On the Species of the Genus Primula in Essex. 153
nor in the size, shape, or colour of the calyx or corolla,
except that the tube of the latter accommodates itself slightly
to suit the difference in the position of the anthers. It is
probable that the two forms never appear on the same plant
(although this is stated to have occurred in one locality),
and the plants invariably bear, year after year, flowers of the
same form. Although there are two forms of flowers, each
form is hermaphrodite—i. e., it possesses both male and
female organs.
The heterostylism of the Cowslip was first noticed by
Perzoon in 1794,8 but for many years it was disregarded as
an accidental phenomenon, until Mr. Darwin came with his
keen observation and untiring energy to enquire into the
matter. He showed in a paper read before the Linnean
Society,9 and more fully in his 'Forms of Flowers' (1877),
that heterostylism is one of the most curious and interesting
of all the many contrivances by which cross-fertilization is
ensured in the vegetable kingdom. His first idea was that
Primulas were on their way to become dioecious—that the
long-styled flower with its prominent stigma was the most
feminine form, and the short-styled flower with its prominent
anthers was the most masculine form ; but the discovery that
the short-styled flowers were actually the most fertile upset
this idea. On making experiments he found that the two
forms require a reciprocal union to ensure full fertility; or in
other words, that a flower, say of the long-styled form, is
very largely sterile when fertilized with pollen from another
flower of the same form (a union which he calls "illegiti-
mate"), but produces its full complement of healthy seeds
when fertilized with pollen from a short-styled blossom
("legitimate" union) and vice versa. Darwin shows that if a
bee visits a short-styled primrose flower it dusts its proboscis
with pollen near the base. If then it goes to flowers of the
same plant, or flowers on another plant of the same form, no
pollen is deposited on the stigmas: on the contrary, more
will, in all probability, attach itself to the insect's proboscis ;
but if the bee visits some flowers on a plant of the opposite
form, pollen is (as required) placed at once on the stigma,
because that part of the bee's proboscis which touches the
anthers in one case touches the stigma in the other, and thus
is reciprocal cross-fertilization effected. Were the anthers
and stigma placed on the same level, the flowers would more
likely than not be self-fertilized; and this Darwin has shown
8 'Forms of Flowers,' p. 14.
9 Proc. Linn. Soc. (Bot.), vol. vi. (1862), p. 77, and vol. x. (1869),
p. 393.
M