476 MANUAL OF BOTANY 



is often accompanied by an increased extensibility of the cell- 

 walls of the turgid region. The growing apex of such a stem will 

 alternately incline first to one side and then to the other, ex- 

 hibiting a kind of nodding movement in the two directions. 

 This is known as nutation, and it is of very frequent occurrence. 

 The region of greatest turgidity, instead of being alternately 

 on one side and the opposite, may pass gradually round the 

 growing zone. The apex in this case will describe a circle, or 

 rather a spiral, as it is elongating all the time, pointing to all 

 points of the compass in succession. This movement has been 

 described by Darwin as circumnutation, and has been said by 

 him to be \miversal in all growing organs. The passage of the 

 maximum turgidity round the stem may be regular or irregular, 

 causing the circle to be replaced by an ellipse. Indeed, the 

 simple nutation spoken of above may be regarded as only an 

 extreme instance of the latter. 



By these movements, incident to growth, and proceeding 

 altogether from internal causes, many advantages are secured 

 to the plant. In the case of climbing stems the circumnutation 

 enables them to reach supports round which they twine, so that 

 with but little expenditure of substance they can secure ad- 

 vantages of light and air which they could not obtain in its 

 absence. The roots by the same method are enabled more 

 easily to make their way through the crevices of the soil. The 

 axis of the embryo shows in one or other of its parts strong 

 hyponastic curvature, forming an arch, which enables it to leave 

 the seed coats and make its way through the soil without damage 

 to the young delicate plumule, its progress being helped by 

 simultaneous circumnutation. On reaching the surface epinastic 

 growth causes it to assume the erect position, whUe circumnuta- 

 tion of the apical region replaces that of the arched portion of 

 the axis. 



During the period of growth the young organ is extremely 

 sensitive to changes in its environment, responding to such 

 stimulating influences by further modifications of its behaviour ; 

 these will be considered in detail in the succeeding chapter. 



Besides the hydrostatic tension set up in the cells of the 

 growing regions, the processes of growth are accompanied by 

 other tensions existing in their interior, which appear to depend 

 upon differences between their tissue systems as these develop. 

 If a petiole of Ehubarb be taken, and a thin strip be peeled 

 from one side, it will immediately curl outwards. If it be then 

 placed in apposition with the part from which it was cut, it 



