|The sepals of a rose bud are green and photosynthetic
like fully developed leaves, and like the one on the left,
sometimes even appear to partially subdivided like full
In some archaic flowering plants, such as Magnolias, petals and sepals intergrade, with no clear distinction between the two, and they are called tepals.
|In Clusia, the leaves are opposite with each pair at right angles to the
preceding pair. A series of small bracts and two pairs of colored sepals
continue the pattern. it's truly hard to see where bracts end and
The development of similar, but modified, organs, from outgrowths of the apical meristem, is called serial homology. Homology in general refers to different body organs that were the same in ancestral species, but have become specialized for different functions in more specialized species. The classic examples in animals are front legs that have become specialized as wings in both birds and bats, and arms with grasping hands in primates. Serial homology can be seen in animals with segmented bodies. Insects and crustaceans, for example, descended from many-legged, centipede-like ancestors, but now have specialized walking legs, reproductive organs, claws, mouth parts, and other specialized organs, all representing modified legs.
|Lilies, like many monocots, have what appear to be six petals,
but three of them are actually petal-like sepals
|Trilliums are monocots only distantly related to the true lilies,
and display three leaf-like sepals, most likely as did the
development of leaves was supplemented with new sets of genes that served to modify the embryonic leaves for specialized functions. The new sets of genes both suppressed the full development of the original leaf size and shape and directed the development of specialized features. Serial homology along a single shoot, from leaves to bracts to flower parts, shows that these different sets of genes are turned on and off in an orderly way.