41 8 TRANSFORMATION OF ENERGY 



LECTURE XXXIII 

 MOVEMENTS DUE TO TURGOR AND GROWTH 



So far as the movements referred to in the last lecture are concerned the 

 presence or absence of protoplasm is of no consequence, for they take place just 

 as well in dead as in living organs. As a rule, as soon as desiccation commences 

 in a tissue the protoplasm dies, and it is only in plants which can endure com- 

 plete desiccation that hygroscopic movements maybe repeated again and again, 

 the plant still remaining alive. This is true of most mosses, and, among higher 

 plants, in Selaginella lepidophylla, which undergoes alterations in form very 

 similar to those described as occurring in Anasiatica. [The branches of many 

 forest trees also exhibit movements of this character, often due to periodic altera- 

 tions in the amount of water present in the cell-walls (Ganong, 1904).] We 

 have now to study movements which are possible only in the living plant, move- 

 ments which are not due to swelling or shrivelling of the cell-membranes nor 

 yet to tension in these membranes induced by the evaporation of imbibition 

 water. The causes of such movements, apart from locomotory movements 

 (Lectures XLII and XLIII) lie rather in alterations in the cells, in which both the 

 walls and their hving contents participate equally — alterations which are condi- 

 tioned either by osmotic pressure or growth in the cells, with both of which 

 phenomena we have already made acquaintance. It will be necessary for us, 

 however, to consider these phenomena somewhat further in detail. 



We have already seen how osmotic pressure is brought about ; we have also 

 seen that in plasmolysis we have a method of determining this pressure, which 

 has this great advantage that we do not need to know what the substances are 

 which are present in the cell-sap and produce this pressure. All we have to do 

 with here is the amount of osmotic pressure and how it acts on the cell-mem- 

 brane. If we assume to start with that the osmotic pressure is insufficient to 

 stretch the cell-wall we may also conclude that the plasmolytic solution is of 

 the same concentration as the cell-sap. If a 2 per cent, solution of potassium 

 nitrate produces plasmolysis then we may conclude that the cell-sap has the same 

 osmotic value, although the cell-sap may consist of a mixture of all kinds of 

 substances such as various sugars and organic acids, &c. Strictly speaking, 

 the plasmolytic method gives us always rather too high a value, for if there be 

 an obvious retraction of the protoplasm from the cell-wall, the plasmolysing 

 liquid must have a somewhat higher value in terms of potassium nitrate 

 than the sap. When we have estimated the concentration of the cell-sap in 

 terms of potassium nitrate we can then calculate the osmotic pressure in the 

 cell, since it is known that a i per cent, solution of potassium nitrate 

 ( =0-1 GM.) exerts a pressure of 3-5 atmospheres. With the aid of the table of 

 isosmotic coefficients we are able to calculate the osmotic pressure value of any 

 other solution we please. As a matter of fact, a potassium nitrate solution is 

 peculiarly convenient for plasmolytic experiments, and very many investigations 

 have been carried out with its aid. The following data with reference to the 

 amount of osmotic pressure in different vegetable cells are taken from Ryssel- 

 BERGHE (1899, p. 23) : — 



Pepcromia (hypoderm of leaf) 

 Plantago amplexicaide (peduncle) 

 Phycomyces (hyphae) 

 Sorbus auctiparia 

 Foeniculum ^ peduncle) 

 Helianthus (medulla) 



