GROUP IV. — PHANEROGAMIA : ANGIOSPERMJE. 



511 



Violaceoe, Echium, Lobelia, Orchidacese, the marginal flowers of 

 the inflorescences in some Umbelliferfe and the ray- florets of some 

 Compositee). The degree of irregularity in these flowers varies 

 widely ; the irregularity may be very slight, due to the more active 

 growth of the leaves (perianth-leaves only, or stamens also) of 

 one half of the flower, either the posterior (e.g. Gladiolus), or the 

 anterior (e.g. Amaryllis), which causes an upward or a downward 

 curvature ; this is more marked in Dictamnus where the calyx 

 and corolla tend to form two lips, an upper and a lower; this 

 bilabiate form of flower is more fully developed in the calyx and 

 corolla of the Labiates, the corolla {personate, the lips being closed) 

 of the Scrophulariaceae, and of the Orchidacese and Lobelia. In 

 not a few cases the irregularity of the flower is increased by the 

 development of spurs from some portion of the perianth (e.g. 

 among Monocotyledons, Or- 

 chis, Rhinopetalum, from the q 

 corolla ; among Dicotyledons, 

 Linaria, Viola, from the co- 

 rolla; Pelargonium, from the 

 calyx). A remarkable mor- 

 phological feature is offered by 

 the flowers of Orchis and of 

 Lobelia which are resupinate ; 

 that is, in consequence of tor- 

 sion of the pedicel, the pos- 

 terior side of the flower becomes 

 anterior. The plane of sym- 

 metry is generally median in 

 these flowers. 



In some few cases the irregularity, leading to doi*siventrality, 

 is due, not to the unequal development of the floral leaves, but 

 to the configuration of the floral receptacle, so that the floral 

 leaves are not developed in a radially symmetrical manner (e.g. 

 Reseda, Papilionese, Fig. 827). 



When in irregular flowers the single plane of symmetry is the 

 median plane, the flower is dorsiventral : but there are other cases 

 (e.g. flowers of some Fumariaceee, Fumaria, Corydalis) in which 

 the single plane of symmetry is the lateral ; these flowers are 

 therefore not dorsiventral, that is, they have not an tero- posterior, 

 but lateral, asymmetry. The zygomorphic symmetry of a flower 

 is indicated in its floral formula by symbols ; when the plane of 



V. s. B. L L 



Fig. 327.— Diagram illustrating dorsiventral 

 symmetry in leguminous flowers: A Vicia 

 Faba (Papilione«e) : B Cercis Siliquastrum 

 (Ceesalpiiiieee) : in both cases tbe odd sepal 

 is anterior : the plane of symmetry is median. 



