THE SEED PLANTS 299 
The essential structures of a flower are stamens (microsporo- 
Phylls) and pistils, or carpels (meyasporophylls). A carpel, 
strictly speaking, is one megasporophyll, and when several 
carpels are united, the result is known as a compound carpel. 
The name pistil is used without discrimination for either a 
simple or a compound carpel. 
The flower and the resulting seed that characterize this 
group of plants has often led people to give to the angio- 
sperms names which suggest these characters. The most com- 
mon of these names are flowering plants and seed plants. The 
name phanerogam, meaning “ plants with visible reproduction,” 
was applied when botanists knew less of the intricacies of the 
reproduction of angiosperms than is now known. In the same 
way eryptogam, which means “ plants with hidden reproduc- 
tion,” was applied collectively to the pteridophytes, bryophytes, 
and thallophytes. These names are still used by many people, 
but it is evident that an interchange of the names would bet- 
ter fit the facts of reproduction in the groups of the plant 
kingdom. 
284. Stamens and pollen. In connection with the discussion 
of the gymnosperms the structures of the stamen are fully 
illustrated (fig. 225). The parts of the angiosperm stamen — 
the anther and the filament —are similar to the same structures 
in the gymnosperms, though of course many variations appear. 
In the young angiosperm anther there are four sporangia, and 
these, when they ripen their spores, unite in pairs, so that 
two pollen sacs are formed from the four sporangia (fig. 108). 
The anthers of the angiosperms may open in a variety of ways, 
the method of opening being called the dehiscence. 
Since the pollen grains are formed by the division of cells 
in a sporangium, it is evident that they are asexual spores. 
When mature each pollen grain consists of a heavy outer wall, 
an inner wall, cytoplasm, and nucleus (fig. 118). Often there 
are starch and oil foods in the pollen grains. The single- 
celled pollen grain sometimes begins to germinate before it 
leaves the anther in which it was formed, and when this has 
