412 
Lewis and Tuttle. —Osmotic Properties of some 
than in comparatively large volumes is indicated by the work of Dufour ( 4 ). 
Direct observations on plant tissues have confirmed this. Thus Muller (2) 
found the lowest sub-cooling point without freezing of the grape was 
— 6.8° to — 7*8° C., while the real freezing-point of the sap was — 3-1° C. 
Dixon and Joly (10) found ice formation in the tracheides of Taxus began 
at — io° to — ii° C. In our experiments we first carried out observations 
on greenhouse plants, and these are recorded in Table V. 
Material 
leaves. 
Treatment. 
Table V. 
Freezing mixture. 
Maximum 
under- 
cooling. 
A of sap 
within cells 
of leaf . 
Impatiens 
Fresh material from green- 
house 
NaCl + Ice — io° 
- i *3 
— 1. 1 
Polypodium 
,, 
- 2.9 
— 2 -5 
Begonia 
>5 
- 2.8 
— 2-1 
Syringa 
— 2.8 
- 2-5 
Pyrola 
Killed by freezing in CO s , 
thawed, and tested 
Solid C 0 2 
- 3-5 
- 3 -i 
Pyrola 
Fresh leaves not previously 
killed 
— 32-1 
-31*65 
All 
these plants give a sub-cooling from - 
- 1'3° C. in 
Impatiens 
to —2-9° 
C. in Polypodium and the A with formation of ice from 
-I-I°C. 
in Impatiens to — 2-5° C. in Polypodium. 
Pyrola leaves show very different values. The material was gathered 
outside during a spell of — 20° F. weather. A thermometer graduated to 
o-i° and registering to — 40 0 C. was used in the Beckman apparatus, as the 
amount of under-cooling was too great to allow the use of the ordinary 
Beckman thermometer. 
The behaviour of the living and dead leaves gave results of some 
interest. The leaves were killed by freezing in liquid C 0 2 , rapidly thawed, 
and the amount of under-cooling and A determined. The fresh leaves 
under-cooled to —32-1° C., rising to —31-65° C. on the formation of ice in 
the tissues. The leaves previously killed with liquid C 0 2 , thawed, and 
tested in the Beckman apparatus using a freezing mixture of solid C 0 2 , 
under-cooled to —3*5° C., rising to —3*1° C. on the formation of ice in the 
tissues. Further observations will be carried out next winter by us using 
the leaves of other northern evergreens. That the condition of the cell 
contents during periods of low temperature determines the resistance of the 
tissue to cold is suggested in a paper by one of us in 1919 ( 11 ). Linnaea 
leaves naturally de-starchified are able to withstand the cold of a northern 
winter, whilst plants in which the fat has been reconverted into starch are 
quickly killed on exposure to temperatures of about — i9°F. ( — 28° C.). 
