PHANEROGAMIA, 935 
distance. In sunflowers, in addition, the marginal flowers 
in the cluster develop an especially showy perianth, sur- 
rounding the whole with conspicuous rays. 
500. Many showy flowers have no nectar (honey) glands, 
but in general some part of the flower secretes a sweet, 
sugary fluid which is attractive to insects and some birds. 
The nectar is always situated in the back part of the flower, 
so that in securing it the insect is obliged to come near to 
the pollen-sacs or stigma. 
501. In this connection the various irregularities of size 
and form in the parts of the perianth, as well as of stamens 
and pistils, have ameaning. Thus the perianth-leaves may 
grow together into a tube, in which case the nectar is at 
its bottom; or they may be of different sizes, as in orchids, 
beans, peas, etc., where they are so placed as to admit of 
access to the nectar from one direction only. In some 
tubular flowers there are two forms in the same species, 
those of some plants having long stamens and short styles, 
while in others the structure is exactly the reverse. Insects 
in getting honey from these will pollinate the long-styled 
flowers with pollen from the long stamens of other flowers, 
and vice versa. There is also very often a greater or less 
difference in the time of maturity of the stamens and pis- 
tils. In some the pollen is set free before the stigma is 
ready for pollination; in others it is the reverse. This (and 
the preceding) arrangement prevents pollination of a pistil 
by pollen from the stamens of the same flower; i.e., close 
fertilization is prevented. 
502. Self-pollinated (autogamous) flowers are much less 
numerous than those which are wind- or insect-pollinated, 
and it is doubtful whether there are any species of plants 
all of whose flowers exhibit constant self-pollination (au- 
