Symmetry 159 



by tracing the system back to the cotyledonary node. Camefort (1956) 

 has presented a full account of Plantefol's theory and has endeavored to 

 reconcile it with the classical concepts of phyllotaxy and modern experi- 

 mental studies. 



The solution of these problems of leaf arrangement is evidently to be 

 sought near the apical growing point where the leaf primordia actually 

 originate, for their relations here will determine those between mature 

 leaves on the elongated stem. This emphasis on the study of primordia 

 is a return to the point of view of Schwendener ( 1878 ) , who believed 

 that mechanical contact and pressure exerted by the primordia on one 

 another accounted for their distribution, and especially of Hofmeister 

 (1868), who proposed the general rule that a new primordium arises 

 in the largest space available to it. This conclusion is generally accepted, 

 but the developmental basis for it is not clear. The essential morpho- 

 genetic problem beneath all this is what determines the origin of a par- 

 ticular primordium at a particular place and time. 



An early idea was that a leaf-forming stimulus passes along the genetic 

 spiral, but the significance of the spiral itself now seems rather slight. 

 Church (1920), concerning himself chiefly with parastichies, believed 

 that the point of intersection of the two major ones determined the point 

 of origin of a primordium. This leaves undetermined the reason for the 

 course of the parastichies themselves. Some workers are inclined to think 

 that stimuli from previously formed leaves or primordia determine the 

 position of new ones. Plantefol assumes that a foliar helix extends up- 

 ward into the meristem to the generative center where the primordia are 

 formed. Sterling ( 1945 ) finds that in Sequoia the procambial strands are 

 always continuous with the older ones below and differentiate acropetally, 

 pushing up into the apical meristem before the emergence of the pri- 

 mordia into which they will pass, and suggests that these procambial 

 strands may influence the position of the primordia. Gunckel and Wet- 

 more ( 1946 ) reach the same conclusion for Ginkgo. Opposed to this 

 idea is some experimental evidence, chiefly derived from isolating part 

 of the meristem from regions below it by incisions without disrupting 

 normal phyllotactic arrangement, a result which suggests that the stimu- 

 lus for the development of a primordium does not come from below. 

 Perhaps in such cases as this it is incorrect to assume that a given devel- 

 opment is the cause of another which succeeds it in time. A series of re- 

 lated structures and processes are part of the same organized whole and 

 should be thought of as developing together rather than each step as 

 inducing the one that follows it. 



The problem of what determines the phyllotactic series is open to 

 experimental attack, and much work has been done on it by various 

 people, among them Wardlaw, Ball, and especially the Snows. They have 



