Curiously these poisonous qualities may also give them antiseptic and antibacterial properties. Scent molecules occurred first among those products that living organisms expel as excreta. Natural selection seems to have ensured, little by little, the retention of the scented molecules among these products and control of their distribution around the organism and their specific functions. Whether flower or leaf scent came first is difficult to ascertain. But, leaf scents being less refined and less varied than flower scents, it is thought that those plants where leaves, petals, seeds, wood and roots are all scented, such as Magnolia kobus and Chimonanthus praecox (syn. C. fragrans), are at some intermediary stage of evolutionary development. In these examples the flower scent can be recognised as the elaboration of the leaf scent, more sophisticated and more delicious. Protection against predatory insects and grazing animals is key to most leaf smells. Aromatic plants wear their essential oil in a layer of glands, on their outer surfaces generally. What Richard Mabey describes as an oil skin in reverse. In extreme dry heat the plant must preserve water to maintain turgidity and carry on photosynthesising. Some Cistus species secrete a resin, labdanum, on to the surface of the leaf whose stickyness reduces the speed of its evaporation into the air and acts as a browsing deterrent. In thyme, sage and many other labiates – which give off their smell at a touch or in bright sunshine – the oil is stored on the surface. Aromatic and repellent-functioning leaves provide us with a bounty of flavours for cooking: bay, sage, rosemary, thyme and hundreds more.
Many marigolds are used in companion planting as their scent wards off whitefly in the air, but the pong in their roots will kill eelworms in the soil up to a metre away and discourage the growth of expansive weeds like ground elder and bindweed (don’t rely on it though). Underground there is so much more to discover to do with scent. The secretions from the roots of potatoes can induce hatching in eelworm eggs, and so it may be that many underground creatures are as specific in their reactions and as sensitive as moths. Immense, complex and barely understood is the network of chemical activity that goes on underground, much of it scent based in the darkness, and unwittingly disturbed by our reckless use of synthetic pesticides and fertilisers.
The essential oil of flowers is invariably more complex than that of scented leaves – compare rose-leaf geranium with the flower smell of a rose. The rounding and shading of floral scents are due to many closely related chemical compounds being present; this provides ‘colour’. The essential oil exists ready formed in the mature flower bud. But if you have ever opened a bud before it is ready, the alchemy of scent has not yet happened. It is thought, therefore, to be the case that the essential oil is stored along with a glucoside, the combination of sugar and oil unlocked by the action of a ferment, which is produced at exactly the right moment within the living cells of the plant. The action of glucosides must be reversible or at least switchable on and off with the opening and closing of flowers. This is most probably regulated by temperature.
A synchronicity must exist between the quarry and the forager, plant and animal, between the activity, the likely presence, the abundance, the desire of the pollinator and the secretion of the nectar, the reward. Scent molecules can travel further in relatively high humidity and warmth. The production of fragrance is costly to the plant, it does not waste it. Warm, humid and still are the optimum conditions for smell to evanesce, and in the landscape these conditions may be found more readily