IRRITABILITY 189 



apply mechanical force to living plants, and to these influ- 

 ences they respond by the increased or modified activity of 

 the living protoplasm. 



The most evident effects of winds are those deformities 

 which usually, however, are more the result of injury than 

 of stimulus. Trees in exposed places are unsymmetrical, 

 their limbs short and broken on the side toward the strong 

 or violent prevailing wind, while on the other side the limbs 

 look as if they had been drawn along with the wind. But 

 the root-syste n of such trees shows the stimulating 'effect of 

 the wind without the deformities exhibited in the branches, 

 the roots being longer and stronger on the windward 

 than on the leeward side, the greatest strength develop- 

 ing where there is the greatest mechanical force to be 

 resisted. 



The ordinary swaying of stems and branches, and even of 

 leaves on their stalks, acts as a stimulus to the living 

 cells of a plant. The difference in the amounts of mechan- 

 ical tissue in plants which carry their own weight and in 

 those which lean, twine, and otherwise climb, is partly due 

 to the difference in mechanical stimulus to form strength- 

 ening tissues. By tying an erect plant so firmly to a sup- 

 port that it cannot sway in the wind, or by supporting its 

 weight on a frame, the plant will be deprived of those move- 

 ments, stresses, and strains which normally stimulate it 

 to develop strength. 



Water-currents exercise similar effects to those of wind- 

 currents, whenever they are rapid enough to develop any 

 considerable force. Even slow water-currents have been 

 found to stimulate and direct growth and movement in 

 rather peculiar fashion. Thus the roots of certain plants, if 

 suspended in clean running water, will bend so that the tips 

 point and grow up stream. This phenomenon is known as 

 rheotropism. * The plasmodia of certain Myxomycetes will 

 grow on a vertical strip of filter-paper always in the direc- 

 tion opposite to that of the current of water which is sup- 



* Juel, H. O. Untersuchungen iiber den Rheotropismus der Wurzeln. 

 Jahrb. f. wiss. Bot., Bd. 34. 1900. Newcombe. F. C. Rheotropism cf 

 roots. Bot. Gazette vol. 33 1902. 



