222 RELATION OF PARTS OF THE FLOWER TO THE AXIS. 
the species of St. John’s Wort (Hypericum), the Geranium, 
and in many other plants, the calyx is quincwncial or imbricate ; 
and the corolla twisted. 
The kinds of eestivation above described are always constant 
in the same individual, and frequently throughout entire genera, 
and even natural orders ; hence they are of great importance in 
Systematic Botany. For a similar reason they are also of much 
value in Structural Botany, by the assistance they commonly 
afford in enabling us to ascertain the relative succession and 
position of the parts of the flower on the axis. 
The term anthesis is sometimes used to indicate the period at 
which the flower-bud opens. ) 
Besides the definite and constant relations which the parts 
of the floral envelopes have to one another in the flower-bud, 
they have also a definite and constant relation in the same 
plant to the axis upon which they are placed. In describing 
these positions we use the terms anterior or wmferior, swperior 
or posterior, and lateral. Thus, we call that organ posterior or 
superior, which is turned towards the axis ; and that next the 
bract from the axil of which it arises, wmferior or anterwr. 
When there are four organs in a whorl, one will be swperior, 
one inferior, and two lateral, as in the petals of the Wallflower 
(fig. 25, p, p). If there are five we have two arrangements. 
Thus, in the calyx of the order Leguminose, two sepals are 
superior, two lateral, and one inferior ; while in the corolla one 
petal is superior, two inferior, and two lateral (figs. 452 and 
477). But in plants of the order Rosacex we have a precisely 
reverse position exhibited by the parts of the two floral enve- 
lopes ; thus, here we have two sepals inferior, two lateral, and 
one superior ; while in the corolla there are two petals swperior, 
two lateral, and one inferior (fig. 476). 
The same definite relation with respect to the axis also 
holds good in many cases in the staminal and carpellary whorls, 
by which important distinctive characters are frequently ob- 
tained, as will be seen afterwards when treating of Systematic 
Botany. 
Section 3. THE FLorRAL ENVELOPES. 
1. THE CALYX. 
We have already stated that the calyx is the outermost 
envelope of the flower, and that it is composed of one or more 
leafy organs called sepals. These sepals are usually green like the 
foliage leaves, by which character, as well as by their position and 
more delicate texture, they may, in most cases, be distinguished 
from the petals. There are numerous instances, however, 
especially when the number of petals is much increased, in 
which there is a gradual transition from the sepals to the petals, 
