GROWTH AND DEVELOPMENT OF THE INFLORESCENCE AND FLOWER 



493 



tactic systems may not be so very diflFerent fundamentally, and even 

 in the same apex a transition from one system to another can readily 

 take place ( Richards, 1951; Cutter and Voeller, 1959; Voeller and Cut- 

 ter, 1959 ) . ( 5 ) The symmetry and orientation of a leaf primordium are 

 determined by the inception of its growth center in the apical meristem 

 and by the rates of growth on its adaxial and abaxial sides; its size and 

 shape are controlled by the genetically and ontogenetically determined 

 apical organization (Wardlaw, 1955). (6) The position of a growth 

 center and the rate of growth of the lateral member formed from it 

 are determined and regulated by the distal region of the apical meri- 

 stem and the adjacent older primordia (Wardlaw, 1949, 1950). (7) 

 The inception and pattern of vascular tissue are directly related to the 

 active growth of the apical meristem, its growth centers, and the pri- 

 mordia. (8) While the apical meristem may change in characteristic 

 ways during the ontogenetic development of the plant, it usually shows 

 remarkable stability in its histological organization and morphogenetic 

 activity under experimental treatments ( Figures 1 and 2 ) . 



Growth centers. The concept of growth centers— i.e., discrete cen- 



Figure 1. Diagrammatic representation of a fern apex as seen from above, 

 showing the leaf primordia one to ten, the positions of the next primordia to 

 be formed, in the order of their appearance (Ij to I3), the apical cell (ac), 

 the approximate positions of bud rudiments (b), the lower limits of the 

 apical meristems {m-m'), and the approximate position of the base of the 

 apical cone (broken circular line). Magnification: X 30. (From Growth, 

 Suppl., 1949.) 



