392 BOTANICAL GAZETTE [JUNE 
With bark on twig the diameter, 7.80™™ expanded to 8.04™™ on thawing; 
difference o.24™™. With bark removed from a small spot for the clamps of the 
measuring instrument, the diameter, 5.05™™, expanded to 5.15™™; difference, 
Lon 
Therefore more than half of the total expansion was in the bark. Thickness 
of the bark was 0.5 ™™ on each side; thickness of the wood and pith, 2.05™™ 
on each side; expansion of the bark, therefore, was 13.5 per cent.; of the wood, 
only 2.5 per cent. 
With thicker twigs, containing more hard wood, the expansion 
would have been still less. 
Where the bark was whittled away entirely around the end of 
the twig and for some distance back, the expansion of the wood 
was not detected; probably because water had passed to the bark 
to freeze and being removed there was none to cause swelling again 
when the twig thawed. 
The explanation of the contraction of twigs on freezing probably 
lies in the following considerations. When the water is extracted 
from the walls of the wood-cells, the latter contract to a slight extent 
just as they do when wood seasons. This accounts for a part of 
the shrinkage. The rest and greater part occurs in the cortex. 
Here the intercellular spaces are quite large and numerous, and 
are normally filled with air. When freezing occurs the ice forms 
in the spaces and the cells collapse, while the air is mostly driven 
completely out of the twig. The contraction in the cortex will 
be approximately equal to the volume of air expelled plus that of 
the air compressed minus the expansion of the ice while freezing. 
This: is for contraction in all directions; only a portion of this will 
be radial, depending upon the structure of the particular species; 
much the greater part, however, is radial in all twigs. 
With buds the study is not quite so easy. The record of buds 
measured at —18° C. and then again after thawing is shown in the 
adjoining table. 
From this table it will seem that in all cases, except in Populus 
and Acer, there was a decided increase in size on freezing and a 
consequent decrease when thawing out. In the two named cases 
there was a slight contraction as in the twigs. It is not quite clear 
why the buds should behave so differently from the twigs. The 
only explanation I can offer at present is that the contraction of 
