IT has become customary of late years to pay a great deal of attention to what is called ‘vocational testing.’ Much has been written on this subject and we do not need to consider it in detail. It may be summarised by saying that psychologists and sociologists have devised a large number of experiments by which the suitability of any boy or girl for any particular occupation may be assessed. It is possible to classify human beings in respect of their equipment in manipulative skill, resistance to fatigue, toleration of repetition, inventive capacity and so on. It is found, for instance, that
people employed in the manufacture of photographic emulsions which are handled in red light are much less subject to depression than those handling the emulsions in green light. It is therefore good business to see that the workers of the green light section are naturally cheerful, leaving the more exhilarating red light to their less cheerful associates.
Qualifications can be considered under a number of headings. Obviously the first of these is anatomical, or we may say, comparing it with our machines, constructional. In other words, has an organism the necessary parts to do a particular job? We can decide at once that a child born blind will not make a publisher’s proof-reader, but not everybody with the power of sight can do that job. In other words, having got the machine with the necessary parts, will it work?
THE WATER BUDGET: HOW A PLANT BALANCES INCOME AND EXPENSES
IT has already been made reasonably apparent1 that for all organisms living on the land there is a water problem. An organism living in an atmosphere which is not saturated with water is liable to lose water into the atmosphere by evaporation and we may remind ourselves yet again that the living substance of all living organisms—the protoplasm —contains more than 98 per cent water, and on the maintenance of this the continuance of the life-processes depends. It is therefore easy to see that a plant subject to a dry atmosphere must be capable of conserving its water-content or it will die. On the other hand, all land-plants maintain a stream of water from the soil into the atmosphere and thus accumulate the necessary supplies of mineral salts. What the plant has to do, therefore, is to balance the income from the soil with the expenditure into the atmosphere, or, since the study of international economics is so popular, to make water-imports slightly exceed water-exports.
Plants living in a dry soil and a dry atmosphere, that is to say, having a very poor water income and the possibilities of a heavy water expenditure must keep a tight hand on that expenditure or die. For the plant living in a very damp situation, in an almost water-saturated atmosphere, the problem is the other way about; how to pass a sufficient
amount of water through the plant body to leave behind the required amount of mineral salts. We cannot consider all the various devices by which these problems are solved, but we may realise that, in a broad way, plants of damp situations have large, that is, good evaporating surfaces, and plants of very dry situations have very small leaves or none at all. As a comparison we may consider that an income which will enable one to live very comfortably in a remote country village will probably be less adequate in a country town, and inadequate in a great city; much depends on the company one keeps.
It is a commonplace to remark that the climate of the British Isles is not a climate of extremes and we shall not expect to find any extreme examples of water income and expenditure. As one of the chief factors in bringing about a high water-expenditure is the sun, we shall be justified in expecting to find our most extravagant examples in the tropical parts of the world. We have neither tropical rain-forests where there is very heavy expenditure of water, nor desert plants which live in conditions conducing to heavy expenditure but, in fact, adapt themselves to a ridiculously small water-income. Nevertheless, it is easy to find in the British Isles plant populations which, for one reason or another, have to make do on very scanty water supplies. Coarse, sandy soils, though they may receive fairly abundant rainfall, are incapable of retaining very much moisture, and so the plants found on them are those which are able to exercise rigid control over water-expenditure. Well-known examples of this kind of thing are the pine-heaths which we often know as commons; they probably would not be common land if they were good for anything else.