1C92 



A TEXTBOOK OF THEORETICAL BOTANY 



The horizontal symmetry of the flower depends in the first place on 

 the mode of arrangement or insertion of the parts of the floral receptacle. 

 Here we can distinguish three types: (i) spiral throughout, (2) cyclic 

 throughout, that is with all the parts in distinct whorls, and (3) a mixture of 

 both types, generally called hemicyclic or spirocyclic. As spiral phyllotaxy 

 is the commonest arrangement in vegetative shoots, many morphologists 

 have assumed that the spiral system in flowers is the earlier, more primitive, 

 type and that the cyclic arrangement has been derived from it. Without 

 anticipating the question as to whether a flower can be truly equated to a 

 shoot, which we shall discuss in a later section, we may point to the fact 

 that spiral and spirocyclic arrangements only occur in the flowers of families 

 which, on other grounds, may be regarded as primitive, e.g., Magnoliaceae 

 Ranunculaceae, Nymphaeaceae, in whose flowers the number of parts is 

 large and indefinite. The more advanced families, including all the Meta- 

 chlamydeae, in which the flowers have a small, definite number of parts, are 

 strictly cyclic. Even in the families mentioned the spiral arrangement is 

 seldom complete (examples: Caltha, Ficaria). For instance, in Nymphaea, 

 although all the other parts are spiral, the carpels form a whorl. 



In Magnoliaceae, the perianth shows a tendency towards tripartite 

 whorls and in Ranunculaceae to five-partite whorls, while in some of the 

 latter family, the sexual parts, though spiral, can nevertheless be analysed 

 into sets of three, which in species with reduced numbers of floral parts 

 become definitelv whorled. 



In spiral flowers the number of parts is nearly always large and they are 

 usually inserted on a large receptacle and are completely free from each 

 other, but in cyclic flowers, with smaller receptacles and closer insertion of 

 parts, modifications by cohesion and adhesion often arise. Cohesion 

 of similar parts afl'ects any of the organs of the flower. The sepals may be 

 united into a calyx tube or the petals into a corolla tube, the degree of 



union varying in extent in both cases 

 from complete fusion, which leaves no 

 trace of the individuality of the mem- 

 bers, to cohesion into a shallow ring 

 at the base only. While the cohesion 

 of sepals, or synsepaly, is a sporadic 

 phenomenon, in regard to which even 

 species within the same genus may 

 differ, sympetaly, on the other hand, 

 tends to run consistently through whole 

 families and is a valuable classificatory 

 character, due no doubt to its impor- 

 tance in connection with pollination. 

 Cohesion of stamens, synstemony, 

 is somewhat rarer. It is characteristic of a number of Leguminosae 

 and is a constant feature throughout the Compositae, in both of 

 which families the stamen tube is an essential part of the pollination 



Fig. 



1060. — Coherent stamens. A, 

 Otiivisia. B, Oxalis. (After Van 

 Tieghem.) 



