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POPESCO (11) credits the first observations on the absorption of solutes by roots to the work of MARCELLUS MALPIGHI in the seventeenth celntury. POPESCO writes that, in his work entitled Anatomne Plantarnin which appeared in 1675, MALPIGHI maintained that nutritive substances were absorbed by the roots at the level of the region of the root hairs. Since not all plants bear root hairs, MALPIGHI stated further that the entire epiderlial surface funetioned in absorption when root hairs were absent.According to POPESCO, many early investigators following MALPIGHI were not in agreement with these conelusionis: GREW in 1682 said t.hat the roots absorbed water and nutritive substances from the earth through their tips, especially through the root cap; HALES in 1727 and DE LA BAISSE in 1733 supported GREW'S contention stating in addition that the role of root hairs was secondary. Many of the observers in the beginning of the nineteenth century also believed that water and solutes were absorbed thlrotuo the very tip of the root but in 1865 SACHS contended that the plant absorbed water from the earth by means of the root hairs, that the surface area of absorption was greatly increased by the preseniee of root hairs, and that volume absorption was proportionally iniereased. Subsequently, SCHWARZ calculated that the epidermal area in roots of corn was increased six times and in roots of peas twelve times when root hairs developed on these roots in moist air (10).Such outstanding investigators of the early part of the twentieth century as JOST, MOLISCH, and PFEFFER also maintained that the principal role of water absorption in hairy roots was played by root hairs but several of their contemporaries disagreed, especially DE LAVISON (11)

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Although many investigations have been made there is, up to the present, no available study which furnishes reliable quantitative data on the distribution of the rates of water absorption in any one root. Only recently (1, 8) has there been any attempt to make the investigations strictly quantitative in character but all studies are open to adverse criticism since none were carried out under adequately controlled conditions. It appears that the only method at present available which provides for complete control of conditions using the intact, uninjured, and unstimulated root is that used in the present study.Since reviews have been presented by other investigators (1,4,8,9) no comprehensive survey of the literature will be attempted. It is important, however, to call attention to certain inadequacies of the methods employed by the more recent investigators. POPESCO (4) made determinations of the rates of water absorption on single roots of a number of different plants, but his results were only relatively quantitative. He employed both direct and indirect methods which involved: (1) the isolation of given regions of a root by covering these with cocoa-butter and then placing the root in a U-tube, one arm of which, drawn out to a capillary, was graduated in arbitrary units; (2) the use of dyes and microchemical reagents; and (3) plasmolysis. HOHN (1) made unsuccessful attempts to use a modified form of PoPEsco's "cocoa-butter" procedure. He was unable to obtain a perfect seal with rings of cocoa-butter and he points out the impossibility of employing inelastic material to cover the region of elongation. To cover various regions of the root HOHnN substituted oil for cocoa-butter. His experiments were carefully carried out under known conditions of temperature and humidity. The chief criticism of his work is given by SIERP and 1 www.plantphysiol.org on May 9, 2018 -Published by Downloaded from

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