80 CARNEGIE INSTITUTION OF WASHINGTON. 



movements usually less than 10°. These motions are clearly related 

 to the soil-moisture conditions. The downward motion takes place 

 slowly, the lowest position being reached in from 4 to 6 weeks after 

 the last rainfall, while the upward motion begins within a few hours 

 after the first significant rain and is completed within a few days. 

 A week or so of drought will start the downward motion, which may 

 be in turn interrupted by a rain or by artificial irrigation, but no 

 downward motion begins so long as the water-content of the soil 

 remains high. All evidence thus far is that the motion is a purely 

 mechanical one caused by the varying balance between the forces 

 of gravity, turgidity, and elasticity of the tissues. 



In addition to this seasonal motion, small plants which have been 

 kept in the green-house for a few months in moist soil show an 

 upward motion of from 2° to 20° during daylight and usually a 

 downward motion during the night. In two cases hot-house-grown 

 plants have shown a slow rise during the daylight hours of several 

 days until a vertical position was reached, and then a sudden drop 

 through 180° to the opposite side, and then again a slower rise to 

 the vertical, the whole story being repeated several times. By the 

 use of mirrors and shades it has been shown nearly conclusively that 

 this upward motion takes place only when the upper half of the 

 stem is exposed to direct sunshine ; a downward motion was induced 

 by shading the top and reflecting sunlight on the lower half. These 

 facts, together with the shape of the curve of bending, seem to indi- 

 cate that this daily motion is due to a slight shrinking of the tissues 

 on the illuminated side, caused by the faster evaporation on that 

 side, and a consequent unequal set of stresses which tend to erect 

 the stem. However, the rate and even the existence of this motion 

 are greatly influenced by several other factors, among which the 

 inter-relation of the moisture-content of the plant, air, and soil, the 

 temperature of the air, and the geotropic erection of the growing- 

 point have been shown to be important. The united operation of 

 these factors makes the experimentation diflScult to carry on under 

 controlled conditions. Relative transpiration of potted individuals 

 was measured under different conditions of water-content of both 

 plant and soil, a record being kept of the motions of the branches 

 at the various times. The amount of relative transpiration, as de- 

 termined by comparison with the Livingston atmometer, was found 

 to vary greatly with the water-content of plant and soil, the sun- 

 transpiration of a plant which had been kept well watered for 

 several months being 92 per cent greater than that of the same plant 

 when it had been three months without irrigation. In the first case 

 the relative transpiration was greater for the day than for the night, 

 but was a very appreciable amount for the night, while in the second 

 case the plant actually gained in weight, during the night, the same 



