136 BuLi^ETiN 79 



relatively lower freezing point. Corn, with a cooling point of 

 — -7.35°C. (18.77° F.) and a freezing point of —2.60° C. (27.3° F.), 

 and fruits of grapes, with a cooling point of — 7.85° C. (17.87° F.) 

 and a freezing point of — 3.10° C. (26.4° F.), may be cited as 

 examples. 



The above data follow very distinctly the laws of the freezing 

 of solutions, as stated by Raoult, who says: "If one dissolves 

 in any selected solvent equimolecular quantities of different sub- 

 stances, the freezing point is lowered the same amount in all 

 cases." Also, " the lowering of the freezing point is proportional 

 to the size of the molecule of the dissolved substance."* 



The different temperatures at which cell-sap freezes is here 

 very apparent. Corn and grapes contain a high percentage of 

 organic and inorganic matter in the water of the vacuoles. It 

 is the great amount of sugar in the grape which gives to the fruit 

 so low a freezing point. , 



Opuntia maxima, with which Muller-Thurgau worked, and the 

 spineless cacti, with which the writer worked, on the other hand, 

 have freezing points very little below that of pure water. It follows, 

 therefore, that the dissolved matter in the large amount of water in 

 the cacti is very small. This is not surprising when we consider that 

 the water in these plants is principally storage water, which has 

 been gathered by the plants during the rainy seasons and which 

 enables the plant to carry itself through long droughty periods 

 such as are characteristic of the arid plains of the southwestern 

 parts of the United States and of adjacent Mexico. 



On the other hand, the water of corn, grape, and similar 

 plants, is imbibition water which serves to carry the inorganic solu- 

 tions from the roots to the young cells, and also assists in distrib- 

 uting different organic soluble substances from one part of the plant 

 to another. It also takes part in the synthesis of carbohydrates. 

 In other words, it is water that is used for the time being and must 

 be replaced regularly. This constitutes the important difference. 



Another factor worth noting is the length of time required to 

 convert all the water of the plant into ice, as is shown in the table 

 and in Figs. 6, 7, and 8. It is not the same with all species of cacti. 

 Opuntia fusicaulis took 45 minutes or more, and in one case 64 

 minutes, while 0. ficus indica, which was introduced from the island 

 of Sicily, required only 12 to 18 minutes. 



*See also W. Nernst, Theoretical Chemistry. 1899, pp. 116 and SOO. 



