THE FLOWER AND INFLORESCENCE 89 
single ovule is inserted on the floor of the ovary the 
placentation is said to be BAsAL. 
55. Aestivation.—This term is applied to describe the 
manner in which the sepals and petals are folded in the 
flower bud (cf. vernation, par. 37). Just as we have 
valvate, imbricate and contorted vernation of leaves in 
a bud so we can have valvate, imbricate and contorted 
aestivation of the calyx or corolla. In addition, the 
margins of the floral leaves may be folded inwards on 
themselves. In this case the aestivation is Inpvu- 
pricaTE. If the corolla or calyx contains five leaves, 
two internal, two external, and one partly internal, 
partly external the aestivation is QUINCUNCIAL. VEXIL- 
LARY aestivation is characteristic of the corolla of 
Leguminosae (see fig. 98). 
56. Regular and Irregular Flowers.—It is important 
to note the variations in symmetry of a flower, as this 
often indicates how pollination is brought about. If all 
the petals and sepals are the same shape and similarly 
arranged with regard to the centre of the flower, the 
flower is said to be REGULAR, e.g. the Peach. If the 
sepals or petals differ in size or shape or arrangement 
from one another the flower is said to be IRREGULAR, 
e.g. the Pea. 
Regular flowers are radially symmetrical or actino- 
morphic, i.e. they can be divided into similar halves by 
two or more planes passing through the axis. Irregular 
flowers may be IsOBILATERAL, ie. divisible into similar 
halves by two planes through the axis at right angles 
to each other, ZYGoMoRPHIC, divisible into similar 
halves by one plane only, e.g. the Pea, Violet, Salvia ; 
