) 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.
(10/49. 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.) 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 plant against
their enemies; but I have never seen any reason to believe that this is
so with the three species observed by me, namely, Prunus laurocerasus,
Vicia sativa, and V. faba. 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.
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