IIO THE PLANT WORLD. 



Such exudations as above described may be accomplished either 

 by the actual contraction of the protoplasm (cold rigor), or more 

 likelv bv a sudden change in permeability of the membrane to the 

 solute, either over its whole surface or at special points, or per- 

 haps both of these conditions may occur together. 



Although from the foregoing it appears that, in special cases 

 at least, water may be excreted by the protoplasm, and that such 

 excretion mav, in certain plants at least, be induced by the appar- 

 ently stimulating action of temperatures near the freezing point, 

 still there are some important objections to assuming this to be 

 the way in which all water gets out of the cell to be frozen. 



In the first place, the fact that a few cases are known where 

 this particular temperature stimulates the protoplasm in this par- 

 ticular way does not warrant us in assuming that all cells are stim- 

 ulated in a similar way at the same temperature. Such a universal 

 phenomenon does not seem probable. 



The first ice-formation in plant tissue does not take place at 

 o° C, nor even at the freezing point of the tissue, but at a tem- 

 perature several degrees lower, known as the over-cooling point 

 of the tissue. This point is usually lower in proportion to the 

 inaccessibility of the water. Therefore if the water is already 

 present in the intercellular spaces before freezing begins, the over- 

 cooling point will naturally be much raised or will entirely dis- 

 appear under such conditions. It seems necessary to assume then 

 that the critical stimulating temperature corresponds with the 

 overcooling point of the tissue, and it may even be assumed that 

 the overcooling point is determined by this critical temperature. 

 Since the overcooling point is nearly at the same temperature in 

 the majority of plants, the range of the critical temperature would 

 be still further limited, and therefore the theory becomes still 

 less probable, especially since all these phenomena may appar- 

 ently be explained in a much simpler manner. 



A third objection lies in the fact that similar ice crusts and 

 ice masses form in connection with dead material of various sorts 

 where there can be no question of the activity of the protoplasm 

 in excreting water. 



