For more than three decades, since the completion of Feistmantel's classic work on the Indian Gondwana flora in 1886, the study of fossil plants had suffered a serious set-back as Indian geologists were sceptical regarding their value in geological chronology. Geologists were largely influenced by the attitude which W. T. Blanford took up in 1876 that the evidence founded upon fossil plants should be received with caution, and that such evidence was in some cases opposed to the evidence furnished by marine faunas. The year 1920, in which Seward and Sahni's volume on the revision of Indian Gondwana plants appeared, is a landmark in the history of Indian geology and palaeobotany: this year marks the revival of palaeobotanical research in India with plant fossils coming more and more into the picture of Indian geology. Prof. Sahni was a rare combination of the botanist and the geologist and the unique position he held in both the sciences made him eminently suited to bridge the gulf that separated them. He did more than anyone else to convince the geologist that study of plant fossils yielded results of a far-reaching nature which the geologist could not afford to ignore.

To Prof. Sahni plant fossils were not just chance relics of ancient floras; they had a deeper significance to him. Their geological background and implications were always present in his mind. His work, at every stage, impinged upon the domain of geology, and the field of palaeobotany became a meeting ground for botanists and geologists in this country. In a memorable address delivered to geologists in 1926, he expressed that fossil plants represent the debt that botany owes to geology. In return, palaeobotanical research which he initiated has not only been helpful in solving stratigraphical problems, but has also thrown light upon questions of palaeogeography, past climates and even earth movements. At the same time it has made its contributions to economic geology.

It is difficult to give an adequate idea, within the space of a brief article, of the extent to which he influenced geological thought and research in India within the last two decades and more. It was in this country that the first Glossopteris was discovered and the great problems in geology concerning the Gondwanaland were raised and discussed. The Gondwana problems naturally attracted a good deal of his attention. Two other chapters of Indian geology where his researches had their repercussions were the Deccan Traps and the Punjab Saline Series. He realized the importance of micropalaeontology, both in its academic and applied aspects, and the micropalaeontological technique which he had already employed in his work on the Saline Series was extended to other problems: in elucidating the Tertiary sequence of Assam and as an aid to the measurement of geological time in India.

Few problems in Indian geology have aroused greater controversy than those connected with the classification and age limits of the Gondwana formations. Feistmantel's original classification into Upper, Middle (with Parsora stage as a transitional stage) and Lower has been questioned by later geologists, particularly Cotter and Fox. Fox thought there was no justification for Middle Gondwanas as there was a floral break above the Panchet stage. He further included the Parsora stage in the Jurassic. Prof. Sahni, however, did not agree that the Parsora flora was Jurassic and, on the other hand, he thought it was not younger than the Trias and possibly as old as the Permian. The geologists, again, considered the upper age limit of the Gondwanas as Lower Cretaceous, as Lower Cretaceous ammonites are found associated with the east-coast Gondwanas. In this connection the silicified flora of the Rajmahals received a good deal of attention from Prof. Sahni. Feistmantel's account of this flora was mostly confined to leaf impressions and in recent years many fossil-bearing localities had been discovered. From a critical examination of the petrifactions, Prof. Sahni came to the conclusion that the flora was Jurassic with not a single species characteristic of the Cretaceous.

A problem in which he took a great interest for many years was that of continental drift. Whereas Wegener thought that continents had broken up by drifting apart, Prof. Sahni, on palaeobotanical evidence, elaborated a complementary theory that continents once separated by oceans had drifted towards each other.

In 1934 his first contribution on the silicified flora of the Deccan Intertrappean beds appeared, and with this was revived a geological controversy which dates back to the time of the pioneer geologists Hislop and Hunter. As against the Cretaceous age put forward on geological grounds by Blanford and others, Prof. Sahni found that the flora was distinctly Eocene, and it is gratifying to note that the Eocene view later received its strongest support from the geologists themselves.