IMORPHOLOGY OF REPRODUCTIVE ORGANS 133 



among Dicotyledons, although the tetramerous is also by no 

 means rare ; while the trimerous is generally found in Mono- 

 cotyledons. 



Although a symmetrical flower, as above described, neces- 

 sarily infers that the parts in each whorl are equal to, or some 

 multiple of one another, still it was very common for botanists 

 to call a flower symmetrical when the three outer whorls corre- 

 spond in such particulars, while the parts of the gyncecium are 

 unequal to them ; as in Staphylea pinnata {fig. 248), where the 

 three outer whorls are pentamerous, while the pistil is dimerous. 

 The gyncecium of all the organs of the flower is that which least 

 frequently corresponds in the nmnber of its parts to the other 

 whorls. 



By most botanists, however, a flower is said to be symmetri- 

 cal when it can be divided by any vertical section into two 

 precisely similar halves, the different halves produced by every 

 such section being exactly alike. Such flowers as those of the 

 Lily, or Crassiila {fig. 245), are instances of this symmetry. A 

 flower which is symmetrical in this sense is also called actino- 

 morpTiic. Flowers whicli can only be divided by one such 

 section into two similar halves are called zygomorphic. Such 

 flowers are found in the Aconite, the Pea, the Dead-nettle, &c. 

 The line through which the section must pass to bring about the 

 result is called theplame of symmetry ; it may be antero-posterior, 

 as in the flowers mentioned ; or lateral, as in the Fumitory ; or 

 obUque, as in some members of the Solanacese. When a divi- 

 sion into two similar halves is not possible by a section in any 

 plane, the flower is said to be asymmetrical. 



Various other terms are used in describing flowers, which 

 wiU be best alluded to here. Thus a flower is said to be complete 

 when the four whorls — calyx, corolla, androeoium, and gyncecium 

 — are present, as in the Rue {fig. 282) ; where one or more of 

 the whorls is absent, the &o-wer is incomplete {figs. 255 and 256). 

 When the parts of each whorl are uniform in size and shape, as 

 in the Rue, the flower is regular; under other circumstances it 

 is irregular, as in the Pea {fig. 303). 



In a normal arrangement of the parts of the flower, the 

 successive whorls alternate with each other, as shown in fig.- 

 ^247 ; the sepals are here alternate with the petals, the petals 

 with the stamens, and the stamens with the carpels. 



We have seen that, when the leaves of the vegetative branch 

 are arranged in whorls, these whorls are developed in acropetal 

 succession, and the leaves of any whorl arise in s^ich a position 



