162 REVIEWS. 



A striking illustration of the amount of space that may be 

 swept over is afforded by a case in which Mr. Darwin allowed 

 the top of a Ceropegia to grow out almost horizontally to the 

 length of 31 inches, — three long internodes terminated by two 

 short ones. The whole revolved at rates between 5^ and 6|- 

 hours for each revolution, the tip sweeping a circle of above 

 five feet in diameter and 16 feet in circumference, traveling 

 therefore at the rate of at least 32 inches per hour. " It was 

 an interesting spectacle to watch the long shoot sweeping, 

 night and day, this grand circle, in search of some object 

 around which to twine." 



As to the nature of this revolving movement, Mr. Darwin 

 clearly shows that it is not a torsion of the axis, but a succes- 

 sive bending (similar to that by which ordinary stems bend 

 toward the light), the direction of which is constantly and 

 uniformly changing. " If a colored streak be painted (this 

 was done with a large number of twining plants) along, we 

 will say, the convex line of surface, this colored streak will, 

 after a time depending on the rate of revolution, be found to 

 lie along one side of the bow, then along the concave side, 

 then on the opposite side, and, lastly, again on the opposite 

 convex surface. This clearly proves that the internodes, dur- 

 ing the revolving movement, become bowed in every direction. 

 The movement is, in fact, a continuous self-bowing of the 

 whole shoot, successively directed to all points of the com- 

 pass." It is an automatic movement, of the same character 

 as those which these and other parts of plants effect in chang- 

 ing position or direction, sometimes slowly and sometimes with 

 a visible motion. The movement may be likened in one case 

 to that of the hour-hand or the minute-hand of a clock, in the 

 other to the second-hand, but in both is as truly a vital move- 

 ment as is the contraction of an involuntary muscle. It must 

 be effected — as Mr. Darwin recognizes — either by the con- 

 traction of the cells on the concave side, or by the turgescence 

 and elongation of those on the convex side of the internode, 

 or by both, — probably the former, as various facts go to show ; 

 but questions of that kind are not investigated in the present 

 essay. 



