REPRODUCTION AND DISPERSAL 827 



phyte, thus permitting the migrating male cells to reach the neighbor- 

 hood of the egg (figs. 533, 599). 



The more or less broadened terminal part of the axis, which bears the 

 floral organs, is the receptacle (fig. 1137). Most flowers are subtended 

 by leaflike organs, known as bracts (b, fig. 1141), into which foliage 

 leaves often grade imperceptibly; a group of whorled or closely arranged 

 bracts is called an involucre (figs. 1193, 1194). Although flowers often 

 are solitary, they more commonly are grouped into an inflorescence 

 (fig. 1141). 



Differences in floral structure. While the sort of flower described 

 above is as representative as any, there are divergences in almost all 

 respects, and since these divergences are relatively fixed, whatever the 

 environmental conditions, they have been made the chief basis for sep- 

 arating seed plants into subdivisions. The kind of flower that is most 

 fundamentally different from the one above pictured is that of the 

 gymnosperms, which has no ovary, style, or stigma, the ovules being 

 exposed directly to falling pollen. Any one of the parts of a flower may 

 be wanting or even all the parts except either stamens or pistils. Often 

 there is but one kind of floral leaves which in the dicotyls is arbitrarily 

 regarded as the calyx (figs. 1159, 1160), but which in the monocotyls is 

 termed the perianth ; sometimes there are no floral leaves, as in the cat- 

 tails, peppers, and hazels (fig. 1161), and in most gymnosperms. 1 Even 

 where the perianth is lacking, one or more bracts commonly are present. 

 The simplest flower is that of the duckweeds, in which the only organ 

 present is a single stamen or pistil. In the dicotyls the corolla may be 

 made up of separate petals (figs. 1136, 1137), or the parts may be united 

 (as in the Sympetalae, fig. 1185). Most flowers are monodinous, that 

 is, with pistils and stamens occurring in the same flower (figs. 1136, 

 1137), but some are diclinous, that is, with stamens and pistils occurring 

 in separate flowers ; diclinous species may be monoecious, having the 

 two kinds of flowers on the same plant (fig. 1161), or dioecious, having 

 the two kinds on separate plants (fig. 1165). 



While the floral whorls commonly are sharply delimited, the calyx 

 and corolla often are much alike, as in many monocotyls and in some 

 dicotyls (e.g. Poly gala). A striking instance of intergrading parts is 

 found in the white water lily (Castalia), where the stamens pass gradually 

 into petals, suggesting to some observers that stamens are transformed 



1 Spikes or catkins of such flowers do not differ essentially in structure from pterido- 

 phyte strobili, though their r61e is radically different. 



