Grafts et al. — 120— Water in Plants 



upon pressing, might easily rupture and yield a sap more nearly like that 

 existing in the vacuoles. It is difficult to see, however, how this would be 

 true for plants such as Taxus. Finally, there could be, as Walter suggests, 

 sufficient filtration of solutes in expressing viscous sap from killed tissue to 

 explain the similarily of osmotic pressures obtained. 



For purposes of comparison, some representative data to be found in 

 Dixon (1914, p. 177, 182) and in Thren (1934) are presented in Tables 

 36 and 37. It is clear that in most instances there is a considerable dis- 

 crepancy between the concentrations of sap from living and killed tissues. 

 Furthermore, the magnitude of the discrepancy varies markedly among the 

 different plants. 



Table 36. — Comparison of saps expressed from living and killed tissue 



(Dixon, 1914) : — 



Freezing point depression °C. 

 Plant Organ Living Killed 



Ilex aqiiifolium leaves 0.667 1.225* 



Hedera helix leaves 0.728 1.031** 



Iris germanica rhizome 0.450 0.829 



Pyrns mains fruit 1.507 1.919 



Citrus limonum fruit 1.033 0.588 



Solamim tuberosum tuber 0.523 0.588 



Vitis vinifera fruit 2.567 3.185 



Chamaerops humilis leaf 0.365 1.529 



Beta vulgaris root 1.473 1.761 



* Heated. 

 ** Desiccated, all others frozen. 



According to Thren the more compactly the leaf tissues are arranged, 

 the greater the difference expected between the two values. That is, Biixus, 

 with compact leaves, showed the greatest deviation. But this would not 

 seem to satisfy all of the results. 



In this connection, the work of Newton (1924) is recalled; he found 

 an inverse correlation between cold hardiness and volume of sap expressed 

 from living wheat leaves. The difference was attributed to a greater amount 

 of bound water and dry matter in the more hardy tissue. Unfortunately 

 no osmotic pressure values were determined. 



Table 27. — Osmotic pressure of sap expressed from living vs. killed {heat) leaves 



(data of Thren, 1934) : — 



Living Killed 



atm. atm. 



Buxus sempervirens 3.1 24.2 



Vinca minor 5.1 14.4 



Linaria cymbalaria 5.4 11.2 



Sedum reflexum 7.9 11.3 



Sedum album 6.2 6.2 



Also pertinent to the problem are data reported by Meyer (1928), the 

 result of sap expression studies on the leaves of Finns rigida, with special 

 reference to cold hardiness. The total water content of the leaves varied 

 but little throughout the year, so that the development of cold resistance 

 during the winter was not due to this factor. The proportion of sap ex- 



