Inflorescence and the Floral Morphology of Angiosperms
The floral morphology of angiosperms describes the functional arrangement of the flowers. The manner of their occurrence and arrangement on the shoot is constant for a species but is variable throughout flowering plants as a whole. We refer to the arrangement of the Flower as the inflorescence of the plant. Flowers may developed singly or grouped. They may be at the apex of the main shoot or one of its branches, in which case vegetative activity at the apex ceases and the vegetative parts are replaced by the reproductive structures. The physiology of this change-over is very obscure. Alternatively, the flower may be produced directly from buds in the axils of leaves. When the flowers are grouped, the main axis on which they are borne is called the peduncle and on this individual flowers are developed, usually in the axils of bracts, but these may be very reduced or even absent. Each flower may or may not possess a stalk or pedicel. Bracts are very variable. They may be ordinary foliage leaves but frequently are much smaller and scal-like. They may be coloured like petals (petaloid). The small outgrowths often found on the pedicels are called bracteoles. These may also be regarded as reduced leaves
The floral arrangement and the floral morphology of angiosperms may be described as belonging to one of two kinds, the racemose or indefinite and the cymose or definite. The distinction lies in whether or not the growing apex of the inflorescence axis is able to produce flower buds In continuous succession, without coming to ahalt by producing one in the terminal position. In indefinite inflorescences, flower buds are initiated at the apex which never loses the power to do so. In definite inflorescences, the apex sooner or later forms a terminal flower and then loses its ability to produce more. In each of these forms of inflorescence, there are several possible flower arrangement.
- The Spike: Individual, unstalked (sessile) flowers, arranged along a single undivided peduncle, e.g. plantain, Orchis. Special forms include the spadix as in Arum, in which the axis is massive and fleshy and protected by a large developing bract. The spathe, and the catkin, a pendulous structure found I may trees such as walnut. Popular, oak, hazel, birch.
- The Raceme: Individual stalked flowers arranged along a single undivided peduncle e.g. wallflower, foxglove, hyacinth, lupin. This may be compounded.
- The corymb; as in the raceme but with the pedicels all of different length, the lowermost longest, so that the flowers are all brought to the same level. E.g. candytuft and many other crucifers
- The Umbel: Stalked flowers with the stalks of about the same length arising apparently from the same terminal point of the axis g. cow parship, carrot and other of the Umbreliferal the ring of bract each associated with one of the flower stalks , it called the involucres. The inflorescence may be compounded.
- The Capitulum: Sessile flower arranged in the same plane at the expanded apex of the axis, e.g. dandelion, daisy, thistle and other of the compositae. The head f flowers is surrounded by a number of sterile bracts collectively termed the involucres. The individual flowers may or may not be in the axils of bracts on the disc-like head.
The definite inflorescences are called cymes
- These Manochasial cyme: Flower production in the terminal position on an axis is followed by the development of another axis from the axil of the bract behind it. There are two main forms of this inflorescence. In the helicoid monochasium all the succeeding axes are developed from bracts on the same side of the preceding axes. So that each successive axis tends to coil the whole further and further towards the first axis, e.g. buttercup. In the scorpioid monochasium the succeeding axes are developed from bracts alternately on opposite sides of the preceding axes, e.g. Myosotis (forget-me-not).
- The Dichasial Cyme: flower production in the terminal position on an axis is followed by the development of two more from the axils of oppositely-placed bracts behind it, e.g. bladder Campion, stitchwort and other of the caryophllaceae. Such a dichasium can be complex if the paired branching is not confined to a single plane, e.g Gypsophila. In addition to the more easily recognizable forms described above, there are numerous apparently special forms. On careful analysis, these all conform to one or other of the general patterns but the condition is confused due to shortening of the axes or crowding of the flowers for example, the somewhat globular heads of Hydrangea, Viburnum and hawthorn are really cymes arranged corymbose fashion. The heads of flowers of pelargonium and some narcissi are similarly cymes arranged in an umbellate form. The apparent whorls of flowers in the dead
Nettles are really two oppositely placed cymes. The whole arrangement is known as a verticillaster. It is not unusual to find a mixed inflorescence, that is, partly indefinite and partly definite. Such as case arises in the figwort in which the main axis of indefinite growth produces lateral branches bearing flowers of definite arrangement. Another case may arise in which a plant may produce a single flower at the end of the main vegetative axis, e.g. wood anemone, tulip. Such a flower is said to be solitary and terminal.
In describing the floral morphology of angiosperms, we must first examine the nature of the flower. The flower may be regarded as a vegetative shoot in which the parts are highly specialized for the purpose of reproduction. The organs essential to this purpose are the stamens forming the androecium, and primarily with protection and the attraction of insect visitors. There are many variations of the generalized flower pattern. When any particular flower is being examined and described, it is advisable to be aware of the nature of the variations which may possibly occur. The major differences in structure between flowers of different species are summarized in the following categories
The manner of insertion of the parts (floral phyllotaxis)
The numbers of parts; reduction and multiplication
The freedom or fusion of parts
The relative position of parts on the receptacle
The distribution of the sexual parts within the flower
Flower parts may be inserted on the receptacle in the continuous spiral or in separate whorls. The first condition, as in flowers of the cactus, is described as the spiral or acyclic arrangement. The whorl arrangement is described as cyclic and is by far the commonest condition. The hemi-cyclic arrangement can be seen in the buttercup, for example, where the calyx and corolla are in two separate whorls whilst the stamens and carpels are spirally inserted.
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