hods been needed? And how have the more complex arrangements of so
many flowers been brought about? Before attempting to answer these
questions, and in order that the reader may appreciate the difficulty of
the problem and the nature of the facts to be explained, it will be
necessary to give a summary of the more elaborate modes of securing
cross-fertilisation.

(1) We first have dimorphism and heteromorphism, the phenomena of which
have been already sketched in our seventh chapter.

Here we have both a mechanical and a physiological modification, the
stamens and pistil being variously modified in length and position,
while the different stamens in the same flower have widely different
degrees of fertility when applied to the same stigma,--a phenomenon
which, if it were not so well established, would have appeared in the
highest degree improbable. The most remarkable case is that of the three
different forms of the loosestrife (Lythrum salicaria) here figured
(Fig. 29 on next page).

(2) Some flowers have irritable stamens which, when their bases are
touched by an insect, spring up and dust it with pollen. This occurs in
our common berberry.

[Illustration: FIG. 29.--Lythrum salicaria (Purple loosestrife).]

(3) In others there are levers or processes by which the anthers are
mechanically brought down on to the head or back of an insect entering
the flower, in such a position as to be carried to the stigma of the
next flower it visits. This may be well seen in many species of Salvia
and Erica.

(4) In some there is a sticky secretion which, getting on to the
proboscis of an insect, carries away the pollen, and applies it to the
stigma of another flower. This occurs in our common milkwort (Polygala
vulgaris).

(5) In papilionaceous plants there are many complex adjustments, such as
the squeezing out of pollen from a receptacle on to an insect, as in
Lotus corniculatus, or the sudden springing out and exploding of the
anthers so as thoroughly to dust the insect, as in Medicago falcata,
this occurring after the stigma has touched the insect and taken off
some pollen from the last flower.

(6) Some flowers or spathes form closed boxes in which insects find
themselves entrapped, and when they have fertilised the flower, the
fringe of hairs opens and allows them to escape. This occurs in many
species of Arum and Aristolochia.

(7) Still more remarkable are the traps in the flower of Asclepias which
catch flies, bu