk on the flat spongy top of the thalamus shaped like an inverted cone, while in Rosaceae they are arranged on the bottom of the cup-shaped thalamus.
The syncarpous gynoecium is much more common and involves union of different degrees, e.g., in Solatium the carpels are completely united ; in china-rose the ovaries are united to form a five-locular compound ovary, the styles are united completely while the five stiginas are free; in Linum usitatissimum of Linaceae and in the pink flower the ovaries are united but the styles and stigmas are free.
In Apocynaceae and Asclepiadaceae the two carpels have free ovaries and styles uniting only at the stigmas at the gynostegium. Rather interesting is the cafe of the union of the gynoeciums of two different flowers.
In Lonicera of Caprifoliaceae (honey-suckle or woodbine) the ovaries of two adjoining flowers may fuse while the other parts of the Sowers remain free. This is termed syngynia. Ovaries of banana, etc., fuse abnormally in a like manner giving rise to paired fruits.
Style and Stigma:
The style connects the ovary with the stigma and usually arises from the top or the summit of the ovary, i.e., it is apical. In some cases, however, the ovary apex itself may be deflected so that the style may appear to originate from near the base (basillar) or from the side (lateral) as in Alchemilla and mango .
In plants belonging to the Labia- tae the Ovary is peculiarly, four-lobed so that the ovary apex is depressed at the centre of the four lobes. As a result, the style appears to arise from the central base of the ovary and is termed gynobasic. In Gloriosa superba the style is placed at right angles to the ovary axis instead of being its direct prolongation.
The style is usually deciduous, dropping off after fertilization. But, in some cases, as in Naravelia zeylanica, Clematis, Digitalis, etc., it may be persistent . The style of Carina , Iris, etc., is petaloid. The base of the style in the family Umbelliferae is swollen forming what is known as the stylopodium.
The styles of a compound pistil may sometimes be completely united as in china-rose . But, even when united, anatomical study shows separate vascular supplies for the different carpels. Thus, a cross-section of the china-rose style shows five vascular traces corresponding to the five carpels. When the styles are free, usually there is one style for each carpel but sometimes the style may be branched as in some Euphorbiaceae .
The stigma is usually placed on the style. Sometimes, there may be no style, the stigma being placed on the top of the ovary as in Sambucus , Berberit, lotus, etc. Then it is termed sessile. The stigma top is usually rough, papillose or even hairy and somewhat sticky due to secretions. This shows a receptive surface where the pollens alight and germinate.
In a syncarpous ovary there may be separate stigmas as in china-rose or the stigma may be lobed when it is described as bifid (e.g., Compositae ), trifid, etc.
Usually, the number of lobes correspond to the number of carpels but, monocarpellary flowers of Graminaceae show bifid feathery stigmas . The stigma of poppies (Papaver) is sessile as well as striate showing a star like radiate appearance .
The Begonia stigma is highly branched. The three stigmas of Crocus have peculiar funnel-shaped forms. These are the stigmas which, when dried, form the saffron (zafran) of commerce.
Ovary:
Ovary is the most important part of the carpel as it contains the ovules which develop into seeds. A carpel without a functional ovary is sterile. The foliar origin of the ovary is rather clear in the simple ovary of pea .
It is even clearer in the inflated ovary of its relative Colutea arborescens . One may easily realise that a leaf like carpel, i.e., a megasporophyll as seen in the Gymnosperms, is folded about its midrib and forms a chamber by the fusion of the margins.
There is a special tissue called placenta along the margin and the marginal line along which the carpel fuses is called the ventral suture, the midrib being the dorsal suture . Ovules develop from this placental tissue and remain within the ovary chamber.
When more than one carpel form the ovary of a compound gynoecium, the different carpels may fuse along the margins giving the appearance of a one- chambered overy .
Another way of joining of the carpels may be as shown in . In this latter case each carpel first fuses along its own margin and then the three carpels fuse so that the ventral sutures all meet together at the axis of the ovary which latter is clearly divided into as many loculi or chambers as there are carpels.
The ovaries of syncarpous pistils are frequently formed as in the second case. But, sometimes the number of chambers does not correspond to the number of carpels due to the development of false partition walls or septa within the ovary chamber.
Thus, many bicarpellary flowers of Solanaccac (Datura , brinjal, etc.) show four-chambered ovaries. Such false partition walls usually develop as outgrowths of the placenta.
