74 THE FLOWER (. SYMMETRY) 
enlarge into a conspicuous lower lip, forming the landing- 
place for insects, as in most Labiatae and allied orders, 
Orchidaceae, &c. If the lower half of the corolla, on the 
other hand, assumed the protective duties, we should get 
such a case as seen in the Leguminosae. This fact, that 
the flower has now only one track to the honey instead of 
many, is probably one of the chief advantages of zygo- 
morphism, as it enables the anthers and stigmas to be kept 
in the one place only and thus favours economy of pollen, &c. 
The two-lipped condition, however, is not the only form 
of irregularity. Very often one petal differs from the rest in 
shape, e.g. the lower petal of Viola is drawn out into a spur 
in which the honey is protected from short-lipped insects. 
Similar phenomena occur in Delphinium, many Orchids 
(Angraecum is the most remarkable), and many Sympetalae, 
e.g. Centranthus, Linaria. In the irregular outer flowers of 
the condensed inflorescences of many Cruciferae, Umbelli- 
ferae, Compositae, Dipsaceae, &c., the advantage seems to 
lie chiefly in greater conspicuousness. 
Gravity and other causes seem to have had a good deal 
to do with the determination of zygomorphism, and it must 
not be regarded without further proof as entirely an adapta- 
tion to insect-pollination. It occurs in the wind-pollinated 
Podostemaceae 1 . 
Descriptive Terms , &c. Flowers may be regular or irregular , 
actinomorphic , zygo?norphic , or asymmetrical (see above). In zygo- 
morphic flowers the plane of symmetry is usually the antero-posterior 
plane. Transverse zygomorphism occurs in some Papaveraceae (y.z/.), 
Haemodoraceae, Anigozanthos, &c., diagonal in Solanaceae, &c., right- 
and left-handed in Exacum, Saintpaulia, Cassia, &c. Twisting of the 
stalk of the flower sometimes alters the plane of the symmetry, as in 
Lobelia, Orchidaceae, Fumaria, Impatiens, &c. The terminal flower 
of an inflorescence has sometimes a different symmetry from the laterals, 
e.g. in Ruta, Acer, Adoxa, Acalypha, &c. (cf. also the peloria of 
Linaria). In Ariopsis, Cyclanthus, Morinda, Lonicera, &c. more or 
less union of different flowers occurs. Mention may also be made of 
the polymorphism of the flowers of some Orchids, e.g. Catasetum. 
Irregularity is most common in the perianth, and the terms used are 
given below. 
The Flower Bud. In the young bud the floral organs 
1 Goebel, Organography of Plants ; Vochting in Pringsh. Jahrb. f. 
wiss. Bot. xvn, 1886; Robertson in Bot. Gaz. 1888; Willis (dorsiven- 
trality of Podostemaceae) in Ann. Perad. I, J902, pp. 426, 434. 
