204 PLANT PHYSIOLOGY 



tected.* Since we conceive all living cells to be affected by 

 external influences, whether they give a response which 

 can be seen at any time or not, there is every reason to 

 believe that all the living cells in a body are connected to- 

 gether. Indeed, Strasburger's investigations go far toward 

 proving that such is really the case.f By Nemec's fibrils and 

 Strasburger's continuity of protoplasm the living parts of a 

 plant are united in one system. Granting this to be the 

 case, it is nevertheless difficult to see how these structures 

 convey a purely mechanical stimulus like that of gravita- 

 tion. Nemec's fibrils suggest the nerve structures in ani- 

 mals. Like these structures, their manner of working is a 

 mystery. We may therefore cling to our diffusion hypothe- 

 sis as comprehensible, if difficult of proof, leaving the future 

 to determine the value and the functions of protoplasmic 

 fibrils and protoplasmic continuity. 



Sachs asserted that when a root-tip is horizontal it is in 

 position to be most strongly stimulated by gravity, and 

 that then the curvature of the growing region will be most 

 rapid and most pronounced. Until Czapek J determined the 

 necessary duration of the stimulus, Sachs's assertion was un- 

 challenged. Czapek ascertained that the root-tips of vari- 

 ous plants require from fifteen to fifty minutes' exposure 

 to the action of gravity in order to bend. The longer the 

 exposure, the more pronounced in rapidity and angle is the 

 bending. Thus roots of Lupinus albus will bend if laid 

 horizontal for twenty minutes, but the maximum effect will 

 follow an exposure of four hours. When the root-tip points 

 upward at an angle of 45, not when it is horizontal, the 

 root bends most, the time of exposure and other conditions 

 being the same. Above this angle, the spontaneous growth- 

 movements, known as nutation or circumnutation, interfere 



* Haberlandt. G. Uber Reizleitung im Pflanzenreich. Biolog. Central- 

 blatt, XXI.. 1901. tber fibrillare Plasmastructuren. Ber. d. Deutsch. Bot. 

 Gesellsch., XIX., 1902. 



f Strasburger, E. fiber Plasmaverbindungen pflanzlicher Zellen. Jahrb. 

 f. wise. Bot.. Bd. 36, 1901. 



J Czapek, F. Untersuchungen iiber Geotropismus. Jahrb. f. wise. Bot.. 

 Bd. 27, 1895. Weitere Beitrage zur Kenntniss der geotropischen Reizbewe- 

 gungen. Jahrb. f. wise. Bot., Bd. 32 1898. 



