GROWTH IN TREES. 19 



The daily procedure in elongation during September may well be 

 illustrated by the following excerpts from the notes: 



The temperature of the air varied between 13 and 22 C. during the greater 

 part of the time, this equable feature of the climate being accompanied by 

 fogs and high humidity. 



The record of the first day was made on a 24-hour clock cylinder to give 

 opportunity for minute analysis. This showed that the stem contracted as 

 much as 0.3 mm. in the forenoon period, which in this case was clear and sunny, 

 and in the periods ending the next morning an increase of 12 mm., or a net 

 total of 8 mm., was made. This behavior was duplicated in the following 10 

 days, during which time the period of elongation of the stem came to a close. 

 Greater activity being displayed by the laterals, attachment was transferred 

 to one of these and a similar procedure was recorded for 7 days, at which time 

 it also came to a state approaching quiescence on September 8. 



On October 15 the instrument was attached to the lateral of another plant 

 continuously in the shade and the record for the succeeding 20 days shows 

 daily variation, including shortening at midday, but with little total growth 

 except on days on which the temperature of a large trunk rose to 17 or 18 C., 

 or the air was humid by the effect of fog or rain. Such a case was produced by 

 fog on October 21 and 22, which were followed by a period of a week of clear 

 days with trunk and air temperatures not widely different in a range from 15 

 to 22 C. during which time the daily variations did not result in any actual 

 increase. 



The influence of humidity and water supply is illustrated still more strikingly 

 by the behavior of No. 5, an unbranched plantlet 40 cm. high, probably about 

 16 months old. Attachment was made to the tip of this plant on September 

 8 and on the two succeeding days a midday decrease and a following larger 

 increase of 0.5 mm. or less were displayed. Beginning at midday on the 

 llth, rain fell continuously for 50 hours and an enormous acceleration ensued. 

 The temperature during this time was 19 to 20 C. The total increase during 

 this two-day period was over 10 mm., which was 5 or 6 times the previous rate, 

 in cool, dry days and cooler nights. On the day following the rain the increase 

 fell to 2.5 mm., after which the daily program of elongation and shrinkage 

 followed, with a net increase, however, of 2.5 mm. in 13 days, the rate diminish- 

 ing on September 28, at the end of this period, which probably soon passed 

 into the condition already described for No. 3. This record illustrates 

 markedly the accelerating effect of humidity on growth and suggests that 

 warm rainy periods of even a few day's duration may have a permanent effect 

 on the structure of the trunk. 



These features are in accordance with a procedure which has been studied 

 and described in great detail in a large number of morphological types of 

 plants. Excessive or maximum water-loss during the daylight period lessens 

 the rate and amount of enlargement in growing stems and causes fluctuations 

 in volume of quiescent ones. 



It is to be noted that the imbibitional conditions in stems a year old are 

 much different from those of large trunks. Thus, in such a stem the epidermis 

 is still unbroken, the cortex is chlorophyllose, over a millimeter in thickness, 

 and the numerous long leaves, which still are present, may withdraw a large 

 amount of water. The growing layer is nearly as thick as the cortex, while 

 the central pith is 3 or 4 mm. in diameter, the rays through the wood being 

 much in evidence. One-third of the diameter of a stem 24 mm. in thickness is 

 thus made up of thin-walled cells in a high state of hydration and in a condition 

 in which the water balance of the system may be quickly modified by evapora- 

 tion or temperature changes. 



