304 RELATION OF PHYLLOTAXIS TO MECHANICAL LAWS. 



to the eye, this being especially well seen in the case of 

 leaf -scars. 



Similarly, the exaggeration of the primary bilaterality of a 

 primordium may follow the path of the so-called orthostichy 

 spiral, which intersects the spiral of dorsiventrality orthogonally. 



Fig. 105. — Acacia cuUri/ormis. Young shoot of phyllodes, system (5 + 8). 



The insertion of phyllodes and " isobUateral " members in packed 

 asymmetrical phyllotaxis constructions does not necessarily follow 

 an exactly vertical plane corresponding to any direct stimulus 

 of vertical light, but follows the path of the spiral diagonal, which 

 is very nearly vertical. The spiral path becomes more approxi- 

 mately radial (vertical) as the phyllotaxis ratios approach equality, 

 but in Fibonacci systems the spiral nature of the path is obvious 



