Chap. \ll. SLEEP OF LEAVES, 3GiJ 



the previous day at an unusually high temperature ; the little 

 lateral leaflets were also jerking at this hour, but whether 

 there was any connection between this latter fact and the sub- 

 horizontal position of the terminal leaflets we do not know. 

 Anyhow, it is certain that the lateral leaflets do not sleep like 

 the terminal leaflets; and in so far they may be scid to be 

 in a functionally rudimentary condition. They are in a similar 

 condition in relation to irritability; for if a plant be shaken 

 or syringed, the terminal leaflets sink down to about 45° be- 

 neath the horizon ; but we could never detect any effect thus 

 produced on the lateral leaflets; yet we are not prepared to 

 assert positively that rubbing or pricking the pulvinus produces 

 no effect. 



As in the case of most rudimentary organs, the leaflets are 

 variable in size ; they often depart from their normal position 

 and do not stand opposite one another; and one of the two is 

 frequently absent. This absence appeared in some, but not in 

 all the cases, to be due to the leaflet having become completely 

 confluent with the main petiole, as might be inferred from the 

 presence of a slight ridge along its upper margin, and from the 

 course of the vessels. In one instance there was a vestige of 

 the leaflet, in the shape of a minute point, at the further end of the 

 ridge. The frequent, sudden, and complete disappearance of one 

 or both of the rudimentary leaflets is a rather singular fact; but 

 it is a much more surprising one that the leaves which are first 

 developed on seedling plants are not provided with them. Thus, 

 on one seedling the seventh leaf above the cotyledons was the 

 first which bore any lateral leaflets, and then only a single one. 

 On another seedling, the eleventh leaf first bore a leaflet; of the 

 nine succeeding leaves five bore a single lateral leaflet, and 

 four bore none at all; at last a leaf, the twenty-first above the 

 cotyledons, was provided with two rudimentary lateral leaflets. 

 From a widespread analogy in the animal kingdom, it might 

 have been expected that these rudimentary leaflets would have 

 been better developed and more regularly present on very young 

 than on older plants. But bearing in mind, firstly, that long- 

 lost characters sometimes reappear late in life, and secondly, 

 that the species of Desmodium are generally trifoliate, but that 

 some are unifoliate, the suspicion arises that D. gyrans is 

 descended from a unifoliate species, and that this was descended 

 from a trifoliate one ; for in this case both the absence of the 

 little lateral leaflets on very young seedlings, and their sub- 



