188 D. P. Penhalloiv — Tendril Movements. 



tendril have already been discussed incidentally, and it is only 

 necessary to repeat in concise form, that coiling of this nature 

 is simply due to increasing disproportion of tension in the vari- 

 ous tissues, through maturity and cessation of growth in the 

 wood cells. 



It only remains for us to now call attention to the fact that 

 the motions of leaves, petioles and tendrils are all due to one 

 and the same cause, and that the much greater activity and 

 range of movement manifested in the latter, finds ample ex- 

 planation upon histological grounds : (1) in the much greater dis- 

 proportion between length and diameter, (2) in the more strict 

 localization of the vibrogen tissue, (3) in its greater flexibility 

 through being comparatively solid, and (4) the greater effect of 

 unequal tension when exerted longitudinally through a more 

 filamentous structure. 



In the nutation of the terminal bud of the squash vine, we 

 have to deal with some conditions which do not generally ob- 

 tain in vines, i. e. the horizontal position of the plant. 

 The growing extremity for a distance of two or three feet from 

 the tip, under the influence of negative geotropism, has a strong 

 tendency to an upright position, which results in a gradual 

 curve and the elevation of the terminal bud from . twelve to 

 eighteen inches above ground. As growth continues, the tip 

 constantly ascends until that portion of the vine unsupported 

 by the earth, has attained such length that it can no longer 

 overcome the direct action of gravitation upon its mass. The 

 bud is then seen to be rapidly depressed, but this downward 

 movement is succeeded by an upward growth as before. These 

 vertical oscillations continue as long as growth of the vine, 

 and constitute the chief movements of the extremity. 



A second class of movements is found in the irregularly 

 ellipsoidal nutations of the bud. This may be directly referred 

 to unequal growth on opposite sides of the stem. While in 

 the bud, the internodes are very nearly in a straight line with 

 one another. Later, however, when the development of leaf, 

 flower and tendril proceeds at a much more rapid rate than the 

 growth of the stem at the same node, the excessive formation of 

 tissue on the side where these organs are inserted, causes a 

 bending of the stem in the opposite direction. This must, there- 

 fore, throw the tip of the vine to one side of its former position- 

 As this condition of unequal growth occurs at every node 

 within the section of growth, the tendency is to throw the ex- 

 tremity first to one side and then to the other, and the final direc- 

 tion of movement is controlled by the excess of growth upon one 

 side, over that on the opposite side. This conclusion was reached 

 by me independently in the summer of 1875, and it was grati- 

 fying to notice somewhat later, that Sachs had been led to simi- 

 lar conclusions, thousrh at a somewhat earlier date. 



