[Illustration: Fig. 49. Floating form of _Ranunculus sceleratus_ grown under control.] _Helianthus annuus_ has been used for other series of experiments, in which alkaline salts or different soils are employed to vary the water-content. These are more complex and hence are not as satisfactory as the series described above, but they are valuable for the light they throw upon the behavior of plants in similar conditions in nature. In the case of soil, however, the adaptation may be referred to water-content alone, if thoroughly leached sands and gravels are used, so that the difference is solely one of water-retaining power. =197. Light series.= Cloth tents have been found the most satisfactory means of obtaining different light intensities in the planthouse. The cloth permits the air to circulate to a considerable degree, and in consequence the equalization of humidity and temperature is much more complete than in the glass houses first employed. The cloth tents, or shade tents as they are called, are cubical, each dimension being 1 meter. The series which has been most used consists of three tents: the first is made of cheesecloth and has a light value of .1; the second is of thin muslin, and has a value of .04, while the third is made of dark cambric and the light is reduced to .01. A more desirable series is one with five tents, which have approximately the following light intensities: .1, .05, .01, .007, .003. Plants grown in shade tents should be repotted as often as they will permit in order to increase the aeration of the soil. The amount of water given them must also be decreased as the shade increases. Mesophytic species give the best results in shade tents, xerophytes thrive less well, and amphibious plants do not grow at all except in the brightest light. Excellent results have been obtained with _Helianthus_, _Taraxacum_, _Gaura_, and _Onagra_, while _Ranunculus sceleratus_ is unable to produce flowers and seeds in a light intensity of .01. A number of important supplementary experiments have been made in connection with light tents. These do not result in the production of new forms, but they throw much light upon it. Plants have been placed in the shade tents so that certain leaves would be in the sun and others in the shade. Young leaves have been fixed at various angles with the stem, and they have been revolved 90° or 180° in order to change the relation of their surfaces. Soils of different colors, e. g., loam and sand, have been used to determine the effect of light reflected from their surfaces. Shade tents make it possible to illuminate plants from the top, bottom, or side, and to carry on a large number of fundamental experiments in adjustment and adaptation. CHAPTER IV. THE PLANT FORMATION METHODS OF INVESTIGATION AND RECORD