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Let us make an in-depth study of the floral apex and vascular anatomy of the flower with diagram.
The Flower:
Though the nature of the flower from morphological and anatomical points of view and its phylogenetic relation to other parts have been subjects of long drawn controversy, majority of the botanists consider the flower as something equivalent to a shoot—a modified shoot, and the floral parts homologous to the leaves.
The arguments in favour of the above statement have been discussed in detail in the chapter on flower in Part I. Like a vegetative shoot the flower has an axis—the thalamus, and the lateral appendages—the floral leaves or parts.
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The latter may be put into two categories—the fertile ones which take active part in reproduction, the stamens and carpels, and the sterile ones—the sepals and petals which only help the process and hence are called helping or accessory whorls.
The Floral Apex:
Structurally and developmentally the floral apex resembles the shoot apex in almost all fundamental features. Slight differences that exist are due to the fact that flower is a determinate stem with closely crowded appendages and very much suppressed internodes.
It is at any rate assumed that the shoot apex is organized into the floral apex and the two are merely the growth forms of the same meristem.
Some workers asserted that in the floral apex there is no tunica-corpus zonation like shoot apex. On the other hand, the floral apex consists of a massive central core of thin- walled vacuolated cells surrounded by a mantle of smaller and actively dividing cells; and that the floral members arise from the mantle by predominantly periclinal division.
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Further works on the subject reveal that the peripheral mantle is derived from the cell layers corresponding to the tunica and part of the corpus of the shoot apex. The elongation of the apex ceases and growth activity is confined to the peripheral region due to determinate nature of the flower.
Periclinal division at any depth of the floral apex is associated with the broadening of the thalamus and origin of the appendages. Thus floral apex is regarded as the ontogenetic modification of the shoot apex.
The shoot is now considered a single unit composed of the stem and leaves; and in that light it is assumed that development of the flower is “parallel to that of the vegetative shoot, and not as being derived from it”.
Vascular Anatomy:
The vascular organization of flower may in a general way be compared to that of a vegetative shoot (Fig. 662).
The axis exhibits three zones—epidermis, ground tissues and vascular system. The latter is really a complex of traces which run to the floral parts—often branch and again combine with each other in the axis in an irregular manner.
The accessory parts—sepals and petals resemble the leaves in internal structure. The ground tissues hardly show any differentiation into palisade and spongy cells. The epidermis is normally made of papillose cells, often with intercellular spaces covered by cuticle.
Trichomes and stomata may occasionally occur. Special cells containing volatile oils may be present in the epidermis of petals imparting fragrance to the flowers. In spite of some variations it may be said that a sepal has as many traces as the leaf of that plant and it resembles a bract anatomically.
A petal of dicotyledon normally has one trace, whereas a perianth member (tepal) of a monocotyledon has one to many traces. Like those of foliage leaves the vascular bundles form complex systems in the sepals and petals and the veinlets are usually dichotomously branched.
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The stamen is a simple structure with the filament surmounted by the anther. A single vascular bundle runs through the filament and ends bluntly in the connective which is located between two anther lobes.
In some families three traces are present, what is considered a primitive condition. The bundle of the filament is usually amphicribral and remains surrounded by vacuolated parenchyma cells without conspicuous intercellular spaces.
The carpel, either a simple or an apocarpous one, is a conduplicately folded structure, at the basal part forming the ovary and the upper sterile part—the style, which ends in the receptive part, the stigma.
This is an advanced condition in the phylogenetic line. The ovules occur on the ovarian wall attached to the parenchymatous outgrowths—the placenta, normally at the regions where the margins of the folded carpels meet. Commonly a carpel has three veins—one dorsal and two ventral.
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The carpellary bundles run through the style. The ovules get the vascular supply from the ventral bundles or from placental branches of these bundles. Normally a single trace enters through the base and. reaches the chalaza; branches may extend to integuments but never to the nucellar region.
The ovules are made largely of parenchymatous tissue with primary vascular system which may be functioning during maturation of the seed. The ovary and style are simple structures from histological points of view being composed of cuticularised epidermis, parenchymatous ground tissues and vascular bundles.
Abscission:
Abscission of floral parts, the petals in particular, is similar to what happens in case of leaves. The petals fall off either with the complete opening of the flower or may remain attached temporarily or permanently till the fruit stage. They often undergo constriction at the zone of abscission.
The separation layer is poorly formed of small closely packed vacuolated cells. As in leaves the separation is due to softening of the middle lamella, but formation of cork or deposition of suberin for protecting the scar is lacking.
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Other members—the sepals, stamens, and style may also absciss in the same manner. Male flowers normally fall off after shedding of pollen grains and both female and bisexual flowers may also drop in case fertilisation fails.
