Onlooker

Crannied walls

Walls have a mess of folklore and other attributes attached to them. Fundamentally, they are structures designed to divide space, for a variety of reasons. They denote the limits of territory, and so promote the idea of possession and privacy. They denote the denial of freedom. They serve as barriers to the unwelcome forces of nature, protecting our persons and property from wind, rain, heat and cold. At the same time, walls sustain a population of plants and animals, particularly in rural surroundings. We are assured in a 17th century proverb that they have ears, but we may dismiss this as metaphor.

Alfred Tennyson made a point regarding what he called "flowers in the crannied wall", saying that if he could understand them, root, branch and blossom, he would know what God and man are. In the bustle of modern living we overlook the lessons which we could learn from considering the things that rely upon walls, ancient and modern, for their continued existence.

To the student of natural history, walls offer a rich harvest. Britain shows more than 50 different species of mosses and liverworts alone on its walls. Where aerial pollution is not serious, lichens, too, abound. There is a sequence in the colonisation of exterior walls by plants, starting with green algae, followed by white, grey or yellow lichens, then by mosses and liverworts. After these come ferns, notably the spleenworts and wall rue, and polypody. Growth usually starts at the base of the wall where moisture abounds and there is shade from the direct sun, and the process may continue for 50 to 100 years before reaching its peak. In the course of centuries, an old wall may support bushes and even small trees.

The flowering plants that take over from the mosses vary with the type of mortar in a wall. Wall pennywort and pellitory are often luxuriant until the winter frosts begin to bite, particularly in the West Country. Yellow corydalis and red valerian abound locally long into autumn. Highly decorative are the stonecrops and toadflaxes, notably the beautiful ivy-leaved toadflax, which sometimes develops its white variety. They may be accompanied by the cranesbill, herb Robert.

Then the animal life of a wall must not be overlooked. Yellow and black ants thrive in cavities, together with nesting bees and wasps, deriving their building material from leaves of whatever plants are handy. More than 60 species of spiders are reputed to adapt themselves to crevices and perforations in mortar and stone. Some prey on woodlice, which also build up colonies in cracks. The jumping spiders are able to leap upon any small insect that moves within their range. Particularly fascinating are the spiders of the Amaurobius and Segestria genera, which make funnel webs issuing from a crevice and surrounded by a frilled edge or loose detector thread which acts as a signal for the spider to emerge when a small insect comes into contact with it.

There is a regrettable tendency for owners of ancient walls, particularly those which form part of an ancient monument, to clean them periodically by means of a sand blast or a herbicide spray, simply for the sake of what they imagine is tidiness. Naturalists deplore this. However, they do recognise special cases, such as old bushes of buddleia, which sometimes plant their roots firmly into the cavities of wall and make such vigorous growth that they cause severe material damage. Otherwise, I feel we should respect the sentiments of Tennyson and look upon these things with a sense of wonder and admiration.

Chemistry for survival

In Science for January 5 there is an argument by Terry Collins of Carnegie Mellon university, in Pittsburgh, United States, that the proper teaching of chemistry plays an important role in helping us to achieve a sustainable civilisation on this planet. Our economy at present involves a massive consumption of natural, often non-renewable resources and the reverse flow of harmful spent matter into the ecosphere. Current methods used in teaching chemistry to students largely ignore the necessity for achieving a balance of these human activities.

When students are taught about chemical processes, their economic value and the hazards they pose to human health — and, indeed, continued existence — the dangers should receive as much emphasis as the benefits. It is essential that we develop safe chemical technology which is also profitable to our activities. Any truly sustainable civilisation relying on high technology must observe the need to study renewable energy. The reagents resorted to by chemists working in industry have hitherto largely been derived from fast depletable oil reserves, and must in future come from renewable sources. Polluting technologies must be replaced by benign alternatives.

Chemists who develop new processes must try to utilise reactions where the desired end product is the sole outcome, without a host of hazardous by-products. Whereas human chemists tend to resort to nearly all the elements in the periodic table in carrying out their work, nature manages to perform an enormous range of biochemical processes using a handful of elements which commonly occur in the environment. Persistent pollutants which accumulate in the ecosphere are a real threat to human and other creatures' sustainability. First come the essentially toxic elements, then the molecules which resist natural degradation reactions. The chlorine industry, for instance, has been responsible for polychlorinated dibenzodioxins and dibenzofurans which persist in nature. The bleaching of wood pulp with chlorinated oxidants is a source of massive pollution. Compounds such as polychlorobiphenyls, endosulfan and atrazine produce alarming damage to the reproductive system of animals by disrupting endocrine processes.

There is a need for stringent regulation of the chemical industry based on the precautionary principle, and there is a need for a satisfactory definition of "persistence" as it applies to toxic by-products. Chemistry students need to be properly informed of the toxic compounds formed in trace quantities when, for example, phenols are chlorinated.

Such information is at present lacking in the teaching curriculum. Many hazards have been recognised and largely eliminated, such as lead additives to paints, fuels and electrical batteries. Improvements in the paper pulp industry have been achieved by substituting chlorine dioxide for chlorine as a bleaching agent. But many home and industrial disinfectants still rely upon chlorine, although products based on hydrogen peroxide are available.

In short the principle of "green" chemistry must become part of chemical education and practice. Laboratory methods must take environmental questions into consideration. There must be no more misrepresentation of hazardous chemical processes as "green" to create a public image favourable to commerce. Chemistry is, indeed, inextricably intertwined with sustainability and ethics.