With this reversal of the relationship between cause and effect it is not, however, intended to represent the commonly accepted order of things as entirely incorrect. In the realm of life, cause and effect are not so onesidedly fixed as in the realm of mechanical forces. We may therefore admit that a reverse effect of the soil-elements upon the plant does take place. This is plainly demonstrable in the case of phosphorus which, however, by reason of its appearance in the soil in proportions hardly to be called a mere 'trace', represents a borderline case. What may apply within limits to phosphorus is wholly valid for the trace-elements - namely, that they are playing their essential role while they are themselves about to assume ponderable form.