492 PLANT GROWTH AND PLANT COMMUNITIES 



genetic, and other factors in floral development, may also yield new 

 criteria of comparison in phylogenetic investigations. 



Needless to say, it remains to be seen whether, from investigations 

 of the extensive and varied mass of angiosperm materials, general laws 

 or principles of a unifying and simplifying character can indeed be 

 discovered. It is at least a reasonable conjecture that if the presence 

 of common, or basic, factors in floral morphogenesis can be substan- 

 tiated, some of the conspicuous and seemingly important morphologi- 

 cal differences may be seen to be not so very different after all: i.e., 

 they may be envisaged as variations on a theme rather than as major or 

 fundamental differences. 



Flowers may occur singly or associated together in more or less 

 complex inflorescences, the latter exemplifying characteristic morpho- 

 logical patterns which are often of a remarkable degree of geometrical 

 regularity. Flowers may be hypogynous, perigynous, or epigynous, may 

 be of radial, bilateral, or zygomorphic symmetry, with a relatively 

 large or a relatively small number of parts, and they may have the an- 

 droecium and gynoecium present in the same flower or separated in 

 different ways. Some species are evidently much more suitable for 

 particular observations or experiments than others; hence, a not in- 

 considerable but fascinating aspect of new investigations is the search 

 for what Lang ( 1915) described as "favorable materials." 



An actinomorphic, polypetalous, hypogynous, apocarpous flower, 

 with numerous stamens and carpels, may be provisionally accepted as 

 exemplifying the primitive condition in dicotyledons, and for conven- 

 ience this will be referred to as the prototypic flower. 



Theoretical considerations 



Floral meristems originate, directly or indirectly, from vegetative- 

 shoot apical meristems or from axillary buds. Accordingly, what has 

 been ascertained or conjectured from causal investigations of the shoot 

 apex, especially during recent years, makes some of the problems of 

 floral morphogenesis considerably more amenable to investigation than 

 seemed possible three or four decades ago. Indeed, our information 

 and ideas on the organization and morphogenetic activity of the shoot 

 apex are essential in any new approach to the problems of floral mor- 

 phogenesis. 



The shoot apical meristem. The following features of the shoot 

 apex are relevant. ( 1 ) The organogenically active apex consists of sev- 

 eral embryonic regions. (2) The apex functions as a whole and posses- 

 ses specific organization. (3) All shoot apices (with rare exceptions) 

 produce a characteristic ( phyllotactic ) pattern of growth centers 

 which typically become leaf primordia. ( 4 ) Seemingly different phyllo- 



