Biological Papers. 99 



position. At one o'clock the jar was removed. In a few minutes 

 the blades hung limp on their pulvini, showing that the stomata 

 were all open. The change was so sudden that they were not able 

 to close, as they would under' normal conditions. Transpiration 

 went on, therefore, and the leaf had soon given up too much water 

 to stand erect. 



Three plants in the open on which measurements had been 

 taken turned their leaves each day, at about eleven o'clock, into the 

 hot-sun position, with angles ranging from eighty-five to ninety 

 degrees, the ground being dry. A heavy rain came on and soaked 

 the soil, and for the next two days the leaves did not take up the 

 hot-sun position, although the temperature was the same, twenty 

 to twenty-three degrees centigrade, and the light strength the same. 



A potted plant in the window turned its leaves each day to the 

 light in the hot-sun position. Then the plant was supplied with 

 plenty of water. During the next two days the leaves did not as- 

 sume the hot-sun position. The temperature in each case was 

 about the same, and the light readings for the second day showed 

 more light than before. 



From these experiments the conclusion is that the hot-sun posi- 

 tion depends on the amount of moisture in the soil and not on the 

 condition of the atmosphere, and that the opening and closing of 

 the stomata depend on the relative humidity of the atmosphere. 



As will be more clearly shown later in the accounts of the ex- 

 periments, nutation took place in almost the entire stem in the 

 young plant, being most noticeable in the pulvinus and decreasing 

 towards the roots. As the plant grows older the region of nutation 

 grows less and less. The pulvinus is so arranged as to permit the 

 leaf to twist in any direction, so that the blade may stand at almost 

 any angle to its petiole. 



The movement originated in the leaf blade and extended from 

 there down the stem. That the leaf was the source of the move- 

 ment was proven by shading the part below the lamina, when the 

 nutation proceeds the same as before. When the lamina above 

 was shaded between dark papers there was no nutation. This, 

 agrees with Schaffner's experiments on Heliantlius. 



A number of experiments were made to find whether the move- 

 ments of the plants and leaves depend on the light only, or whether 

 they simply had a regular daily movement. A few experiments 

 were sufficient to show that the plants would move in any direction 

 to face the light, and that light alone was the cause of the normal 

 day movements. 



