The flower is generally defined as a highly specialized reproductive shoot, comparable to leaves- bearing shoot.
The sepals and petals may be regarded as modified leaves. Stamens and carpels also show some resemblance to leaves in position, arrangement, internal structure and development.
Based on these similarities, they are comparable with leaves which bear reproductive organs.
The terms microsporophylls and megasporophylls are often applied to stamen and carpel respectively. The flower, then, can be defined as a group of sporophylls usually surrounded by perianth.
That “flower is a modified shoot” can be explain d citing the following facts:
(I) Homology of the floral bud:
A floral bud and a vegetative bud appear quite different externally, yet there is great similarity between the two.
(i) Floral and vegetative buds both emerge either in terminal or in axillary position (Fig. 6.17A),
(ii) The floral buds may sometimes get modified in to vegetative buds or bulbils (e.g. Agave, Allium). Thus proving that the two are analogous structures.
(II) Axis nature of receptacle:
That the receptacle (thalamus) is in fact an axis producing flower can be proved by the following:
(i) Though in majority of cases, the internodes in floral axis remain highly reduced, yet in a number of plants such as Capparis, Gynandropsis (Fig. 6.17C), Passiflora etc. the receptacle shows prominent nodes and internodes.
(ii) Second evidence comes from monstrous development of foliage leaves in some flowers (e.g., rose, larkspur, pear etc.). In the flowerpot’ these plants, the receptacle of the flower continues its growth even after producing all the four types of floral appendages and then produces normal foliage leaves (Fig. 6.17B&D).
(iii) A third evidence comes from Michelia champaca where the thalamus elongates like an ordinary stem beyond carpels and bears aggregate fruit.
(III) Foliage nature of floral appendages:
That floral appendages, calyx, corolla, androecium and gynoecium are modified leases can be proved by the following facts:
Foliage leaves and floral appendages have identical arrangement on the stem.
Pre-foliation of foliage leaves and aestivation of floral leaves (calyx and corolla) is virtually identical.
(iii) Foliar nature of sepal:
That sepal is a modified leaf can be clearly seen in Mussaenda. Hen- one of the sepals enlarges abnormally and becomes green like any foliage leaf.
(iv) Transition of floral leaves:
In nature, in many cases, such as Nymphaea (water lily) all degrees of transition from sepals to petals and from petals to stamens can be seen (Fig. 6.18). In Canna, the stamens and the style become petaloid. In Zinnia, some of the stamens and carpels become petaloid or sepaloid. In Hibiscus rosasinensis (China rose) it is believed that stamens have modified into petals. This point has been proved by identical vasculature of the two.