208 NOTES.
some substances, which if not expelled from the tissues, might have an injurious effect.
It is a subject well worthy of a more careful investigation than it has yet receiver],
and the following remarks from the works of two great biologists are highly
suggestive :—"There are, however, a considerable number of plants which bear
small glands on their leaves, petioles, phyllodia, stipules, bracteae, or flower
peduncles, or on the outside of their calyxes, and these glands secrete minute drops
of a sweet fluid, which is eagerly sought by sugar-loving insects, such as ants,
hive-bees, and wasps, In the case of the glands on the stipules of Vicia sativa,
the excretion manifestly depends on changes in the sap, consequent on the sun
shining brightly ; for I repeatedly observed that as soon as the sun was hidden
behind clouds the secretion ceased, and the hive-bees left the field ; but as soon
as the sun broke out again, they returned to their feast.* I have observed an
analogous fact with the secretion of true nectar in the flowers of Lobelia erinus.
Delpino, however, maintains that the power of secreting a sweet fluid by any
extra floral organ, has been in every case specially gained for the sake of attracting
ants and wasps as defenders of the plants against their enemies ; but I have never
seen any reason to believe that this is so with the three species observed by me,
namely Primus laurocerasus, Vicia sativa, and V fata. No plant is so little attacked
by enemies of any kind as the common bracken-fern (Pteris aquilina) ; and yet,
as my son Francis has discovered, the large glands at the bases of the fronds,
but only whilst young, excrete much sweetish fluid, which is eagerly sought by
innumerable ants, chiefly belonging to Myrmica ; and these ants certainly do not
serve as a protection against any enemy." (Darwin's "Cross and Self-fertilisation
in the Vegetable Kingdom," pp. 402-4.) "The number of plants which have
nectaries on their leaves appears to be by no means large ; at any rate, up to
the present time few such cases are known ; but the possibility always remains
that further investigations may discover many more. Those which have been
longest known are Vicia faba, V. septum, and V, sativa, Acacia longifolia,
Primus avium, and P. laurocerasus, Catalpa syringaefolia, Impatiens tricornis,
Ricinus, Viburnum tinus, and V. opulus and Clerodendron fragrans, in all of
which the leaves undoubtedly secrete nectar. In some of these, again, the secre-
tion is produced by special groups of epidermal cells, placed on the under-side of
the leaf or stipules, and transformed into glandular tissue, as is the case for
instance in Clerodendron, Primus laurocerasus, and the above-mentioned species
of Vicia ; while in others the nectar is secreted by disk-shaped or knot-headed
trichomes, which are found springing from the surface of the leaf or of the petiole,
as is the case in Catalpa, Viburnum opulus, and Prunus armeniaca. In Impatiens
tricornis the two small stipules of each leaf are entirely transformed into nectaries.
One of these is very small and stunted ; the other, on the contrary, forms a fleshy
disk, slightly convex above, and below shaped into a semi-globular protuberance,
which is partly adherent to the base of the petiole, partly to the epidermis of the stem,
and lies transversely in front of the axil from which the flower-stalk springs. The
nectar secreted by the tissue of this fleshy disk collects in the form of a drop on the
apex of the semi-globular breast-shaped swelling on the under-side of the disk.
Any insects that creep along the stem must, if they would get at the flower, of
necessity pass over this disk with its drop of nectar ; thus what they would have
sought, and moreover would have found, in the flower, is already offered them here
in rich abundance. The creeping insects are not fastidious. Nectar in one place
is the same to them as nectar in another. They are content with that which is
first offered, and so do not trouble themselves to climb further up to the flowers.
In Impatiens tricornis the stipules are so frequented by Myrmica laevinodis, Nyl.
that I have often seen three of these ants upon a single stipule ; and yet, though
I have examined hundreds of plants of this species, and though its nectariferous
flowers have no other protection whatsoever to keep out these little creatures, I
have never seen a single ant inside a blossom. They would, indeed, be very un-
* "I published a brief notice of this case in the "Gardeners' Chronicle," 1855, July 21, page 487,
and afterwards made further observations. Besides the hive-bee, another species of bee, a moth,
ants, and two kinds of flies sucked the drops of fluid on the stipules. The larger drops tasted
sweet. The hive-bees never even looked at the flowers which were open at the same time ;
whilst two species of humble-bees neglected the stipules and visited only the flowers."