F. F. Blackmail. 
358 
the natural proteids can, by the addition of concentrated neutral 
salts to solutions of them, be “ salted out ” as precipitates, which at 
first can be redissolved on dilution but on standing become insoluble 
and “denatured.” It occurred to Gorke that when the plant is 
frozen and water is withdrawn from the cell, the salts present must 
become more concentrated and might well in time act on the soluble 
proteids of the cell and cause their precipitation. This expectation 
was realised and he found that barley which has been put out at 
—7"C for a night and been frozen solid, yields, on expression of its 
sap after thawing, a juice which contains only 2/3 of the amount of 
proteid in solution which the expressed sap of the unfrozen control 
culture of barley yielded. 
He went further and showed that the low temperature required 
to precipitate proteid from the expressed sap of different normal 
plants varied considerably; that it was mild in the case of those 
plants that suffered severely from cold, but that a very low temper¬ 
ature was required in the case of very resistant plants. Thus the 
freshly expressed sap of Begonia or Pelargonium (which are easily 
injured at —5 n C) yields a denatured precipitate of proteid at —3°C, 
in time, while winter-rye is only precipitated at —15°C and the 
sap of Pine needles not until —40"C. The production of this effect 
will of course vary with the nature of the proteids present and also 
with the nature and amount of the salts present in addition. Gorke 
made up solutions of egg-albumin with mixtures of salts like those 
in the cell and found that after freezing to —20 n C there was a large 
and permanent precipitate. All this work deals with the soluble 
proteids of the cell, but one may reasonably transfer the idea to the 
structural proteids of protoplasm to account for the killing effect. 
It had been previously observed that on lowering the temperature 
of solutions of certain acids like tribasic phosphoric acid, the acidity 
slightly increases, so that more alkali is required to neutralise it, and 
that therefore a mixture with alkali, neutral at ordinary temperatures, 
would develop acidity on cooling. Gorke found that the sap of 
plants also shows an increased acidity on cooling, which would of 
course help in precipitation and denaturation of proteids at low 
temperatures. 
We will now pass to Lidforss’ work on his Swedish winter-green 
flora. The plants which in mid-winter are to be found still bearing 
green leaves belong to a variety of ecological types. None of these 
appears to show any obvious adaptation for protection from winter 
