208 PLANT PHYSIOLOGY 



to a horizontal position, an almost continuous bending takes 

 place. Similar to this is Elfving's observation* that when a 

 fully grown grass internode is revolved horizontally on a 

 clinostat, its meristem resumes its activity, its cells divide 

 uniformly under the stimulus of gravity applied successively 

 to all its parts, and the internode passes through another 

 period of growth in length. 



The force which a geotropically bending organ exerts is 

 manifestly considerable, e.g. that employed in restoring a 

 prostrate grass haulm to the vertical position. The force 

 thus exerted is developed by the growing and bending part, 

 and we have seen (p. 176) that growing organs may exert 

 force equalling a pressure of ten atmospheres. As in ordi- 

 nary growth, stems and roots bending because of geotropic 

 stimulation will develop only the amount of force needed to 

 accomplish the bending. This has been proved experiment- 

 ally by Meischke.t He ascertained the force ordinarily 

 exerted by geotropically bending parts, and found that, 

 when compelled to do so, they developed from several to 

 many times this force in order to accomplish the bending. 

 Thus 



Grass internodes can develop 4 times the ordinary bending force. 

 Cucurbita seedlings 13 " 



Lupinus 17 " 



Phaseolus 28 ' 



Helianthus " 30 ' 



If any evidence of geotropic stimulus were needed, more 

 striking than the geotropic curve itself, these figures, in- 

 dicating the increase in power under stimulus, would fur- 

 nish it. 



INFLUENCE OF LIGHT 



Light as a form of energy does certain kinds of work and 

 accomplishes certain chemical changes within as well as out- 

 side living cells. Living protoplasm is composed of chemi- 



* Elfving. F. Verhalten der Grasknoten am Klinostat. Ofversigt af 

 finska Vet. Soc., Forhandlinger, Bd. 26, 1884. 



t Meischke. P. Uber die Arbeit sleistung der Pflanzen bei der geotro- 

 pischen Kriimmung. Jahrb. f. wiss. Bot.. Bd. 33, 1899. 



