460 SPERMATOPHYTES (SEED PLANTS) 
characteristic of Angiosperms. The stamens are microsporo- 
phylls and the pistils are megasporophylls. A typical flower is, 
therefore, essentially an association of sporophylls surrounded 
by a perianth, and, in so far as a flower is an association of sporo- 
phylls, it does not differ 
fundamentally from a stro- 
bilus. In passing from the 
simplest Angiosperms, 
where there are flowers 
that have no perianth, to 
e those Angiosperms having 
. typical flowers, all grada- 
c tions between a_ typical 
strobilis and a_ typical 
% flower can be found. Itis, 
b, ~ therefore, impossible to de- 
aI a fine a flower so as to in- 
4 clude the flowers of all 
Angiosperms and at the 
same time separate the 
thd flower from the strobilus. 
The flowers of Angiosperms 
and the strobili of the Gym- 
nosperms and Pterido- 
phytes differ in the char- 
acter of their sporophylls 
more than in any other 
feature. 
\ 
ag 
Fig. 407. — The floral structures of a 
typical flower. The floral structures com- 
prise a perianth (a) composed of calyx : . 
and corolla, a number of microsporophylls Perianth. — The _ peri- 
or stamens each consisting of anther (ec) anth, usually consisting of 
and filament (c), and a pistil (b) composed both sepals and petals, not 
of one or more megasporophylls with the only protects the sporo- 
megasporangia or ovules (d) enclosed in an phylls during hei develop- 
ovary. i 
ment but also serves in 
pollination, which in Angiosperms is done largely by insects. 
At the base of the perianth occur nectar glands, which are further 
adaptations to insect pollination. The perianth seems to have 
arisen in two ways. In some cases there is evidence that the 
parts of the perianth are modified sporophylls, while in other 
cases they are apparently modified foliage leaves. 
