The existence of daily growth-rings in the cell wall of cotton hairs

Abstract

From studies of the growing cotton plant in Egypt the author was led some years ago to the conclusions that the wall of the cotton-seed hair-cell was “probably composed of concentric layers, laid down during the active growth of each successive night, and numbering about twenty-five in all . . . they would thus, at the most, be about 0∙0004 mm. in depth, so that their resolution by the microscope is highly improbable without some previous treatment.” Various methods were tried with the intention of bringing these layers into the limits of microscopic vision, but it was not until five years later that an accidental observation gave the clue to a method by which the limitations of microscope observation may be extended, and these layers made actually visible. The observations which followed, demonstrating the existence of con­centric layers in the wall of the cotton-hair as well as in the “fuzz-hairs,” would have been interesting in any case on account of their bearing on all the physical and chemical problems which this typical cellulose presents. C. F. Cross has insisted on the necessity for considering cellulose problems in terms of “the ultimate fibre," but it now seems probable that the ultimate unit components must be the single layers composing the wall of the said fibre. The bare fact of the existence of such layers would have had no particular significance if it could not have been connected with previous precise study of the growth of cotton-hairs. By counting the number of layers in material previously preserved at known dates during the course of those studies, and remembering the cardinal fact that growth is daily arrested by the sunshine effect under Egyptian conditions, we have been able clearly to show that these layers are actually the growth-rings whose existence we had ventured to postulate. Knowledge of their real existence must materially affect some of our views concerning the physical properties of such hairs.