o raised
up about 12 deg. When I saw that it possessed such complex powers of
movement, I thought it would utilise its power to protect the leaflets
from rain. Accordingly I syringed the plant for two minutes, and it was
really beautiful to see how each leaflet on the younger leaves twisted
its short sub-petiole, so that the blade was immediately directed at
an angle between 45 and 90 deg to the horizon. I could not resist the
pleasure of just telling you why I want to know the name of the Cassia.
I should add that it is a greenhouse plant. I suppose that there will
not be any better flowers till next summer or autumn.

LETTER 722. TO T. BELT.

(722/1. Belt's account, discussed in this letter, is probably that
published in his "Naturalist in Nicaragua" (1874), where he describes
"the relation between the presence of honey-secreting glands on plants,
and the protection to the latter secured by the attendance of ants
attracted by the honey." (Op. cit., pages 222 et seq.))

Thursday [1874?].

Your account of the ants and their relations seems to me to possess
extraordinary interest. I do not doubt that the excretion of sweet fluid
by the glands is in your cases of great advantage to the plants by means
of the ants, but I cannot avoid believing that primordially it is a
simple excretion, as occasionally occurs from the surface of the leaves
of lime trees. It is quite possible that the primordial excretion may
have been beneficially increased to serve the plant. In the common
laurel [Prunus laurocerasus] of our gardens the hive-bees visit
incessantly the glands of the young leaves, on their under sides; and I
should altogether doubt whether their visits or the occasional visits of
ants was of any service to the laurel. The stipules of the common vetch
secrete largely during sunshine, and hive-bees collect the sweet fluid.
So I think it is with the common bean.

I am writing this away from home, and I have come away to get some rest,
having been a good deal overworked. I shall read your book with great
interest when published, but will not trouble you to send the MS., as I
really have no spare strength or time. I believe that your book, judging
by the chapter sent, will be extremely valuable.

LETTER 723. TO J.D. HOOKER.

(723/1. The following letter refers to Darwin's prediction as to the
manner in which Hedychium (Zinziberaceae) is fertilised. Sir J.D. Hooker
seems to have made inquiries in India in consequence