What is Flower

Secret of success of flowering plants is the presence of well developed sexual reproductive organs in the form of flower and mechanism of cross-pollination. Their sudden appearance in Cretaceous rapid evolution and diversification in short period is puzzling. A large population of early angiosperms has become extinct and many modern species having disappeared more or less completely from their original home land, have occupied large tracts of altogether new areas. Charles Darwin (1959) commented that development and evolution of flowering plants is "An abominable Mystery" This phrase has become a symbol of a key gap in our understanding of plant evolution. Various problems related to flowers are, how ancient flower is; what is its cradle home; how an ancient flower looks like; who are their immediate ancestors; what is the morphological nature of flower and its organs; nature and morphology of placentation; evocation of flower and evolution of flower from simple to complex flower, etc. Answer to all these questions are searched in studying the comparative morphology, fossil records, cladistic studies with the aid of computation and molecular analysis of living plants. The discovery of fossil flower Archaefructus lianingensis (Sun et al. 1998) from Jixin province of North-East China belonging to Jurassic or early Cretaceous strata, estimated about 145 million years old has solved the problems to some extent. This flower is so far the oldest discovered fossil remain and it resembles the primitive extant flower of Magnolia. Recently established family Hydatellaceae is considered the most primitive family of angiosperms and its genus Trithuria is closely related to Archaefractus.

Regarding the cradle home of flowering plants evidences are more in favor of their origin from South-East-Asia-Australia. However Sun et al.( 1998) proposed that North-East China can also be the center of origin because of the discovery of Archaefructus from this region.

Ancestors of flowering plants have been looked into various plant groups of pteridophytes and gymnosperms. Cladistic studies and molecular analysis of nuclear, plastidal and mitochondrial genomes have revealed that members of Gnetales are closest living relatives of primitive living angiosperms and their phylogeny is rooted through families like Amborella, Nymphaeaceae, Illiciales, Trimeniaceae and Austrobailayaceae (ANITA) and genera Trithuria and Ceratophyllum are close to Archaefructus.

Goethe (1790) stated that, "the flower is a metamorphosed shoot and later DeCandolle (1827) laid the foundation of classical concept of flower morphology and stated that flower is a condensed shoot and floral organs are morphologically foliar in nature. Although, later many other concepts came but the classical concept is strongly supported by most of the workers on the basis of evidences collected from comparative morphology, teratological studies and vascular supply of the flower. The carpels are either conduplicate folded or induplicate folded. In Ochna multiflora having a multicarpellary and syncarpus ovary the carpels are conduplicately folded (Govil and Kumar, 2010). Which placentation, axile or parietal is primitive was question marked by Puri (1952), however Saxena and Govil (1995) proposed that parietal placentation is primitive and axile is advanced. To facilitate cross-pollination there is a co-evolution of flower from simple to complex flowers and biotic pollinators. Genes have been identified for the expression of different floral organs. Many more mysteries of flower will be solved as more and more data is collected on fossils and molecular analysis of primitive flowering plants.