230 
F. J. Lewis and G. M. Tuttle 
such as a hormone, may be responsible for initiating the change and 
further work to test this hypothesis will be carried out. 
Sinnott( 5 ) has recorded the fact that change of temperature alone 
is quite insufficient to account for the seasonal change in food 
reserves and records that these changes occur in trees growing in the 
frostless area of the Gulf of Mexico. 
Further it has been found that the position of the chlorophyll in 
the cell varies with the temperature. At moderate temperatures of 
about o° F. the chlorophyll remains irregularly diffused in the cell, 
but showing a tendency to collect in the neighbourhood of the 
nucleus. At extremely low temperatures of — 20° F. or below, it 
becomes closely retracted round the nucleus, whatever position the 
latter may take up in the cell, to such an extent as completely to 
mask the nucleus. On a rise of temperature the chlorophyll tends 
again to become more diffuse, although the change of position may 
take more than 24 hours. 
The browning of the chlorophyll during the winter season is a 
very marked feature, and it is even noticeable in leaf sections under 
the microscope as well as in the foliage. 
Experiments have been carried out to ascertain what part, if 
any, light plays in the conversion of the chlorophyll from the winter 
to the summer condition as well as the first appearance of starch in 
the spring. For this purpose a number of branches of Picea were 
covered during February and March with bags of closely-wo veil 
black material made in double thickness, the mouth of the bag being 
tied tightly round the branch. Trees were selected growing in different 
aspects, some exposed to sun and others in a shady position on the 
northward facing slopes of the Saskatchewan Valley. Some of these 
observations are recorded in Table II. The light-tight covering was 
placed on lateral branches of the same main branch used as a control, 
so that in all cases the experiment and control were side by side. 
Leaves were withdrawn for examination during February, March 
and April without the bags being removed. 
No change took place either in experiment or in control until the 
critical date of April 5, when starch appeared both in leaves exposed 
to the light and those in darkness. 
Both in laboratory experiments described in a former paper (2) 
and in field experiments, the first-formed starch in the spring is not 
the direct result of carbon assimilation, as it appears in the dark at 
the same time and in the same amount as in leaves in light. 
Complete reorganisation of the chloroplasts in the field experi- 
