416 STKUCTUEE OF Chap. Vll. 



as an embryological habit, probably the result of Meli- 

 lotiis being descended from some form which slept like 

 a Trifolium. This view is partially supported by the 

 leaves on old and young branches of another species, 

 M. Messanensis (not included in the above 15 species); 

 always sleeping like those of a Trifolium. 



The first true leaf of Mimosa albida consists of a 

 simple petiole, often bearing three pairs of leaflets, all 

 of which are of nearly equal size and of the same 

 shape : the second leaf differs widely from the first, 

 and resembles that on a mature plant (see Fig. 159, 

 p. 379), for it consists of two pinnae, each of which 

 Ijears two pairs of leaflets, of which the inner basal 

 one is very small. But at the base of each pinna 

 there is a pair of minute points, evidently rudiments 

 of leaflets, for they are of unequal sizes, like the two 

 succeeding leaflets. These rudiments are in one sense 

 embryological, for they exist only during the youth of 

 the leaf, falling off and disappearing as soon as it is 

 fully grown. 



With Desmodiwm gyrans the two lateral leaflets are 

 very much smaller than the corresponding leaflets in 

 most of the species in this large genus ; they vary 

 also in position and size ; one or both are sometimes 

 absent ; and they do not sleep like the fully-developed 

 leaflets. They may therefore be considered as almost 

 rudimentary ; and in accordance with the general prin- 

 ciples of embryology, they ought to be more constantly 

 and fully developed on very young than on old plants. 

 But this is not the case, for they were quite absent 

 on some young seedlings, and did not appear until 

 from 10 to 20 leaves had been formed. This fact 

 leads to the suspicion that D. gyrans is descended 

 through a unifoliate form (of which some exist) from 

 a trifoliate species ; and that the little lateral leaflets 

 reappear through reversion. However this may be, 



