SYMMETRY OF OEGANS. 363 



form. Like leaves, too, they produce buds in their axil. - The mon- 

 strosity called Hen and Chickens Daisy depends on the development 

 of buds in the axil of the leaves of the involucre. The sepals 

 frequently present the appearance of true leaves, as in the Rose. 

 The petals sometimes become green like leaves, as in a variety of 

 Eanunculus Philonotis mentioned . by DecandoUe, and in a variety of 

 Campanula rapunouloides noticed by Dumas. At other times they 

 are changed into stamens. DecandoUe mentions a variety of Capsella 

 Bursa-pastoris, in which there were ten stamens produced in conse- 

 quence of a transformation of petals. The stamens in double flowers 

 are changed into petals, and in Kymphsea alba there is a gradual 

 transition from the one to the other. Sometimes the stamens are 

 changed into carpels, and bear ovules. This has been seen in Wall- 

 flower, some Willows, Poppy, etc. Petit-Thouars noticed a plant of 

 House-leek, in which the one-half of the anthers bore ovules, and the 

 other half pollen. The carpels, as in the double Cherry, may be seen 

 in the form of folded leaves ; in double flowers they are transformed 

 into petals, and in other cases they are developed as stamens. In a 

 monstrosity of Wallflower the placenta gave origin to flowers. It is 

 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, etc. 

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

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

 flowers only when they reach the prime of life. 



' Symmetey of Organs. — In the progress of growth the plants 

 belonging to the difierent divisions of the vegetable kingdom follow 

 certain organogenic laws (ogySiioi', an organ, and ymdca, I 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 alternat- 

 ing portions. This orderly and similar distribution of a certain 

 number of parts is called symmetry, and flowers are thus said to be 

 symmetrical with various numbers of members. When the number 

 of parts is two the flower is dirkerous-ihii, twice, /(tegos, a part) (fig. 

 630), and the symmetry two-membered. When the number of parts 

 is three the flower is trimerous (rgsTg, three), and when the parts 

 are arranged in an alternating manner (fig. 631) the symmetry is 

 trigonal or friangular {T^iTg, three, yavla, an .angle), as in the Lily. 

 When there are four parts the flower is Utramerous (rer§di, four), 

 and the symmetry is tetragonal or square (flgs. 632, 633), as in Galium 

 and Paris. When there are five parts the flower is pentamerous 



