252 FUNDAMENTALS OF FRUIT PRODUCTION 



twig, prying apart the cells of the cortex in which it lies. The outer cylinder 

 of cortex in such twigs is completely separated from the inner layers when frozen. 

 In a few species instead of the continuous layer, lens-shaped ice masses are inter- 

 polated irregularly throughout the cortex. The cortical cells after the withdrawal 

 of water are as completely collapsed as were those in the bud scales, but they also 

 usually regain their normal condition on thawing. In the wood ice rarely forms 

 in large quantities. It is usually confined to small masses in the vessels them- 

 selves, or, according to some authors, sometimes extends in radial plates in the 

 pith rays. In sectioning twigs I, m.yself, have never seen ice in the wood else- 

 where than in the vessels or wood cells. In the pith the ice, so far as I have 

 been able to observe, always occurs within the cells and therefore in very small 

 masses." As Wiegand points out, Miiller-Thurgau found ice in the large vessels 

 and frequently in the wood cells of pear and most distinctly in the grape. 



Frozen twigs of several species were found to expand on thawing, two apple 

 twigs, for example, increasing in diameter from 2.97 milUmeters and 3.89 milli- 

 meters to 3.03 millimeters and 3.95 millimeters respectively. In the willow, the 

 only species on which this determination was made, more than half the total 

 expansion was in the bark, the percentages being, respectively, 13.5 for the bark 

 and 2.5 for the wood. To explain the contraction of twigs on freezing Wiegand 

 suggests: "When the water is extracted from the walls of the wood-cells, the 

 latter contract to a slight extent just as they do when wood seasons. This ac- 

 counts for a part of the shrinkage. The rest and greater part occurs in the cortex. 

 Here the intercellular spaces are quite large and numerous and are normally 

 filled with air. When freezing occurs the ice forms in the spaces and the cells 

 collapse while the air is mostly driven completely out of the twig. The contrac- 

 tion in the cortex will be approximately equal to the volume of the air expelled 

 plus that of the air compressed minus the expansion of the ice while freezing." 

 Curiously enough in all cases studied, except in Populus and Acer and including 

 apple, pear and plums, Wiegand found that buds increased decidedly in size upon 

 freezing. Prillieux^^^ demonstrated conclusively a loss of air and of weight in 

 frozen plant tissues. 



Freezing, Not Cold, Kills. — Most investigators do not accept the 

 view that, aside from some cases occurring above the freezing point, 

 absolute cold kills any plant, whether by "shock," "cold rigor" or other 

 effects. Again quoting Wiegand: "Most plants are killed by the first 

 ice formation within the tissue. If they survive this, a considerably 

 lower temperature is required to kill them, or they may be capable of 

 enduring any degree of cold. It has been demonstrated . . . that, in 

 the case of delicate tissues at least, death occurs when the ice formation 

 has progressed to a certain extent. . . . Death seems due to the actual 

 withdrawal of water to form ice, not to the cold. The ice formation 

 dries out the cells and the plant suffers therefore from drought con- 

 ditions. Every cell has its critical point, the withdrawal of water 

 beyond which will cause the death of the cell, whether by ordinary 

 evaporation or by other means. It may be supposed that the delicate 

 structure of the protoplasm necessary to constitute living matter can no 



