164 The Phenomena of Morphogenesis 



at right angles to the previous one. This produces two parallel spirals. 

 For a discussion of other recondite aspects of spiral phyllotaxy the reader 

 is referred to the work of Church, Hirmer, Richards, the Snows, and 

 others who have gone deeply into these problems. 



Of some morphogenetic interest is the direction of the genetic spiral 

 itself. Observers generally agree that leaf positions around the stem are 

 as likely to be in a clockwise as in a counterclockwise spiral. Beal ( 1873 ) , 

 studying cones of Norway spruce, found 224 cases of the former and 

 243 of the latter. Allard ( 1946 ) examined 23,507 tobacco plants and found 

 that the two types were almost exactly equal. Direction of spirality was 

 not inherited. How the direction is determined for a given plant is not 

 known, but it is probably in some critical early cell division. This neutral- 

 ity of the phyllotactic spiral is unlike the behavior of climbing plants, 



Fig. 7-8. Diagram of distribution of pri- 

 mordia at the shoot apex, each diverging 

 from its predecessor by the Fibonacci 

 angle. As the primordia increase in size, 

 the recognizable contact parastichies shift 

 from 5 + 8 near the center to 8 + 13 

 farther out. ( From Barthelmess. ) 



in almost all of which a given species climbs in either a clockwise or a 

 counterclockwise manner exclusively. 



The difference in direction of the phyllotactic spiral, however, some- 

 times alters from one part of the plant to another in conformity to a 

 general pattern of symmetry, much as in the case of floral structures 

 (p. 167). In shoot growth of Citrus, for example, Schroeder (1953a) re- 

 ports that there are successive "flushes" with a dormant period between 

 them and that a regular alternation occurs between right and left 

 spirality in successive shoots. Secondary shoots have spirality opposite 

 from their parent one. Thorns develop to the left of the petiole in left- 

 handed shoots and to the right in right-handed ones. The direction of 

 spirality in axillary shoots of View also takes place in a precise and 

 alternating order which depends on their position (Dormer, 1954). 



Spirality. Spiral phyllotaxy involves a number of problems as to the 

 spacing and relative position of leaves which are not present in decus- 



