Plant Cells at Low Temperatures . 413 
Condition of Cell Contents at Low Temperatures. 
Pfeffer (12), in describing the work of Mohl, states that the brownish- 
green colour of evergreens during winter is due to a partial disorganization 
of the chloroplastids, accompanied by certain changes in the chlorophyll 
pigments. In some cases the browning only takes place when the cells are 
fatally affected, but with the restoration to normal conditions of light the 
chloroplastids in most conifers recover, turn green, and become functionally 
active. 
During the progress of the present investigation we decided to examine 
the condition of the leaf cells of some of the material with a view to deter- 
Fig. 1. Picea canadensis. Feb. 21. Mesophyll cells showing ‘laking ’ of the chloroplast. 
Identity of individual chloroplasts completely lost, nuclei are prominent and very granular, nn. = 
nucleus, ppl. = protoplasm, chi. = chlorophyll. 
mining whether similar features prevailed in this region. Microscopic 
examination of the mesophyll cells of Picea reveals a distinct localization of 
the cell contents during winter (Fig. 1). In this condition, which pre- 
vailed during the coldest weather, the identity of the individual chloro- 
plast was completely lost. The diffuse chlorophyll mass, light yellowish 
green in colour, was segregated in a relatively small portion of the cell, 
closely associated with the very granular nucleus. The major portion of the 
cell was occupied by a large vacuole filled with fat (Fig. 2). All trace of 
starch disappears early in the autumn. 
Leaves were examined frequently during the late winter and early 
spring, in order that the change to the summer condition might be observed. 
It was found that the change began quite early and took place within a short 
time. Cells examined on April 7 differed in several particulars from 
those of earlier preparations. The chlorophyll mass, though distinctly 
localized, gave evidence of the formation of distinct chloroplastids. Starch 
formation had commenced, but was localized in the region of the chloroplasts 
(Fig. 3). The central vacuole, though smaller, still gave a strong reaction 
when tested for fat with osmic acid. Three days later the change 
