H2 PLANT PHYSIOLOGY. 



72. Can mechanical resistance, or pressure, be overcome in 

 Growth, and if so, to what extent ? 



Answer by Experiment 41. 



EXPERIMENT 41. This may be settled by placing young growing 

 parts under conditions requiring them to overcome measurable 

 pressure in order to expand. Start six horse-beans in sphagnum 

 in the germination-boxes. When the roots have reached 3 or 4 cm. 

 in length, fit to each one near the cotyledons a cork pressure-jacket 

 made as follows: Take small corks of about i cm. diameter and one 

 half that length, and through their axes burn holes, about the diam- 

 eter of the hypocotyls of the beans; split each cork lengthwise into 

 two equal halves, and make all smooth. Around each cork place 

 a rubber band whose tension is such, as determined by hanging 

 weights to one of the halves, that the six corks form a series re- 

 quiring tensions of from about ten to about five hundred grams 

 to force them apart. Place these corks upon the beans, replace the 

 latter in the sphagnum, and examine from time to time to de- 

 termine which of the jackets are forced apart, and hence what 

 pressures can be overcome. 



The epicotyls of young Ricinus plants give good results with these 

 jackets. 



Do you know of any cases in Nature where mechanical pressure 

 is exerted by growing- parts? 



What is the probable physical basis of this power to overcome 

 resistance? 



E. Movements. 



A familiar fact about Growth is that it is accompanied by 

 movements other than those of mere increase of size. It is 

 important to ascertain whether these are determined by internal 

 or external influences, or by both. Some of these movements 

 are plainly responses to light, gravitation, etc., and these are 

 to be studied presently under the section on Irritability. The 

 question then remains whether there are movements connected 

 with the process of Growth itself and independent of external 

 stimuli. One way to test this is by determining whether, in 

 elongating structures, when the one-sided influence of light, 

 gravitation, etc., is shut out, the growth is in straight lines. 



