1894. ] Crystals of Ice on Plants. 323 
c. Formation of crystals. 
1. The form and fabric of the crystals indicate that the 
latter grow, and that they are somatropic. The end of the 
crystals were first formed, and as the column grew, the pri- 
mary ice-layers were pushed out into the places where they 
are afterwards found. 
2. The central parts of the water-supply layers do not 
freeze. 
3. The formation of the crystals does not result from the 
organic nature: of the substratum; it depends entirely upon 
molecular forces. 
That capillary forces are active in carrying water from the 
ground up to where the crystals are formed may be seen @ 
Priort; but the capillary forces a/one cannot explain the phe- 
plicated. The contraction of the cells is one important factor, 
and the permeability of the membrane is another. 
_ Von Mohl’s explanation of the appearance of ice-crystals 
is “that the cold, before it pervades the branches, first 
Causes a contraction in the outer layers, by this contraction the 
fluids are driven out into the already formed fissure, and there 
it freezes.” That contraction of the layers in the lower parts 
of the stem of Cerasus does take place, I observed in the fol- 
lowing way. The lower parts of the stem, measuring I10™ in 
length, and about 4™ in diameter was cut off from the rest of 
the young tree, and kept for three days ata temperature of 18° 
©. On the fourth day this piece, which had been hitherto kept 
in wet rags, was dried and placed outside the window where 
the temperature measured —5°C. After a while big drops 
of fluid were observed on the cut surface of both ends, cover- 
ing the entire surface; this fluid rapidly froze, and formed, in 
ve hours, a layer of ice in the place where it had been forced 
cut. A similar experiment was made by von Mohl on 
Rhamnus. (Cf. Sachs, l. c., 14). The ice-column at the ends 
of the stem were of a slight white color; they containeda great 
number of minute air-bubbles. 
‘ While Atkinson thinks ‘‘that there is a degree of root ac- 
tivity which furnishes the necessary water,” and that ‘‘a spe- 
Cific variation in the root activity of different plants as re- 
lated to different temperatures” is a factor in the development 
of the crystals, Mac Dougal is of the opinion that capillary 
forces are the only active agents. In regard to the first opin- 
