ANGIOSPERMS. 



535 



Magnolia, the subsequent development of the organs appears also not to show any zvgo- 

 morphic nor indeed generally any kind of actually symmetrical arrangement 



The zygomorphic and monosymmetrical form occurs, on the contrary, very com- 

 monly m those flowers the parts of which are arranged in whorls. A very distinctly 

 zygomorphic arrangement is not unfrequently united with a partial or entire abortion of 

 particular members, as, e.g., in Columnea, Fig. 385, and other genera of Gesnerace^, 

 where the posterior stamen is transformed into a small nectary ; while in LabiatcT it is 

 entirely wanting. This abortion is carried still further in Orchidcce, where, of the six 

 typical stamens, only the median anterior one of the outer whorl or the two lateral 

 anterior ones of the inner whorl are developed (see Fig. 379, p. 526). The final mono- 

 symmetrical arrangement is sometimes to a certain extent indicated by the order of 

 their formation, even in the rudimentary condition of the parts of the flower, when 

 their origin is not simultaneous in the whorl, and does not progress wich a definite 

 angle of divergence, but is so arranged that the development commences with one 

 anterior or one posterior member, and then advances simultaneously right and left from 



Fig. 386.— Zyi,'oinnrphic flower of ro/j'^<7/a s>(i>'difIora : A entire flower seen from the side after removal of one 

 sepal /: ; A flower divided syniiuetrically without the frynjeceum ; C the gyn:eceum magnified ; D horizontal section of 

 the ovary ; /:" median longitudinal section of the ovary ; /•" horizontal section of the flower ; k calyx, c corolla, 

 St staiiiinal tube. </ yynophore, / ovary, .4-- style, n stis,'ma, si ovules, xx the tube formed by the adhesion of tlie 

 petals and stamens. 



the median line towards the opposite side of the w^horl. Examples have already been 

 given of this arrangement in Papilionaceae in the one case and Resedaccce in the 

 other. 



In the zygomorphic flowers of Fumariaceae, the diagram (Fig. 380, p. 526) is, as we 

 have already pointed out, symmetrically divisible in different ways by two planes. The 

 anterior and posterior halves, symmetrically similar to one another, are unlike the right 

 and left halves, which again are symmetrically alike. This is the arrangement of the parts 

 in the mature flower of Dicentra ; in Fumaria and Corydalis the right side is developed 

 differently from the left, one producing a spur, the other not ; while the anterior and 

 posterior sides remain symmetrical. In this case therefore the plane of symmetry coin- 

 cides with a lateral section. In the zygomorphic flowers of some Solanacege the plane of 

 symmetry and the median plane intersect at an acute angle. But by far the greater 

 number of zygomorphic monosymmetrical flowers are so constructed that the median 



