396 PHYSIOLOGY OF GROWTH. 



from Arsenic-free Zinc and dilute Hydrochloric acid. To purify 

 the gas, it is passed through vessels containing solutions of potash 

 and Potassium permanganate. The water intended for diluting 

 the Hydrochloric acid, and for making the purifying solutions, 

 may be boiled before being used and allowed to cool in closed 

 bottles. That our seedlings do not grow in the atmosphere of 

 pure Hydrogen is easily determined by measuring the distance 

 between the marks on the root, at intervals of a few hours, by 

 means of the horizontal microscope (see 153). If we open the 

 apparatus, the growth of the plants at once begins again. 



The seeds of different species of plants appear to behave 

 differently in germination towards pure Oxygen. 2 I found that 

 wheat grains germinate as rapidly in pure Oxygen as in atmo- 

 spheric air. The necessary Oxygen is prepared by heating a 

 mixture of Potassium chlorate and Manganese dioxide in a retort, 

 purified by passage through potash solution, and led into the 

 vessels provided, in the manner above indicated, with air-free 

 water and seeds. 3 



1 See Detmer, Landwirthschaftl. Jahrbiicher, Bd. 11, p. 225. 



2 The older literature is collected in my Vergleichende Physiologie d. 

 Kcimungsprocesses der Samen, 1889, p. 272. 



3 For details, see Wieler and Jentys in Untersuchungen aus d. lot. List, zu 

 Tubingen, Bd. 1, p. 216, and Bd. 2. 



161. The Influence on Growth of Pressure and Tension. 



From, the theory of growth it directly follows that pressures, 

 whose action on the turgescing cells consists in a compression of 

 the stretched cell parts (hyaloplasm and cell-wall), must retard 

 growth. On the contrary, tension of these parts will result in an 

 acceleration of the rate of growth. Similarly, pressures and 

 tensions are not without influence on the direction of most vigorous 

 growth. 



According to the researches of Scholtz 1 and Hegler, 2 which, 

 however, we cannot examine in detail, the acceleration of growth 

 by tension is by no means always in correspondence with its 

 mechanical equivalent, because if stems, for example, are subjected 

 to tension, their growth at first becomes slower, and does not till 

 later undergo an acceleration corresponding with the mechanical 

 equivalent of the tension. The tension at first acts as a stimulus 



