355 
Vegetation and Frost. 
I propose in this article, therefore, to give some account of the 
present state of our knowledge on this matter, which was materially 
advanced, not long ago, by the publication of the detailed results of 
Lidforss’ investigations on the plants that remain green through the 
winter (the “winter-green” flora) in South Sweden. 
A critical account of the earlier literature upon the subject 
does not come within the scope of this article and we will begin 
with the pamphlet published by Molisch a dozen years ago and then 
pass to the work of Mez, 1905; Gorke, 1906; and Lidforss, 1907. 
The various works by Muller-Thurgau on this subject are of great 
importance, but published in 1880, 1882 and 1886, they are hardly 
recent enough for detailed notice. 
The well established fact that Yeast, Bacteria, and some moulds 
can withstand prolonged exposure to the cold of liquid hydrogen, 
teaches us that there is no primary essential injurious effect in 
chilling protoplasm. 
Nevertheless most sappy parts of the higher plants are killed 
by cooling much below 0 n C. We have to enquire what this injurious 
indirect effect of cold may be and why some plants are so much 
more resistant than others. One general secondary effect of cold 
is the formation of ice within the tissues of the plant, and another 
is alteration of the balance of the chemical changes that are going 
on in the cells. As a tertiary effect we have the dessication that 
results on withdrawing water as solid ice and this again might give 
rise to a quaternary class of effects due to the very concentrated 
solutions of the cell-constituents in which the protoplasm would be 
bathed in a partly frozen cell. 
To Molisch we owe the first direct observations, with microscope 
and freezing mixture, of the formation of ice in plants. He noticed 
that in the living plant ice-formation does not always take place 
immediately the temperature has fallen to the freezing point of the 
sap, which of course is below zero just in proportion to the amount 
of dissolved substances in it. Indeed he found a good parallel with 
the phenomena observed on freezing salt-solutions in general. 
Thus just as drops of water suspended in olive oil may be over¬ 
cooled many degrees below zero without ice-formation, till suddenly 
they solidify completely and the temperature rises to the true 
freezing point of water, so the hairs of Tradescantia stamens may 
be cooled to —6’5"C and then suddenly the cells fill with crystals of 
ice. Further as water in a fine capillary tube *3 mm. diameter 
