308 



SYMMETRY OF ORGANS. 



said that increase of temperature, and luxuriance of growth, sometimes 

 make flowers produce stamens only. In plants having unisexual 

 flowers, this is more liable to take place, as in Melon, Cucumber, &c. 

 Increased vigour seems to be required for the development of stamens, 

 for some fir trees in their young state bear cones, and produce male 

 flowers only when they reach the prime of life. 



643. Symmetry of Organs. In the progress of growth, the plants 

 belonging to the different divisions of the vegetable kingdom follow 

 certain organogenic laws (S^yavov, an organ, and ysymifiv, to produce), 

 the operation of which is seen in the definite arrangement of their 

 organs. The flower consists sometimes of three, at other times of four 

 or five equal sets of organs, similarly and regularly disposed. Thus, 

 the Iris has three straight parts of its perianth, and three reflexed ones 

 alternately disposed, while the Fuchsia has four parts of the calyx 

 alternating with four petals, and the Rose has five alternating portions. 

 This orderly and similar distribution of a certain number of parts is 

 called symmetry, and flowers are thus said to be symmetrical with vari- 

 ous numbers of members. When the number of parts is two, the 



flower is dimerous (Sif, twice, 

 and ftfos, a part) (fig. 531), 

 and the symmetry two-mem- 

 bered. When the number of 

 parts is three, the flower is 

 trimerous (T^S, three), and 

 when the parts are arranged 

 in an alternating manner (fig. 

 532), the symmetry is trigonal 

 or triangular (TJ?, three, and 

 yaviet,, an angle), as in the Lily. 

 When there are four parts, the 

 flower is tetramerous (rer^eis, 

 four, and the symmetry is 

 tetragonal or square (figs. 533, 

 534), as in Gah'um and Paris. 

 When there are five parts, the 



flower is pentamerous (vivrt, five), and the symmetry pentagonal (fig. 

 535), as in Ranunculus. The number of parts in the flower is indi- 

 cated by the following symbols: Dimerous $ Trimerous $ Tetra- 

 merous /<y Pentamerous \/. 



Fig. 531. Diagram of the dimerous flower of Circaea Lntetiana, Enchanter's Nightshade. 

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

 flower is Isostemonous. 



Fig. 532. Diagram of the rrimerous Isostemonous flower of Cneorum tricoccum. The floral 

 envelopes are arranged in sets of three, and so are the essential organs. 



Fig. 533. Diagram of the tetramerous Isostemonous flower of Zieria. The organs are ar- 

 ranged in verticils of four parts each. 



Fig. 534. Diagram of the tetramerous Diplostemonous flower of Ruta graveolens. There are 

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



