322 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. 

 Boots by the same method are enabled more easily to make 

 their way through the crevices of the soil. 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 direction of growth is vertical. 

 Coincidently with this change, circumnutation of the apical 

 region replaces that of the portion which was at first arched. 



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 be- 

 haviour. 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 Ehubarb 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 original petiole. In such a petiole the central part is 



