]^S THE PLANT WORLD. 



mav be termed for convenience the " expulsion " theory and the 

 " attraction " theory. 



The first of these seems to have been the more popular in the 

 past and has been upheld by a number of plant physiologists. In 

 this case it is assumed that, in order to freeze in the spaces, the 

 water must be in those spaces before freezing begins ; in other 

 words, that the cell actively gives up water at low temperatures, 

 probably as a protection against ice-fcrmation within the cell. 

 Kerner* states the theory thus : " In order that the water shall 

 get from the interior of the cell into the adjoining intercellular 

 spaces a pressing and squeezing is necessary, and the pressure can 

 only proceed from the living protoplasm in the cell-chambers ; con- 

 sequently the process of freezing can be most correctlv repre- 

 sented in this way, viz., that the protoplasm becomes stimulated 

 and roused by the lowering of the temperature to transport a por- 

 tion of the water from the interior to the exterior of the cell by 

 means of contraction and pressure. What happens then is accord- 

 ingly not unlike the excretion of watery sap into the intercellular 

 spaces in the stimulated pulvini on the leaf-stalk of Mimosa ; but 

 the advantage obtained by the excretion of water in the two cases 

 is very dififerent. In the cooled leaves the benefit, of course, is to 

 be sought for in the fact that the living portion of the cells is 

 protected from destruction as long as possible by the production of 

 ice crystals in the intercellular spaces. If the water were forth- 

 with frozen inside the cells between the groups of molecules of the 

 living cell body and its wall by a few degrees of cold, fundamental 

 displacements and disorganizations of the groups of molecules 

 would be unavoidable. On the other hand, the ice crvstals on the 

 exterior of the cells do not produce such destruction." 



Whether cells do or do not extrude water is of course no longer 

 a question. It seems fairly well established that such is the case 

 in Mimosa and the sensitive staminal filaments of the C\"nare:e 

 (Pfefifer), and to the loss of turgidity thus caused, is probably 

 due the movement of the various organs. Water seems to be 

 extruded in nectaries, but there osmosis often seems to plav the 



* Kerner and Oliver, The Natural History of Plsnts, i: 541. 



