316 VEGETABLE PHYSIOLOGY 



incident to growth, and proceeding primarily from internal 

 causes many advantages are secured by the plant. In 

 the case of a climbing stem, the circumnutation enables it 

 to reach a support, round which it twines, so that with 

 but little expenditure of substance it can secure access to 

 more light and air than it could obtain in its absence. 

 Eoots 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 

 seedcoats 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 an erect 

 position, the arch opening out till the axis is straightened. 

 Coincidently with this change, circumnutation of the apical 

 region replaces that of the portion of the axis which was at 

 first arched. 



During the period of growth the young organ is 

 extremely sensitive to changes in its environment, respond- 

 ing to such stimulating influences by further modifications 

 of its behaviour. These will be considered in detail in a 

 subsequent chapter. 



Besides the hydrostatic tension set up in the cells of 

 the growing regions, the processes of growth are accom- 

 panied by the development of other tensions in the 

 interior of the growing member. These appear to depend 

 upon differences between the turgidities of their several 

 tissue systems as these develop, and upon different rates of 

 growth of different internal parts. If a petiole of Rhubarb 

 is taken, and a thin strip is peeled from one side, it will 

 immediately curl outwards. If it is then placed in apposi- 

 tion with the part from which it was cut, it will be found 

 to be appreciably shorter than the rest of the petiole. If 

 the petiole is carefully measured, and then deprived of its 

 cortical covering by the separation of successive strips, the 

 central part will be found to be slightly longer than the 



