l60 THE PLANT 



Hclianthus anmitis 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 i meter. The series which 

 has been most used consists of three tents : the first is made of cheese- 

 cloth and has a light value of .i ; ^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 approxi- 

 mately 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 Hclianthus, Taraxacum, Gaura, and Onagra, while Ran- 

 unculus scelcratus 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 iiave been revolved 90 or 180 in order to change the rela- 

 tion of their surfaces. .Soils of different colors, c. 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 adjust- 

 ment and adaptation. 



