162 The Phenomena of Morphogenesis 



this fact and has returned to methods of mathematical analysis in ap- 

 proaching the problem of the development of the primordia at the meri- 

 stem. He emphasizes the importance of the plastochron ratio, the ratio of 

 the radial distances from the center of the meristem to two consecutive 

 primordia. Where this distance increases considerably in each plasto- 



Fig. 7-7. Diagram showing a spiral succession of points, each separated from the next 

 by the Fibonacci angle, or about 137.5°. Parastichies can be recognized by intersec- 

 tions of approximately 90° between them. At the center there are five counterclock- 

 wise ones and eight clockwise. The counterclockwise series soon shift from 5 to 13, 

 and later the clockwise ones from 8 to 21. (From F. J. Richards.) 



chron, both the genetic spiral and the spirals of the parastichies will open 

 out rapidly. The meristem itself under these conditions will tend to be 

 relatively steep, the primordia few and the parastichy numbers low. On 

 the contrary, when the radial distance increases but little from one 

 primordium to the next, the primordia are packed closely, the meristem 



