THE ANGIOSPERMAE : LEAVES 



i)>i~ 



case, the number of primordia which exist simuUaneously at any given level 

 of the apex, and must naturally be related to the size of the apex itself. These 

 primordia are not, of course, except in whorled phyllotaxis all formed simul- 

 taneously. For example in stems with decussate phyllotaxis, although there 



Fig. 974- — SidutH reflexion. Trans\erse 1-ic;. 975. — Eup/iorhia uiilfenii. Transxcrsc section 



section through the stem apex showing through the apical bud showing the leaf 



the genetic spiral of the leaf rudiments. rudiments in parastichies. 



{From Cliurcli, " On tl.e Relation of Phyllotaxis to Mechanical Lazvs" Williams atul Norgate, Ltd.) 



are four orthostichies, only the primordia of one pair of leaves are formed 

 at the same time. They leave only a small flat apex unoccupied between 

 them and it is not until after an interval or plastochrone, during which the 

 apex again grows forward into a dome, that another pair is formed. 



As the primordia approach each other towards the apex, secondary spirals 

 become evident in their arrangement. These are called parastichies (Fig. 

 97^), and thev are purely a geometrical consequence of the packing of the 

 rudiments together. They can be traced both clockwise and anticlockwise. 

 In whorled constructions the number of parastichies in each direction is 

 equal. Spiral phyllotaxis always produces an unequal number. 



The mathematical deductions which may be derived from these arrange- 

 ments, however interesting, leave untouched the primary biological question 

 of why a given rudiment arises at a given point on the apex, since its inception 

 must be determined before it appears. The apex is a dome, the surface of 

 which is undergoing uniform expansion outwards from its apex as centre 

 (Fig. 976). On this growth-field secondary centres are established which 

 become primordia of leaves, and we have still no better account of the 

 phenomenon to offer than that originally proposed by Hofmeister, namely, 



