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 8mm., 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 orrain. 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 
11th, 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 astem 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. 
