While connoting both the social as well as biological consequences of an entity that has plagued mankind for millennia, this sentiment recognizes the central role of the immune system in wound healing, or, in this context, tumor elimination. The critical role that the immune system plays in tumor regression, and therapeutic strategies harnessing the host immune response against tumor, have been recognized since the advent of Coley’s toxin over a century ago—based on observations that patients with severe postoperative skin infections after their sarcoma surgery would spontaneously achieve cancer remission. Bacillus Calmette–Guérin (BCG) vaccine has shown durable effcacy in localized bladder cancer with reported responses in etastatic cancers as well. Decades of innovation in medical science would be required to further refne cancer immunotherapy for clinical use.

Flowers, the reproductive organs of angiosperms (flowering plants), are more varied than the equivalent structures of any other group of organisms. Because of this variation, interest in plant sexual diversity has a long and venerable history in biology. Linnaeus used variation in sexual structures of plants as the basis for his classification, Darwin wrote three volumes on plant reproductive biology and Fisher provided the population-genetic principles for understanding the evolution of plant mating systems. However, despite this attention, it is still not obvious why flowering plants have evolved such extraordinarily diverse means to achieve only one primary function — mating success. A central problem for evolutionary biologists is therefore to understand the selective forces that are responsible for the origins and maintenance of sexual diversity in flowering plants . Here, I review recent advances in our understanding of the evolution and functional significance of sexual diversity in flowering plants. Several developments make it an opportune time to explore this topic. First, a rich theoretical framework and new advances in genetics and molecular biology have meant that sexual systems have become a unique testing ground for both phenotypic and genetic models of natural selection1–3,as well as for phylogenetic analysis and comparative biology4–6. Second, because plants are so amenable to experimental manipulation, they can be exploited as model systems for testing adaptive hypotheses on the function of sexual traits7–9. Last, the recent unification of pollination biology and mating system studies10, two areas of reproductive biology that remained separate for most of the past century, has led to new experimental findings on the mechanisms that drive sexual diversification in plants.

The integration of trees, especially nitrogen fixing trees (NFTs), into agroforestry and silvo-pastoral systems can make a major contribution to sustainable agriculture by restoring and maintaining soil fertility, and in combating erosion and desertification as well as providing fuelwood. The particular advantage of NFTs is their biological nitrogen fixation (BNF), their ability to establish in nitrogendeficient soils and the benefits of the nitrogen fixed (and extra organic matter) to succeeding or associated crops. The importance of NFTs leads to the question of how we can maximise or optimize their effects and how we can manage BNF and the transfer of nitrogen to associated or succeeding plantings. To be able to achieve these goals, suitable methods of measuring BNF in trees are necessary. The total nitrogen difference (TND) method is simple, but is better suited for low than high soil N conditions.