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In the below mentioned article, we will discuss about the insertion of floral leaves on thalamus.
Thalamus (also called torus) is the axis of the floral shoot which is the direct prolongation of the pedicel and bears four sets of floral members.
Usually, it is a slightly swollen knob-like structure but sometimes it may be somewhat elongated and conical bearing floral leaves spirally as in Artabotrys , Michelia , etc., or like an inverted cone with a spongy flat top as in lotus , or it may be slightly convex as in brinjal, etc. In perigynous and epigynous flowers the thalamus becomes flat or cup-shaped as explained under leaf insertion.
The thalamus of most flowers has three internodes which are very much condensed. But, in certain flowers these internodes are elongated. The first inter-node (between calyx and corolla) is called the anthophore (anthos=flower), the second internode (between corolla and androecium) is the androphore (also called gynandrophore) and the third internode (between androecium and gynoecium) is called the gynophore.
Cleome gynandra shows both androphore and gynophore (gynandrophore). Capparis sepiaria and Pterospermum acerifolium show gynophores only. Passion-flower (Passiflora suberosa of Passifloraceae) shows an androphore .
The thalamus is terminated by the pistil except in cases of monstrous development. But, in the family Umbelliferae the thalamus normally projects into the ovary and the carpels remain attached to it separating at maturity.
This prolongation of the thalamus is called the carpophore. This is seen in the fruits of anise, Foeniculum, Coriandrum, etc.
Any portion of the thalamus may be modified in certain flowers into characteristic structures called discs. The discs often bear nectar glands. Thus, the family Rutaceae (orange, lemon, etc.) has fleshy annular discs surrounding the bases of the pistils .
In grape vine (Vitis sp. of Vitaceae) the disc is formed of some scaly structures . The disc may be hypogynous, perigynous or epigynous according as it is below, around or on the top of the ovary.
Insertion of Floral Leaves on the Thalamus:
In a typical flower the thalamus is a slightly swollen (conical, convex, etc.) structure with the calyx, corolla, androecium and gynoecium developed successively on it. It is terminated by the pistil so that the gynoecium is on the top .
Such a flower is hypogynous (i.e., other members below gynoecium). The ovary here is superior and other members inferior. This condition may be seen in many common flowers like magnolia, mustard, brinjal, etc.
If the thalamus be flattened out to form a flat or a cup-shaped concave top, the gynoecium will be placed not on the top of the flower but at the centre. The other three whorls are usually inserted on the rim of the flat or the concave halamus.
Such an ovary may also be partially sunken in the thalamus. This is called the perigynous condition as the other members surround the gynoecium. Examples of the first (i.e., thalamus more or less flat) type may be found in the peas, strawberry, etc., while the second type (thalamus forming a concave receptacle) is found in rose, etc.
Here also the gynoecium is usually described as superior although it is not actually
on the top of the flower. Sometimes, the term half-inferior is used instead of superior.
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In a third type the cup-shaped thalamus as seen may fuse with the wall of the ovary giving the appearance as in . It now appears that the gynoecium is the lowest or the inferior whorl of the flower while all other members are superior, i.e., the condition is epigynous.
Epigyny may be seen in the unisexual female flowers of cucurbits and in plants like banana, guava, sunflower, etc. It should be noted that in an epigynous flower the wall of the ovary has also got the thalamus fused with it.
Thus, any fruit developed out of this ovary has this thalamus tissue on the outside. This statement is further strengthened by the fact that in some gooseberry one or two foliage leaves are sometimes seen to develop on the fruit wall. This is possible because of the stem nature of the thalamus.
