126 INFLUENCE OF THE EXTERNAL CONDITIONS ON GROWTH 



He mostly employed the method of imbedding in plaster-casts, and measured either 

 the longitudinal pressure (Fig. 22) or the radial pressure (Fig. 23) by causing it to 



act against a spring. The pressure is 

 given by the distance between the needle 

 points at f, the spring having been pre- 

 viously tested by known weights. In some 

 cases the resistance was kept constant 

 during growth by means of a special ar- 

 rangement (Pfeffer, 1. c., p. 261). 



The turgidity was determined by 

 comparison with non-diosmosing osmotic 

 solutions, and the gradual disappearance 

 of the tension in the cell-wall is shown 

 by the fact that a root which shortens 

 on plasmolysis before imbedding, does 

 not do so if removed from the cast after 

 two or three 'days and tested. 



Water-pressure. Not only air-pressure 

 but also water-pressure is able to retard 

 growth. Thus a submerged plant can 

 only enlarge its air-spaces by overcoming 

 the pressure of the water outside, but 

 where air-spaces are absent and the plant 



is readily permeable, the pressure of the water exercises no perceptible result on 

 growth, for it acts equally on all parts inside and outside, and rapid readjustment 

 takes place to any change of pressure. Bacteria and other organisms grow in fact 



FIG. 22. Apparatus to show the force with which 

 a root elongates. w= radicle of seedling growing in 

 pot (a) filled with sawdust (b). It is imbedded in the 

 plaster-cast (c), but the apex in the separate cast (d\ 

 which is pressed against the spring (/), causing the 

 points at (_/"-) to approach. 



FlG. 23. Apparatus to show radial pressure exerted by a growing root. Lettering as in Fig. 22. 



in the greatest depths of the ocean 1 , and according to Melsens 2 yeast grows in 

 a nutrient fluid under a pressure of 8,000 atmospheres. 



1 Fischer, Centralbl. f. Bact., 2. Abth., 1900, Bd. vi, p. 58. 



3 Melsens, Compt. rend., 1870, T. LXX, p. 831. Cf. also Certes, Compt. rend., 1884, T. xcix, 

 p. in; Roger, Compt rend., 1894, T. cxix, rj. 963. Berthold (Mittheil. a. d. zool. Station in 



