There is an optimum degree of moisture in the soil which promotes the maximum development of root-hairs. If the soil is too wet they are not developed. These facts are of importance as correlated with the ease or difficulty experienced by the roots in obtaining water, and plants such as our ordinary agricultural plants show this very distinctly. Although, as shown in the experiments with wheat, the short roots in dry soil were more densely covered with root-hairs than the much longer roots in moderately moist soil, subsequent closer investigation shows that the total quantity and area of root-hairs is less in the former case than in the latter. The greatest number of root-hairs are developed on roots which are growing at their best: too much moisture may prevent the formation of root-hairs: too little may induce dense growths of root-hairs locally, but the total number is reduced. Another set of events which exerts influence on the development of root-hairs is the composition of the dilute solution--water containing dissolved salts--which surrounds them in the soil. Thus, Schwarz found that when similar oat and wheat plants were grown with their roots in solutions of various salts, the results differed as follows: Oats in a 15 per cent. solution of calcium chloride developed no root-hairs, though they formed in a 5 per cent. solution, and were very numerous in a 0.5 per cent. solution, or in water alone. In a 10 per cent. nutritive solution the plants developed no root-hairs, though they were abundant in a 1 per cent. solution. Wheat plants with their roots in a 15 per cent. solution of potassium nitrate bore no root-hairs, but they were numerous in a 2 per cent. solution of the same salt. These are extreme cases, for, although the roots were not killed, they were strongly inhibited in their growth by the more concentrated solutions. However, experiments of this kind at least bring vividly before us what variations are possible, and suggest that similar events on a smaller scale may occur in a soil which yields large quantities of soluble substances, _e.g._ when freshly manured. Obviously these facts have a practical significance as regards kind of soil, drainage, season (_e.g._ drought or wet), etc. But there are other factors which rule the development of root-hairs, and some experiments by Lesage show that the correlations between the development of root-hairs and roots are probably much more complex than had been suspected; for he finds that if the lateral rootlets of a Bean, in a water culture, are suppressed, the main rootlet develops numerous and very long hairs to compensate the loss in surface, a matter of obvious importance in the discussion of cases where roots have been injured in the soil. Before proceeding further it is necessary to look a little more closely into the structure of a single hair.