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. 
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 
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. Ifa 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 
