In this article we will discuss about the thalamus of a flower.
The thalamus is the short abbreviated axis bearing the four sets of floral leaves. It is the swollen end of the peduncle or pedicel with four nodes and very much compressed internodes. The floral leaves remain inserted on the nodes in whorls or spirally. The axis nature of the thalamus becomes quite evident in some flowers in which it (thalamus) is considerably long and the internodes are distinct.
The internode between the calyx and corolla is known as anthophore, that between corolla and androecium is androphore, and the one between androecium and gynoecium is gynophore. Androphore is distinct in passion-flower and gynophore in Pterospermum (B. Kanak champa). In Gynandropsis (B. Hurhure) both androphore and gynophore develop to form what is known as gynandrophore (Fig. 85).
The thalamus may be a bit elongated, as in Michelia (B. Champaka), convex in flowers like China-rose, Datura, cup-shaped in roses, and it may have spongy flat top, as in lotus. According to the nature of the thalamus and mode of insertion of the first three floral sets with respect to the gynoecium, flowers may be hypogynous, perigynous and epigynous (Fig. 86).
Hypogynous flower is that in which the thalamus is convex and dome-shaped, and the pistil occupies the top position of the thalamus, other whorls remaining inserted below the pistil one after another in normal order. The ovary here is called superior. Examples—China-rose, Datura.
Perigynous flower has concave or cup-shaped thalamus forming rims or flanges, on which stamens, petals and sepals other whorls are placed around it. The ovary is said to be inferior.
Examples—pea, roses. Epigynous flower is that in which the thalamus is hollow and cup-shaped, and completely fuses with the lower part of the pistil (ovary). The stamens, petals and sepals are placed above the pistil. The ovary in epigynous flower is inferior. Examples— Sunflower, Cucurbita (B. Gourd), tube-rose.
Flower is equivalent to a shoot:
It has been stated that the flower is equivalent to a shoot. The following facts may be cited in support of the above statement. The floral buds, like the leaf-buds, develop either at the terminal or axillary positions. The floral members remain on the thalamus in whorls or in spirals resembling the phyllotaxy of the leaves.
The arrangement of the floral parts in respect to each other (aestivation) is also similar to the arrangement of the leaves in the bud stage. Modifications of floral buds into bulbils (buds modified into swollen bodies for vegetative multiplication) is found in plants like Aloe, pine-apple.
Normally the thalamus is a short abbreviated axis with suppressed nodes and internodes. The axis-nature of the thalamus is evident in the flowers like Pterospermum (B. Kanak champa), passion-flower, Gynandropsis (B. Hurhure), etc., where the internodes between the whorls are quite distinct.
A peculiar feature, as found in some roses and pears, is that the thalamus instead of terminating at the flower, continues or prolongs through it and bears normal leaves. Such condition, referred to as ‘monstrous development’, further proves the axis-nature of the thalamus.
The leaf nature of the members of accessory whorls—sepals and petals, is rather clear, in view of the fact that they are usually expanded bodies resembling the foliage leaves in structure and nature of venation.
In Mussaenda (Fig. 88) one spreads out like a leaf. Main difficulty arises in case of the essential members—stamens and carpels. The gradual transition from one member to another—sepals to petals, petals to stamens, is distinct in water-lily (Fig. 87).
In many cultivated flowers the floral leaves in water-lily, stamens are found to be converted into petals, what is commonly called ‘doubling of flowers’. Rose, China-rose, Hibiscus mutabilis (B. Sthal padma) are the examples. The stamens and style become petaloid in Canna, a common garden flower. All these facts go to show that the stamens and carpels are also morphologically equivalent to the leaves.