SYMMETRY OF ORGANS. 309 



644. There are also other kinds of arrangements in flowers, which 

 may be referred to certain modifications in the organogenic law. Thus, 

 what is called oblong or two and two-membered symmetry, occurs in 

 cases where the opposite ends are similar, and the opposite sides as hi 

 the arrangement of the stamens of Cruciferse. Again, simple symmetry is 

 that in which the two sides of the object are exactly alike, without any 

 further repetition, as in papilionaceous, personate, and labiate flowers, 

 as well as in most leaves. The term symmetry, however, is properly 

 confined to cases where the parts are arranged alternately, and are 

 either equal or some multiple of each other, and has no reference to 

 the forms of the different parts. In the very young state, the parts of the 

 flower appear as a shallow rim, from which the petals and sepals arise 

 as mammilla?., in a symmetrical manner. In the case of irregular 

 corollas, the parts at first appear regular, as shown by Barneoud.* In 

 speaking of flowers, it is usual to call them symmetrical when the 

 sepals, petals, and stamens follow the law mentioned, even although the 

 pistil may be abnormal. Thus, many Solanacea? are pentamerous, and 

 have a dimerous ovary, yet they are called symmetrical. In Cruciferaa, 

 the flowers are, properly speaking, unsymmetrical, for while there are 

 four sepals and four petals, there are six stamens in place of four. 

 This depends apparently on the long stamens being in reality composed 

 only of two, the filament of each of which is split by a process of chori- 

 zation (^[ 383), and each division forms for itself by multiplication a 

 perfect anther. In Papilionaceous flowers, the parts are usually sym- 

 metrical, there being five divisions of the calyx, five petals, and ten 

 stamens in two rows. 



645. It will be seen that flowers constituting trigonal or pentagonal 

 symmetry, may present what has been called simple symmetry, when one 



* Annales des Sciences Xaturelles, November, 1846. 



Fig. 535. Diagram of the pentamerous Isostemonous flower of Crassula rubens. cccc c. Parts 

 of the calyx, p p p p p. Petals alternating with the leaves of the calyx e e e e e, Stamens alter- 

 nating with the petals, a, Accessory bodies in the form of scales, or a disk alternating with the 

 stamens. These scales are often an abortive row of stamens, o. Carpels alternating with the 

 stamens, and opposite to the scales. 



Fig. 536. Diagram of the pentamerous flower of Sedum Telephium. The stamens are ten, 

 arranged in two alternating verticils. The flower is Diplostemonous. 



Fig. 537. Diagram of the pentamerous Diplostemonons flower of Coriaria myrtifolia; the 

 parts of the four whorls alternating, the verticil of stamens being double. 



Fig. 538. Diagram of the trimerous Diplostemonous flower of Ornithogalnm pyrenaicum. 

 Stamens six in two alternating verticils. 



