Chap. XII. CONCLUDING EEMARKS. 547 



In some cases, as with the hypocotyls of Brassica, 

 the leaves of Dionsea and the joints of the Gramineje, 

 the cireumnutating movement when viewed under tlie 

 microscope is seen to consist of innumerable small 

 oscillations. The part under observation suddenly 

 jerks forwards for a length of "002 to "001 of an inch, 

 and then slowly retreats for a part of this distance ; 

 after a few seconds it again jerks forwards, but with 

 many intermissions. The retreating movement appa- 

 rently is due to the elasticity of the resisting tissues. 

 How far this oscillatory movement is general we do 

 not know, as not many cireumnutating plants were 

 observed by us under the microscope ; but no such 

 movement could be detected in the case of Drosera 

 with a 2-inch object-glass which we used. The pheno- 

 menon is a remarkable one. The whole hypocotyl 

 of a cabbage or the whole leaf of a Dionaea could not 

 jerk forwards unless a very large number of cells on 

 one side were simultaneously affected. Are we to sup- 

 pose that these cells steadily become more and more 

 turgescent on one side, until the part suddenly yields 

 and bends, inducing what may be called a micro- 

 scopically minute earthquake in the plant ; or do the 

 cells on one side suddenly become turgescent in an 

 intermittent manner ; each forward movement thus 

 caused being opposed by the elasticity of the tissues ? 



Circumnutation is of paramount importance in the 

 life of every plant ; for it is through its modification 

 that many highly beneficial or necessary movements 

 have been acquired. When light strikes one side 

 of a plant, or light changes into darkness, or when 

 gravitation acts on a displaced part, the plant is 

 enabled in some unknown manner to increase the 

 always varying turgescence of the cells on one side ; 

 so that the ordinary cireumnutating movement is 



