xi INTRODUCTION TO BOTANY. 
Thus, a flower is, 
dimerous, trimerous, etc., if symmetrical, according as there are 2, 
3, etc., parts in each whorl. 
80. Flowers are unsymmetrical, or anisomerous, strictly speaking, when 
any one of the whorls has a different number of parts from the others ; 
but when the carpels alone are reduced in number, the flower is still 
frequently called symmetrical, or isomerous, if the calyx, corolla, and 
staminal whorls have all the same number of parts. 
81. Flowers are irregular when the parts of any one of the whorls are 
unequal in size, dissimilar in shape, or do not spread regularly round the 
axis at equal distances. In descriptions, it is more especially irregularity 
of the corolla that is referred to; a slight inequality in other parts does 
not hs saaaoe the flower being classed as regular, if corolla or perianth be 
regular. 
§ 10. The Perianth, and Calyx or Corolla. 
82. The Calyx or outer whorl of the perianth is usually green, smaller 
than the corolla, and of coarser texture; sometimes very minute, rudi- 
mentary, or obsolete altogether ; sometimes imperfectly whorled, or not 
whorled at all, or composed (as in Cactus) of a large number of sepals, of 
which the outer ones pass gradually into bracts, and the inner ones into 
petals. 
83. The Corolla or inner whorl of the perianth is usually coloured, 
larger than the calyx, and of a more delicate texture, and in popular lan- 
guage is often called the flower. Its petals, except in double flowers, are 
rarely indefinite in number, and the whorl more rarely broken than in the 
calyx. Sometimes the petals are very small, rudimentary, reduced to 
scales (as in Thymelee), or absent altogether. 
84. In very many cases the so-called simple Perianth is one in which 
the sepals and petals are nearly similar in form and texture, and present 
apparently a single whorl. The real nature of such a perianth may be 
detected by examining an unopened flower-bud, when one half of the 
parts will be found placed outside of the others (as in Anthericum, Ornitho- 
galum, Rumex, etc.), indicating an arrangement in two whorls, or calyx 
and corolla. Hence different authors may describe the same flower diffe- 
rently, either as having a single or a double perianth. 
85. In the following terms the prefixes expressive of the modifications 
of the corolla and its petals are equally applicable to the calyx and its 
sepals, or to the perianth and its segments. 
86. The Corolla is, 
monopetalous (sometimes called gamopetalous), when the petals are 
united or soldered together, either entirely or at the base only, 
into a cup, tube, or ring. 
polypetalous (or dialipetalous), when they are all separate or free 
from the very base. 
87. When the petals are partially united in a monopetalous corolla, the 
lower consolidated portion of the corolla is called the tube, whatever be its 
shape, and the free upper portions of the petals are called the teeth, lobes, 
or segments, according as they are short or long in proportion to the whole 
length of the corolla. When the calyx or corolla enlarges after flowering 
it is called accrescent ; when it falls early, deciduous or caducous. 
88. The flat expanded portion of a petal, corresponding to the blade of 
the leaf, is called its Jims or lamina ; and the stalk, corresponding to the 
petiole, its claw. When there is no claw, the petals are sessile. 
89. The estivation of a corolla is the arrangement of the petals, or their 
free portions, in an unexpanded bud. It is valvate, when the edges of the 
petals touch, but do not overlap ; imbricate, when the edges overlap each 
other, at least near the top ; twisted, contorted, or convolute, when each petal 
