It was not enough in the first place. The Green Revolution succeeded in tripling the food supply, but even that was not enough to feed the growing human population. Population growth outran the gain.

Where its gains actually came from. Increased yields were partly due to improved crop varieties, but mainly due to better management practices and the use of agrochemicals — fertilisers and pesticides. So the achievement rested largely on inputs, not on the plants themselves.

Why that cannot be continued. Two reasons are given. First, for farmers in the developing world, agrochemicals are often too expensive — so the method fails exactly where food is most needed. Second, further increases in yield with existing varieties are not possible using conventional breeding: traditional breeding techniques failed to keep pace with demand and failed to provide sufficiently fast and efficient systems for crop improvement.

And there is an environmental reason. The chapter asks directly whether there is a way to minimise the use of fertilisers and chemicals so that their harmful effects on the environment are reduced. Tissue culture and genetic modification were developed as answers to both the biological and the environmental limit.

The tissue to use is the meristem — apical and axillary.

Why it works. Even if the plant is infected with a virus, the meristem is free of virus. The meristem is the actively dividing growing region; viruses spread mainly through the vascular tissue, which is not yet differentiated there, and the rapid division of meristematic cells tends to outrun viral replication. So one can remove the meristem and grow it in vitro to obtain virus-free plants.

Why the rest of the technique is needed. A meristem tip is a tiny fragment, not a plant. It can be grown into a whole plant only because of totipotency — the capacity to generate a whole plant from any cell or explant — and only in a special nutrient medium under sterile conditions, supplying sucrose as a carbon source along with inorganic salts, vitamins, amino acids and growth regulators such as auxins and cytokinins.

Where it has been done. Scientists have succeeded in culturing meristems of banana, sugarcane and potato — all crops propagated vegetatively, in which a virus would otherwise be passed on indefinitely from one generation of planting material to the next.

Why the wall must go. Fusion requires two plasma membranes to come into direct contact and merge. A plant cell is encased in a rigid cellulose wall that holds the membrane away from any neighbour and cannot itself fuse. As long as the wall is present, the two cells can be pressed together but never joined.

The resulting cell. After digesting the cell walls, what remains is a naked protoplast, surrounded only by its plasma membrane.

The technique and its product. Isolated protoplasts from two different varieties of plants, each having a desirable character, can be fused to get hybrid protoplasts, which can be further grown to form a new plant. The hybrids are called somatic hybrids and the process somatic hybridisation. Fusing a tomato protoplast with a potato protoplast produced the pomato, although this plant did not have all the desired combination of characteristics for commercial utilisation.

Why the technique matters despite that failure. It crosses the barrier between species without pollination or crossing, which no breeding programme can do. Its limitation is that it transfers two entire genomes, so the outcome cannot be predicted or controlled — which is the problem single-gene transfer was developed to solve.

(i) Reduced reliance on chemical pesticides, through pest-resistant crops. When a plant carries a gene that makes it lethal to its own pest, the insecticide does not have to be sprayed at all. The toxin is produced only within the plant tissue and only in the quantity the plant makes, instead of being distributed over the whole field, the soil and the water that drains from it. The chapter describes this as, in effect, creating a bio-pesticide.

(ii) Increased efficiency of mineral usage by plants. A plant that takes up and uses minerals more efficiently needs less fertiliser for the same yield, and the chapter notes the specific benefit: this prevents early exhaustion of the fertility of the soil. Less fertiliser applied also means less run-off into rivers and lakes.

Why this matters to the argument of the chapter. The question the chapter asks is whether there is a way to minimise the use of fertilisers and chemicals so that their harmful effects on the environment are reduced. These two benefits are the direct answer — and note that both work by changing the plant rather than by adding something to the field. That is the conceptual shift from agrochemical to genetically engineered crop-based agriculture.

A fair qualification. The chapter also warns that genetic modification of organisms can have unpredictable results when such organisms are introduced into the ecosystem, which is why the GEAC was set up. Reduced chemical use is a real environmental gain, but it is not the whole environmental account.

The conclusion. They are not rivals but stages. Genetic modification or somatic hybridisation creates the new plant; micro-propagation is then what turns one successful plant into a plantation, quickly and uniformly. And for the many traits that no one can specify as a list of genes, conventional breeding is still the only route.