26 OUTLINE OF STRUCTURAL BOTANY 
These external organs, corolla and calyx, are largely protective 
to the essential organs but they have, especially the corolla has, 
an important function aside from the protective one, that of 
attracting insects or birds and it is this function which undoubt- 
edly lends to the flower its charm of form, hue and perfume. 
When the several petals of a flower are distinct, one from the 
other to the base as we see them in the buttercup (Fig. 28) the 
corolla is said to be dialypetalous, i. e., separate petaled and the 
sepals being thus distinct the calyx is dialysepalous; or the corolla 
is said to be polypetalous, i. e. many petaled, and the calyx poly- 
sepalous. ‘This arrangement of distinct petals is by no means 
constant and we see the parts of the corolla more or less united, 
from those corollas in which there ‘is a faint union at the base to 
those in which, like the blue bell, the petals are all united to form 
a single envelope. The corolla of which the petals are thus joined, 
is said to be gamopetalous or monopetalous. (Fig. 33). In the 
great family of composite flowers, asters, thistles, etc., the heads 
are made up of from few to many of these gamopetalous flowers 
crowded into colonies, the whole having the appearance of a single 
flower. 
The great majority of showy flowers are entomophilous, that is, 
they are attractive to insects. The primary source of this attraction 
is a series of small sacs or glands secreting a sweet liquid or semi- 
liquid substance called nectar. These nectar sacs may be situated 
at the base of the pistil, at that of the stamen, in the inner extrem- 
ity of the pistils or even at the base of the sepals. In the violet 
it is hidden at the end of the spur. Nectaries are indeed sometimes 
situated outside the flowers but they are always enticements to the 
insects which are induced to visit the flower. (Fig. 34.) 
It is from the nectar that the perfume of the flower largely pro- 
ceeds and it is this nectar in which certain insects as well as hum- 
ming birds delight as an article of food. 
It is not always enough that the flower exhales a perfume which 
is pleasant and enticing to the insect, the wind or other agencies 
might prevent the perfume reaching the sensitive organs of smell 
but there is added a display which is attractive to the sight. Hence 
the fine colors and even probably the varied and beautiful forms 
of the corolla are provided as additional attractions to these winged 
visitors. 
The instrumentality of these visitors in the fertilization of the 
flower will be seen as we proceed. 
The various elements of the flower are arranged with a certain 
degree of uniformity even in the most irregular forms, 
