i868 i88i] SLEEPING PLANTS 399 



Cassia. I want much to know its name, as its power of move- Letter 721 

 ment, when it goes to sleep, is very remarkable. Linnaeus, I 

 find, was aware of this. It twists each separate leaflet almost 

 completely round, 1 so that the lower surface faces the sky, at 

 the same time depressing them all. The terminal leaflets are 

 pointed towards the base of the leaf. The whole leaf is also 

 raised up about 12. 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 4 5 and 90 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. 



To T. Belt. Letter 722 



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 pro- 

 tection to the latter secured by the attendance of ants attracted by the 

 honey." (Op. tit., pp. 222 et seg.} 



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 sw*eet 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 1 think it is with the common bean. 



1 See Power of Movement in Plants, Fig. 154, p. 370. 



