GROWTH IN TREES. 7 
12. Estimates of the range of daily equalizing variations in a Mon- 
terey pine taken from bearings on a thin layer of cork external to the 
bast of a trunk which had ceased to grow for the season show that the 
diameter might vary 1 part in 1,750. That a large share of this varia- 
tion is due to changes in the hydration of the living cells is proved by 
the fact that when bearings are taken on the woody cylinder of the 
trunk internal to the growing layer the variation drops to 1 part in 
8,750 of the diameter. The actual change in volume, in the first 
instance calculated on the basis of a conical trunk 18 meters high and 
35 cm. in diameter at the base, would amount to about 400 cu. cm., 
of which not more than one-fifth, or 80 cu. em., is attributable to varia- 
tions in the wood. It is to be noted, however, that the change in the 
volume of the wood may by no means be taken to represent the water 
deficit in the wood. The woody mass is made up of box-like cells, 
which may include a bubble of gas, the water forming no more than a 
thin film on the wall of the cell and inclosing the gas bubble in the 
condition of extreme water deficit. The withdrawal of water through 
the walls of the cells, which are semi-rigid, increases the surface ten- 
sion of the gas bubble, which results in a slight lessening of volume of 
the whole mass, but in an amount that would constitute no more than 
a small fraction of the total of the water loss. 
13. Of 15 trees which were under dendrographic measurement in 
1920, one each of Pinus scopulorum, Citrus aurantica, and Parkinsonia 
microphylla made no enlargement during the year. Such occurrences 
are to be taken into account in estimations of the ages of trees from 
the annual layers. 
14. The greatest daily equalizing variations were shown by Fraz- 
inus, Pinus, Picea, Pseudotsuga, and Juglans, and lesser variations 
were displayed by Populus, Platanus, Fagus, Quercus, and Citrus. 
No available facts furnish the basis of an adequate explanation of such 
differences. 
15. The final effect of rainfall shown within a few hours is to accel- 
erate growth, but it has been repeatedly observed that actual shrink- 
age may take place while the rain is falling. This action can not be 
traced to any instrumental error. 
16. Irrigation of the soil which had a moisture content of less than 
6 per cent around the roots of a Monterey pine was followed within 
24 hours by progressive enlargement constituting growth at the base 
of the tree, and at a point 8 meters higher. The distance from the 
absorbing surfaces of the roots through which the added water supply 
must enter could not be Jess than 3 meters from the lower instrument, 
and the influence of the added supply was within the day felt at the 
upper instrument, 11 meters from the absorbing surfaces. It does not 
seem possible that water could have been conducted through the 
tracheids this distance within the given length of time. 
