98 THE FLOWERING PLANT. 
stamens. Their exterior is covered with epidermis, which fre- 
quently produces hairs, and may possess stomata. The filament 
and connective are traversed by a central vascular bundle, and 
the ground-tissue is made up of parenchyma. A cross-section of 
a young buttercup bud will give numerous thin slices of anthers. 
In these it will be seen that each lobe contains two compartments 
filled with pollen grains, and hence called pollen sacs (ef. fig. 51). 
The pollen grains have been formed by active division of paren- 
chymatous cells belonging to the ground-tissue. Later on the 
party-wall between the two sacs in each lobe breaks down, and 
their contents escape to the exterior by formation of a slit. 
Pollen grains are mostly spherical. They are invested by two 
membranes, a firm outer one, the eatine, often produced into 
spines, knobs, or ridges, and in the fir into a pair of little air 
bladders (fig. 45, C); and a delicate inner one, the zntine. Within 
the coatings are two or a few small cells in gymnosperms and 
protoplasm with two nuclei in angiosperms. 
The Carpets depart even more widely from the ordinary leaf- 
type than the stamens. They collectively form the piste or 
gynoecitum, which occupies the centre of the flower. 
Number and Arrangement.—In acyclic and hemicyclic flowers 
an indefinite number of carpels are frequently present. The 
ordinary cones of the Scotch fir consist of a large number of 
spirally arranged woody scales (fig. 45). These are carpels, or, at 
any rate, outgrowths from them. We have previously seen (p. 75) 
that a large number of carpels are present in the hemicyclic 
flowers of the buttercup. It is probable that two alternating 
whorls of carpels are typical for the cyclic flower. One entire 
whorl, however, is generally suppressed, and the remaining one 
is very frequently reduced. Dicotyledons often possess five or 
four carpels, as in geranium and holly respectively, but a smaller 
number is extremely common. ‘Thus, columbine has three, 
wallflower, dead nettle and foxglove two, and papilionaceous 
flowers only one. A few British monocotyledons, as flowering 
rush and frogbit, retain six carpels in alternating whorls of three 
each. Far more commonly the inner whorl is suppressed, so 
that three carpels only are present, as in lily (fig. 38), tulip, 
snowdrop, and orchis. Still further reduction takes place in 
most grasses, which have only two carpels, while duckweed, wild 
arum, and maize retain but one. In male flowers carpels are, of 
course, absent, and in this case they may have been suppressed 
or else never have existed. As mentioned previously, neuter 
flowers contain no essential organs. 
Most flowers are bisexual, 2.e., possess both stamens and car- 
pels, some are unisexual ; and are then male, with stamens only, 
ee ee ee a 
