The sporophylls (i.e., stamens and carpels) are the essential parts in a flower so that the flower may be defined as a shoot bearing sporophylls.
An ordinary shoot has an axial stem divided into nodes and internodes, the foliage leaves being borne at the nodes in an alternate or cyclic manner. The flower is exactly a similar shoot, only it has been modified for the special purpose of reproduction.
Here the axial stem is the thalamus which is so much condensed that it is difficult to distinguish the nodes although they are there, and the specialised floral leaves (which again are modifications of leaves—Chapter IV) are borne on these nodes.
The phyllotaxy is spiral in flowers like Nymphaea or Magnolia whereas it is cyclic in a vast number of flowers. In the latter case, at the first node there is a whorl of sepals at the next node a whorl of petals, at the third node a whorl of stamens and at the apex a carpel or a whorl of them. The internodes are indeed small.
We may argue as follows to establish that the flower is a modified shoot:
1. Homology of the Flower Bud:
(a) The flower bud has the same axillary or terminal position as that of a vegetative (shoot) bud and develops in the same way.
(b) Floral buds may sometimes be transformed into vegetative buds or bulbils as in Agave , Globba marantina, Allium sativum, etc.
2. Axis Nature of Thalamus:
(a) The axis nature of the thalamus is not apparent as the internodes are so much condensed. But, in certain exceptional cases, the internodes do develop showing the stem character clearly.
Such development of internodes may be seen in Cleome gynandra and Capparis sepiaria of Capparidaceae, Passiflora suberosa of Passi-floraceae, Pterospermum acerifolium of Sterculiaceae, etc.
(b) The growth of the thalamus is limited by the carpels but sometimes the thalamus grows beyond the gynoecium and bears a leafy shoot or a flower above the first flower. Such monstrous development has been seen in rose , pear and some other cases.
(c) In Michelia champaca of Magnoliaceae ,Polyalthia longifolia of Annonaceae , etc., the region of the thalamus bearing carpels (arranged spirally) elongates like an ordinary stem giving rise to an aggregate fruit.
3. Leaf Nature of Floral Members:
The sepals, petals, stamens and carpels reveal their leaf nature in the following:
(a) The phyllotaxy of floral leaves on the thalamus (cyclic or spiral) strongly resembles that of foliage leaves on the stem. The floral leaves even show ptyxis and prefoliation (aestivation) in the same manner as foliage leaves.
(b) The four types of floral members (viz., sepals, petals, stamens and carpels) and foliage leaves often change from one to the other.
(i) Clear gradual transition of one type of floral member into another is seen in the flower of water-lily (Nymphaea sp.) where there are a large number of floral leaves arranged spirally on the thalamus.
The lowermost sepals are leafy green showing venation, these gradually merge into petals, petals gradually narrow down and merge into stamens developing anthers on the top. This need not signify that petals changed into stamens. More probably it is the stamens which became sterile and changed into petals. The carpels come last after the stamens.
(ii) In peony (Paeonia of Ranunculaceae) one can see transition from leaves to sepals and from sepals to petals showing the sameness of all these .
(iii) In the green rose the sepals look like foliage leaves and even the petals are green and leafy.
(iv) In Mussaenda frondosa of Rubiaceae one of the sepals looks out of proportion in the calyx as it is large and vein-marked like a leaf but pigmented like a petal.
(v) In many flower species one finds ‘single’ and ‘double’ varieties. Doubling is usually the transformation of some stamens into petals as one sees in rose, china-rose, etc. N.B. The doubling of Compositae flowers like marigold is, however, different. Doubling may also be caused by chorisis as explained later.
(vi) Canna flowers show petaloid stamens.
(vii) Carpels also become petaloid or sepaloid in Zinnia, etc.
(viii) In certain flowers like the legumes (peas, etc), the carpel (specially the ovary part) clearly looks like a folded green leaf.
(c) While buds are developed in the axils of leaves, this does not ordinarily happen in the case of floral leaves. But, in some cases of monstrous development, buds are developed at the axils of floral leaves (e.g., petal) and these buds may develop into secondary flowers so that one flower develops above another. This is called prolification.
In a similar way a carpel may develop at the axil of a carpel causing the development of one papaw fruit within another.
(d) In the lower groups like the Pteridophytes and the Gymnosperms, the sporophyll is clearly seen to be an ordinary leaf, modified or not, bearing sporangia.