38 PRACTICAL COURSE IN BOTANY 



jurious to plants or parts of plants when full of sap (water) 

 than when dry. This, in the opinion of the most recent 

 investigators, is because the water in the spaces outside the 

 cells freezes first and as moisture is gradually withdrawn 

 from the inside to take its place, the soluble salts which may 

 be present in the cell sap become more concentrated, and by 

 their chemical action on the contained proteins cause them 

 to be precipitated, or " salted out," as we see sugar or salt 

 precipitated from solutions of those substances when water 

 is withdrawn by evaporation. In this way, it is believed, 

 the fundamental protoplasm of the cell may be so disorganized 

 that death ensues if the freezing is continued long enough, 

 since the protein precipitates become " denatured " and cannot 

 be reabsorbed if kept in a solid state too long. The length of 

 time necessary to produce death from this cause is, of course, 

 different in different plants, according to the kind of salts 

 dissolved in the sap and the nature of the proteins acted on 

 by them. The proteins in the sap of Begonia, or Pelargo- 

 nium, plants which are very sensitive to cold, yield a dena- 

 tured precipitate at, or a little below the freezing point of 

 water, while those of winter rye withstand a temperature of 

 -15 C., and of pine needles, -40 C. 



Mechanical injury through rupture of parts by freezing 

 is not apt to cause serious damage except in cases of sudden 

 and violent cold at a time when the tissues are gorged with 

 sap, as not infrequently happens during the abrupt changes 

 of temperature which sometimes occur in spring after the 

 trees have put forth their leaves. In an extreme case of 

 this kind, the writer has seen the trunk of an oak a foot 

 or more in diameter split in deep seams from the effects 

 of freezing. 



34. The length of time during which seeds may retain 

 their vitality. No direct experiment can be made to test 

 this point, since it would require months, or even years, 

 covering in some instances more than the lifetime of a genera- 

 tion. It has been stated on good authority that seeds of the 



