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cells freezes first and as moisture is gradually withdrawn 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is withdrawn by evaporation.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 
