ICE IN PLANT TISSUE. 33 



the stem completely separating^ the outer layers from the parts 

 within. Small ice crystals usuallv occur within the dead pith 

 cells, within the vessels, and sometimes between the cells of the 

 pith-rays, as Miiller-Thuroau has shown, and as the writer has 

 often observed. 



The structure of the lenticular ice masses in beets, potatoes, 

 etc.. when the tissue has been forced apart to accommodate them, 

 has been studied by Prillieux and later in more detail by jNIiiller- 

 Thurgau. A\'hen examined in cross-section under the microscope 

 they are seen to be composed of two layers of prismatic crystals 

 which in each la}'er are packed closely together side by side. 

 The prisms are more or less regularly hexagonal, and the majority 

 contain at their axis a chain of minute air bubbles as did those 

 in the free ice-incrustation already described. In case the ice 

 mass comes from near the center of the beet or potato the two 

 layers of which it is composed are of nearly equal thickness 

 (Fig. 4, A). If, on the other hand, it comes from near the peri- 

 phery, then the outer layer of crystals is always much thinner, 

 the separate crystals being therefore much shorter (Fig. 4, B). 

 This decrease in thickness of the outer layer continues in pro- 

 portion as one progresses from the center toward the periphery 

 until in subepidermal masses only the inner layer remains. The 

 transition is therefore gradual from the two-layered structure to 

 the superficial ice crust of Sachs composed of the single series of 

 palisade-like prisms. A similar transition occurs in going from 

 the interior toward the simple ice crusts lining the walls of the 

 large cavities within the tissues. 



The cause of this variation in the two layers is probably not 

 due to their position in the organ, but to the amount and source 

 of the water at command. The ice crystals are added to in every 

 case at one end only. Consequently, if the tissues on both sides 

 of the ice mass are filled with available sap then the mass will 

 consist of two layers, and if this water is equally accessible on 

 both sides, that is, requiring the same force to disengage it from 

 the tissues in equal quantities, then the layers will be of equal 

 thickness. Water comes to these masses from quite a distance, 

 hence the character of the tissue round about must be considered. 



