141 



the c'a])illaries, water from witliiii would fill the latter 

 again and a new layer of ice would be formed as before. 



Caspary (1854) suggests that, at freezing temperatures, 

 for one reason or another, an exceptionally abundant 

 amount of sap might ascend the plant through the vessels. 

 That sap would traverse the walls of the vessels to freeze 

 outside. 



Sachs called the attention of the biologists to the fact 

 that a body imbibed with a liquid is always surrounded by 

 a film of that liquid. To show this, he covered with var- 

 nish a piece of a pig's bladder membrane and dipped it 

 into water; the varnish, which before imbibition was ad- 

 hering to the membrane, became loose, on account, says 

 the author, of the formation of a film of water between the 

 membrane and the varnish. Applying this principle to 

 the present problem he assumed that the film of w^ater 

 present on each w^et surface freezes first and that upon 

 formation of a new film, it behaves as a removable layer. 



Prillieux (1869a) objected to Le Conte that the size of 

 the ice columns did not correspond to that of the capil- 

 laries. He objected to Sachs that a surface film of capil- 

 lary size should resist congelation instead of initiating it. 

 He furthermore remarked that the expansion of water 

 between 4° and 0° is not sufficient to explain the exudation 

 observed. Finally he proposed the following explanation. 

 Water is held in the living cells or in the boiled white of 

 egg by the forces of imbibition; the molecules of water 

 w^hich are farther away from the imbibing molecules and 

 are not so strongly attracted by the latter, leave them and 

 freeze. The crystals formed in that manner grow by 

 attraction of new molecules of water. So, as we under- 

 stand the author's interpretation, the columns of ice origi- 

 nate in larger intermolecular spaces and in pores. 



It seems that, during the first half of this century, the 

 attention of the biologists has been attracted by other 

 questions and that the problem of the mechanism of these 

 particular ice formations has been left unsolved. 



6. Tissues. luterceUular Freezing. In 1817, du Petit- 



