In describing the construction and operation of the many factor instruments, there has been no attempt to make the treatment exhaustive. Those instruments which the author has found of greatest value in his own work are given precedence, and the manner of using them is described in detail. Other instruments of value are also considered, though with greater brevity. Some of the most complex and expensive ones have been ignored, as it is altogether improbable that they can come into general use in their present form. While the conviction is felt that the methods described below will enable the most advanced investigators to carry on thorough work, it is hoped that they will be seen to be so fundamental, and so attractive, that they will appeal to all who are planning serious ecological study. WATER-CONTENT =40. Value of different instruments.= The paramount importance of water-content as a direct factor in the modification of plant form and distribution gives a fundamental value to the methods used for its determination. Automatic instruments for ascertaining the water in the soil are costly, in addition to being complicated, and often inaccurate. For these reasons, much attention has been given to developing the simpler but more reliable methods in which a soil borer or geotome is used. The latter is simple, inexpensive, and accurate. It can be carried easily upon daily trips or upon longer reconnaissances, and is always ready for instant use. In the determination of physiological water-content, it is practically indispensable. Indeed, the readiness with which geotome determinations of water-content can be made should hasten the universal recognition of the fact that it is the available, and not the total amount of water in the soil, which determines the effect upon the plant. _Geotome Methods_ [Illustration: Fig. 1. Geotomes and soil can.]