Placentation:
It has already been seen that the placental tissue develops along the margin of the megasporophyll so that, when the latter closes to form a chamber, the placenta is located along the ventral suture.
But, ma-gin is not the only place where placenta develops. Placenta may also develop on a direct prolongation of the thalamus at the base of the carpel. As a result, we get different types of placentation, i.e., distribution of placenta, in different ovaries.
As this placenta is the tissue on which the ovules or the future seeds develop, a study of placentation is of importance in the study of the fruit and the flower.
The following types of placentation are usually met with:
(1) Marginal:
This is the placentation of the simple ovary of pea (Leguminosae). The pea ovary is developed by the union of one megasporephyll along the ventral suture. The placenta forms a ridge along this suture and the ovules are borne on this forming two rows. The ovary is one-chambered and superior .
(2) Parietal:
When two or more carpels unite along the ventral sutures as shown in , the ovules also are located on the placenta along these sutures. The ovary is unilocular as may be seen in papaw. But, in Cruciferae showing this type of placentation, the ovary becomes falsely two-chambered by the development of a false partition wall called replum.
Ovaries of many Cucurbitaceous plants (also parietal) become falsely trilocular, or even hexalocular, because of later projections of the placental tissue .
(3) Axile:
As shown in a number of carpels independently form chambers as in. the marginal type, i.e., folding inwards and then fusing together so that the ventral sutures are all placed along the axis.
This is termed axile as all the placenta and therefore the ovules are axial. The ovary is divided into as many chambers as there are carpels. This is found in Solanaceae, Malvaceae, Rutaceae, Liliaceae, etc.
But, here also the number of chambers may increase by the development of false partition walls .
(4) Basal:
In Compositae, a single ovule is developed at the base of the unilocular simple ovary. The placenta is apparently placed on the tip of the thalamus at the floor of the ovary .
(5) Free-central:
In this type the ovules are found to develop on an axial column which is not connected with the ovary wall .
Such an ovary may arise in two different ways:
(i) In a placentation which was originally axile, the partition walls may break down at a later stage making the ovary falsely one-chambered. This is the case in the family Caryophyllaceae (pink flower, etc.).
(ii) The thalamus may slightly extend into the ovary and placenta may develop on it as in the family Primulaceae .
(6) Superficial:
The multicarpellary flower of Nymphaea (water-lily) develops multilocular ovaries in which the whole inner walls of the chambers are lined with placental tissue so that ovules develop all round.
This is really an overdevelopment of the axile type, the placenta spreading to the chamber walls from the axial column .
The Ovule:
The ovule is the megasporangium contained within the ovary. There may be one or more ovules inside an ovary and these are destined to be the seeds.
When fully formed, the tissues in a typical ovule are as follows: The ovule is attached to the placenta by the funicle or funiculus which meets the ovule at the hilum.
An ovule devoid of any funicle and directly attached to the placenta is termed sessile. Raphe is an extension of the funicle and may extend up to the chalaza which is the base of the ovule. (The ovule commonly remains reversed ).
The general tissue of the ovule is called nucellus and the embryosac (the female gametophyte within the megaspore—its development is described ) is in the top part of it.
The ovule is enveloped by two integuments (inner and outer) which leave an opening called micropyle at the top. Abnormally, there may be a single integument as in some Compositae while in some parasites like Loranthus, Viscum album, etc., there may be no integument at all.
The ovule is the anatropous or reversed type which is the commonest and may be seen in pea, bean, castor, etc. When the ovule remains straight , as in Polygonaceae, Piperaceae, Urticaceae, etc., it is called orthotropous or atropous.
It is amphitropous when placed transversely at right angles to the funicle . This condition is not common but is met with in Ranunculus, Lemna and poppy.
Sometimes, a transverse ovule may be bent like a horse-shoe so that the micropyle is brought nearer to the chalaza . This is the campylotropous (kampylos— curved) condition found in many plants of Cruciferae, Chenopodiaceae, Caryophyliaceae, etc., and some other plants like Mirabilis (Nyctaginaceae).
The position of the ovule in the ovary chamber (Fig. 386), specially in ovaries containing single ovules, may be (A) erect as in Compositae, (B) pendulous (hanging from top) as in some Annonaccae, (C) ascending (rising obliquely from a side of the ovary wall and not the direct base) as in some Ranunculaceae, (D) suspended (hanging from a side of the wall but no! the exact top) as in castor, or (E) horizontal as in Podophyllum.
