CHANGES OF FORM PRODUCED BY CHEMICAL AGENCIES 115 



three to four atmospheres (= the same density of oxygen as in air under a pressure 

 of 14-19 atmospheres) commonly caused a retardation of growth, and usually 

 produced permanent injury when under a pressure of six atmospheres. The 

 experiments were performed upon ordinary terrestrial plants, and the results were 

 clearly due mostly to the action of the concentrated oxygen, for in air at a pressure 

 of three to six atmospheres growth is not retarded, and may even in some cases 

 be accelerated (Jaccard). The same result is also produced by pure oxygen at 

 atmospheric pressure (Wiesner), but a secondary maximum is not shown by 

 all plants. 



A diminution of the atmospheric pressure usually causes a distinct acceleration 

 of growth in strongly aerobic plants, and this appears to attain its maximal value 

 when the pressure falls to between a quarter and a seventh of an atmosphere. 

 Growth may then be accelerated to two or three times its original rapidity in soms 

 plants (Wider), while Jaccard observed that potatoes grew seven times more rapidly 

 under such circumstances. The result is due partly to the decreased density of 

 the oxygen, and partly to the decreased air-pressure. Experiments performed by 

 diluting air with indifferent gases show that a reduced partial pressure of oxygen does 

 act as an accelerating stimulus to growth, and the fact that different authors are not 

 in precise agreement as to the respective parts played by these two factors probably 

 shows that the duration of the experiments and the nature of the plant are of 

 considerable importance. Wieler found that a mere diminution of air-pressure 

 produced no effect upon growth, possibly because his experiments lasted for 

 a shorter time than those of Jaccard and Schaible. That duration is a factor 

 of great importance is shown by the fact that prolonged tension may at first 

 produce a retardation, but subsequently an acceleration of growth. 



It is possible that the decreased air-pressure acts by removing a portion of 

 the external pressure antagonizing turgor, and hence increasing the tension exerted 

 by the internal osmotic pressure upon the cell-wall. In any case, however, the 

 action is a stimulatory one, for a correspondingly increased tension of the cell-wall 

 does not produce any mechanical acceleration of growth. This mechanical theory 

 of Schaible's is in fact based upon an erroneous view as to the importance of turgor 

 in growth. Apparently, therefore, either a decrease of air-pressure or a diminution 

 of the partial pressure of oxygen may act as stimuli accelerating growth, although 

 if the time of exposure is short the period of induction may not be sufficiently 

 prolonged for the production of a perceptible result. [Vochting observed (Bot. 

 Ztg., 1892, p. 94) that a reduction of the percentage of oxygen to 3 per cent, 

 suppressed the formation of root-hairs on the roots of the potato.] 



SECTION 32. Changes of Form produced by Chemical Agencies. 



All growth is necessarily accompanied by the chemical changes 

 involved in metabolism, and it can be influenced to a greater or less degree 

 by chemical agencies of external origin. The quantity of food often 

 influences the shape and growth of a plant to a pronounced extent 1 . For 



1 Cf. Frank, Krankheiten der Pflanzen, 1895, 2. Aufl., Bd. I, p. 278. 



I 2, 



