WINTER KILLING AND HARDINESS 251 



intercellular spaces, frequently making lens-shaped masses of hexagonal 

 crystals, larger at the side which draws on the greater number of cells. 

 As the growing ice needles deplete the water of the intercellular spaces, 

 more is drawn from the cell contents.^^^ The continuance of this process, 

 however, makes the sap remaining within the cells more concentrated 

 and thus increases "the force with which the remaining quantities of 

 water are held."^^"' A still stronger force, operating as a reserve, is that 

 known as molecular capillarity, holding with extreme tenacity a certain 

 amount of water of imbibition. Hence, it is with increasing difficulty 

 that ice formation continues and it must cease sooner or later unless the 

 temperature be lowered further. Moreover, the very process of solidi- 

 fication liberates a certain amount of heat. Therefore it is not surprising 

 that, as the temperature falls, the ice formation for each degree becomes 

 progressively less. Mliller-Thurgau found, at 4.5°C., 63.8 per cent, of 

 the water of an apple frozen, while at -15.2°C. only 79.2 per cent, had 

 frozen. 214 Wiegand-^^ found very little ice in dormant twigs of many 

 species of forest trees at 20°F. At 0°F. ice was plainly visible in buds of 

 19 species out of the 27 examined"; 6 of the remaining 8 showed ice, 

 but in small scattered crystals, at — 15°F. These buds "all contained 

 little cell-sap and small cells with rather thick walls." 



As the ice crystals increase, the cell walls collapse and become packed 

 together in dense masses. Buds and bark of hardy trees show this 

 condition, as do evergreen leaves, but at suitable temperatures they 

 expand, draw back the water and become normal. 



Wiegand"^ reports: "The ice was found to occur always in broad prismatic 

 crystals arranged perpendicular to the excreting surface; and usually formed a 

 single continuous layer throughout the mesophyll of the scale or leaf, to accom- 

 modate which the cells were often separated to a considerable distance. This 

 ice sheet was composed of either one or two layers of the prismatic crystals, 

 depending on the water content of the adjacent surfaces, and was often as thick 

 as the whole normal scale. The cells surrounding the ice, having lost their water 

 content, were in a more or less complete state of collapse, depending upon the re- 

 sistance of the walls, and often occupied a space smaller than the ice itself. These 

 cells were uninjured, however, and would resume their normal condition on 

 thawing. ... In young anthers the ice often filled the entire anther cavity 

 and in it the pollen grains were imbedded in a completely collapsed state." 

 At temperatures between — 23.5°C. and — 18°C. in the apple and pear the 

 tissue was "packed full of ice in shoot and in the mesophyll of the scales." In 

 general, the species in which ice formed most readily had larger cells, a higher 

 water content and a greater proportion of water to cell wall and protoplasm. 



"In the twigs," Wiegand states, "ice is also present in very cold weather, 

 where it may be found in three different localities. The largest quantity occurs 

 in the cortex, where the ice crystallizes in prisms arranged in single or double 

 series according to the law of freezing tissues. The ice is more frequently in 

 the form of a continuous ring, or really a cylinder, extending entirely around the 



