28 THE PLANT WORLD. 



sections cooled quickly produced ice within the cells. The writer 

 has found this to be true also of Xitclla and Spirogyra cells. To 

 this is probably due the finding of ice within the cells bv the older 

 authors. In many of their experiments bits of watery tissue were 

 usually placed abruptly in the freezing mixture, or exposed di- 

 rectly to outside temperature. In the natural process of freezing, 

 however, the fall of temperature is so gradual that ice formation 

 within the cell does not occur. 



In considering the structure and arrangement of the ice masses 

 produced when plant tissue freezes it mav be best to commence 

 with those formed on the free outside surface of organs. 



As shown by Sachs and others, all moist substances, when 

 cooled very gradually and protected from evaporation, at length 

 become covered with an incrustation of ice. This crust possesses 

 a definite structure in that it is composed of innumerable little 

 prisms placed side by side palisade fashion, perpendicular to 

 the substratum. Perhaps the most familiar instance of this 

 sort is in connection with the freezing of damp soil. In little 

 cavities underneath boards or other covering which prevents 

 evaporation, we often find in winter ice crusts of considerable 

 thickness consisting of prisms set rather loosely together and 

 often more or less curved. If the soil is well protected, ice crusts 

 often one to two inches in thickness are formed, the individual 

 crystals of which are quite stout as compared with those formed 

 on plant organs. This is largely because the water in the soil is 

 not held with as much force as in plant tissue. Hugo V. Mohl* 

 has described an especially fine case occurring in the Black Forest 

 where the outer layer of the earth froze and was afterward raised 

 by an ice crust forming beneath it and drawing its water from the 

 unfrozen earth below. The crystals were from two to five cen- 

 timeters long, and from the thickness of a needle to that of a 

 goose-quill. Similar cases were described by ^^liiller-Thurgau, 



Sachs found that succulent plant tissue such as beets, turnips. 



* Hugo V. Mohl. Ueber die anatomischen Veranderungen des Blatt- 

 gelenkes, welche das Al^fall der Blrittor, herbeifiihren. Bot. Zeit. i8: i6. 

 i860. 



See also, Monthly Weatlier Review, U. S. Dept. Ag. 26: No. 5, 217. 1898. 



