178 EVOLUTION OF PLANTS 
rounded by the floral envelopes, corolla and calyx, 
made up respectively of the petals and sepals. The 
base of the carpel forms the ovary, while above it is 
Fic. 43. — Flowers of a pond- 
weed (Naias); the. male 
flower, A, and the female, 
B, are much alike. Each 
consists of a single sporan- 
gium invested with an in- 
tegument, in, the whole 
enclosed in a tubular leaf 
with spiny processes at the 
summit. This sheath-like 
leaf is usually considered 
_in the female flower to be 
acarpel. Fig. C shows a 
section through the base of 
the female flower, with the 
enclosed macrosporangi- 
um, or ovule, ma, and two 
integuments, in, and the 
contained embryo-sac, or 
macrospore, sp. 
prolonged into the style tipped 
by the stigma, or portion upon 
which the pollen falls (Fig. 44, 
A). 
In the number of parts in the 
flower, as well as in their ar- 
rangement and form, the Angio- 
sperms show almost infinite 
variety. The petals are very 
frequently brightly colored, and 
this, together with many modi- 
fications in the other structures, 
is intimately associated with 
pollination through insect aid, 
which has undoubtedly played 
an important part in the evolu- 
tion of the floral structures of 
the Angiosperms. 
The gametophyte in the An- 
giosperms is so much reduced 
and so inconspicuous that it is 
usually quite ignored in the 
ordinary study of these plants; 
but it must be borne in mind 
that the gametophyte is always 
present, although in a very re- 
duced form. As in the Gymnosperms, the ovule cor- 
responds to the macrosporangium of the heterosporous 
Pteridophytes, and within it is formed the single macro- 
