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PART III. PHYSIOLOGY. 



[47 



the prolongation of the longitudinal axis of the fully grown 

 portion. "When the stem is radially symmetrical, the orbit is 

 approximately circular ; but in cases in which the member tends 

 to be bilaterally symmetrical, one diameter of the orbit becomes 

 proportionally elongated, the orbit being then oval, or elliptic, 

 until, finally, when the bilateral symmetry is strongly marked, 

 the orbit becomes a straight line, the growing-point simply oscil- 

 lating from side to side. Whilst the growing-point is travelling 



Fie. 131. Illustration of the epinastic growth of the leaves of the Sunflower (Helianthus 

 animus). A represents the position of the leaves when the plant is exposed to light; J5 

 represents the position of the leaves when the plant has been kept in darkness for twenty- 

 four hours. In A the leaves are expanded in consequence of the directive (diaheliotropic) 

 action of the incident rays of light. In B the leaves, in the absence of light, have become 

 recurved in virtue of their inherent epinastic growth. 



in its orbit, it is at the same time being raised upwards ; so that 

 it describes a path which is, according to the form of the orbit, a 

 circular spiral, an elliptical spiral, or a zig-zag. These changes 

 of position are, however, not permanent ; for example, though the 

 growing-point may be travelling upwards in a spiral, the fully- 

 grown stem does not resemble a corkscrew, but is straight. 



