364 SYMMETRY OF ORGANS. 



(mvn, five), and the symmetry pentagonal (fig. 634), as in Ranun- 

 culus. The number of parts in the flower is indicated by the 

 following symbols : — Dimerous ^, Trimerous ^, Tetramerous ^, Pen- 

 tamerous ^. 



Pig. 630. Kg. 631. Fig. 632. Fig. 633. 



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 in 

 the arrangement of the stamens of Cruciferae. 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 mammiUse, in a symmetrical 

 manner. In the case of irregular corollas the parts at first appear 

 regular. 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 Solanacese are 

 pentamerous, and have a dimerous ovary, yet they are called sym- 

 metrical. In Oruciferae the flowers are, properly speaking, unsym- 

 metrical, for while there are four sepals and four petals, there are six 

 stamens in place of four. This condition of the stamens depends pro- 

 bably on deduplication (p. 210). In Papilionaceous flowers the parts 

 are usually symmetrical, there being five divisions of the calyx, five 

 petals, and ten stamens in two rows. In these flowers there should 

 normally be five carpels, but there are very rarely more than one. 



In Dicotyledonous plants it is common to meet with pentagonal 

 (figs. 634, 635, 636) and tetragonal (figs. 632, 633) symmetry, the 

 parts being arranged in fives and fours, or in multiples of these num- 



Fig. 630. Diagram of the dimerous flower of Circtea Lutetiana, Enchanter's Nightsliade. 

 Tliere are two carpels, two stamens, two divisions of the corolla, and two of the calyx. The 

 flower is Isostemonoua. Fig. 631. Diagram of the trimerous Isostemonous flower of 

 Cneorum tricoccum. The floral envelopes are arranged in sets of three, and so are the 

 essential organs. Fig. 632. Diagram of the tetramerous Isostemonous flower of Zieila. 

 The organs are arranged in verticils of four parts each. Fig. 633. Diagram of the tetra- 

 merous Diplostemonous flower of Euta graveolens. There are four carpels, eight stamens, 

 or four in each verticil, four folioles of the calyx, and four petals. 



