36 CELL-CONTENTS AND CELL-WALLS. 



accumulated, so that the cells have been disturbed and 

 thrust asunder. As the tissue freezes water is drawn from 

 the cells, and this on freezing collects as films on the walls 

 abutting on the intercellular spaces. As the water is 

 withdrawn from the cell- sap these films accumulate and 

 cause disruption of the tissue, the cells at the same time 

 shrinking. 



(o) From experiments like the preceding, it has been 

 suggested that death from freezing is really due to the 

 resulting withdrawal of water from the protoplasm, and 

 that unless and until ice-formation occurs the cold is not 

 fatal that for sudden death on cooling ice-formation is 

 essential, whether it acts on the protoplasm directly or 

 indirectly. The reverse, however, is not true, for many 

 plants readily recover after being frozen solid. It has 

 also been suggested that it is only on the thawing of the 

 cell that the fatal disorganisation occurs, and that if thaw- 

 ing proceeds slowly recovery may take place even in cases 

 where quick thawing would lead to death. But this view 

 has been disproved by experiments, of which the two 

 following should be made. 



(1) The cells of the red sea-weed Nitophyllum, on being frozen 

 to 5C., show orange-red fluorescence; in the living cells the 

 pigment shows no fluorescence, and its appearance is a sign of death. 

 (2) Treat in the same way the leaves of the commonly cultivated 

 Ageratum mexicanum ; on freezing the characteristic smell of 

 coumarin is perceived. This aromatic substance, which occurs in 

 Sweet Woodruff and some other plants, is not present in living 

 Ageratum leaves, but is produced on the death of the cells. 



IV. PROTEINS AND THEIR DIGESTION. 



37. Vegetable Proteins. Various proteins occur in 

 plants. The proteins contain Carbon (50 to 55 per cent.), 

 Hydrogen (about 7 percent.), Nitrogen (15 to 20 per cent.), 

 Oxygen (about 20 per cent.), and Sulphur (O'l to 2 per 

 cent.). The nucleoproteins and phosphoproteiiis contain 

 Phosphorus in addition to these five elements. The pro- 

 teins show various reactions in common. All, except the 



