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 cireumnutation 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 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 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 original petiole. In such a petiole the central part is 
