Some Hydration Reactions and Growth. 153 



Another trio of sections from an older stem measuring 5 mm. in 

 diameter was taken at midday, and when immersed in distilled water 

 at 21 C. increased 7.5 per cent. The entire lot of observations con- 

 firms and supports the conclusion that the stems of Helianthus have 

 their hydration capacity more nearly satisfied in the morning than at 

 noon, when the leaves may be in a wilting condition. This fact would 

 inevitably have an important influence on the rate at which such stems 

 might elongate. 



The growth of stems of Helianthus was measured on stems growing 

 in the soil of a large bed. Heavy wooden bases were placed on the 

 surface of the soil of the greenhouse bench and the stems were brought 

 closely against this and fastened at the base of the growing internodes 

 in such manner that only the elongation above this point would be 

 registered by the auxograph, and the movements of the base due to 

 softness of soil or other features would have no effect. The growing 

 part consisted of one internode approaching maturity and a terminal 

 one less than 3 cm. in length. A fine wire loop was passed around this 

 and carried up over the arm of the auxograph lever. 



Temperatures were taken by thermometers with thin bulbs thrust 

 into the stems of similar plants within a few centimeters of the one 

 being measured. As an example of the rate, the older and the younger 

 internodes, together having a length of about 15 cm., increased 2.7 mm. 

 during an hour at midday at a temperature of 30 C., while in the 2 

 hours immediately afterwards, when, as will be seen, the stem of another 

 plant was showing shrinkage in thickness, the rate was but 1.2 mm. 

 per hour at 29 C. During the next 4 hours the temperature slowly 

 fell to 19 C., but the rate of elongation came up to 2.1 mm. per hour, 

 a fact plainly due to decreased water-loss. 



A similar behavior ensued on the following day, when the rate was 

 2.4 mm. per hour at midday at 29 to 30 C., then fell off to 1.4 mm. 

 per hour during the next two hours at 26 C., and then to 0.6 mm. per 

 hour during the following 2 hours. Such diminished growth might 

 be attributed to the falling temperature if it had not been observed 

 that a higher rate was shown at temperatures as low as 14 to 16 C. 

 This lowered rate in the afternoon was accompanied by a distinct 

 wilting of the leaves. 



An auxograph was now provided with a cork bearing hollowed to 

 fit against a stem about 15 cm. from the apex, and the stem was held 

 firmly in place, so that any variation in thickness would be expressed 

 by the free arm of the auxograph and traced on the revolving cylinder 

 by the pen. The daily action may be exemplified by the following 

 transcript from the notebook : 



"Feb. 8. The temperature had risen from about 16 C. in the morning to 

 23 C. at 10 a. m., at which time an increase in the thickness of the stem at a 

 point 15 cm. from the tip had been in progress for 4 hours. The pen was 



