PHANEROGAMIA 453 



represented sometimes by an outer, sometimes by an inner whorl, 

 while the perianth and gyncecium at the same time exhibit the highest 

 stage of development. 



Flowers in which the andrcscium is formed by a single complete 

 whorl are said to be haplostbmonous. 



In addition to the number of the whorls, the number of the 

 members composing the single whorls is subject to variation, and is 

 due similarly, in many if not in all cases, to their subsequent diminu- 

 tion by reduction or to their multiplication by splitting. 



A decrease in the number of the floral leaves of a single whorl (oli- 

 gomery) is most frequently met with in the gyncecium, which, in flowers 

 with a pentamerous perianth and andrcscium, has usually but three or 

 even two carpels. Next to the gyncecium a suppression of one or more 

 members of a whorl is most frequent in the andrcecium, while the 

 perianth rarely consists of incomplete whorls (Polygala). Multiplication of 

 the members of a whorl (pleiomery) occurs most often in the andrcscium, 

 less frequently in the gyncecium (Malva), still less frequently in the peri- 

 anth (Dryas odopetala). Flowers with incomplete whorls, resulting un- 

 questionably from suppression, are met with, for example, in the family 

 Scraphulariaceae, in which the genus Verbascum possesses five fertile 

 stamens, while in Scrophularia the posterior stamen is represented 

 only by a staminodium, and in most of the other genera it is altogether 

 absent. The origin of a pleiomerous whorl from one consisting of 

 fewer members is equally well shown in the flowers of Tilia, where the 

 numerous stamens are arranged in five groups, which occupy a corre- 

 sponding position to the five simple stamens of allied forms. 



The Symmetry of the Flower. — The flowers of Angiosperms are 

 sometimes actinomorphic (radial), sometimes zygomorphic (mono- 

 symmetrical), or, more rarely, asymmetrical. 



Eadial flowers exhibit probably the more primitive structure, since 

 in them the arrangement of the members varies less from that of the 

 vegetative region. The derivative origin of zygomorphic flowers is 

 apparent in their more complicated structure, metamorphosis, and reduc- 

 tion. Zygomorphism is always indicative of a high degree of adapta- 

 tion to insect-pollination. 



A flower is LONGITUDINALLY zygomorphic when the plane of sym- 

 metry coincides with the median plane of the flower, viz. the plane 

 passing through its axis and the axis of the main stem (e.g. Orchidaceae, 

 Labialae) ; obliquely zygomorphic when it cuts the median plane at 

 an acute angle (Aesculus) ; transversely zygomorphic when it cuts 

 the median plane at right angles (Fumariaceae). The first is by far 

 the commonest. Occasionally a plant which otherwise possesses only 

 zygomorphic flowers produces others of a radial structure. Such ex- 

 ceptional radial flowers are termed PELORIA, and are regarded as 

 the result of reversion to the primitive type. 



Floral Diagrams and Formulse. — The number and arrangement 



2 I 2 



