xiv] SESBANIA AND AESCHYNOMENE 191 



"cottony felt" as serving to hold the delicate bipinnate leaves 

 and the heads of pale yellow flowers above the surface of the 

 water. In Sesbania 1 , again, another Leguminous genus not at 

 all closely related to Neptunia, a similar air-tissue occurs, arising 

 from a cork-cambium in the inner cortex, just outside the 

 endodermis. 



It is a curious fact that among the Leguminosae we not only 

 meet with the case just described, in which an aerenchyma arises 

 externally from a phellogen, but we also find instances in which 

 a tissue of somewhat similar nature is produced internally from 



FIG. 123. Ncptuniaoleracea.'Lour. Floating shoot. The two oldest internodes have 



lost their floating tissue, /, while the three youngest have not yet developed it. 



(Reduced.) [Adapted from Rosanoff, S. (1871).] 



a normal cambium, and is thus of the nature of secondary wood. 

 In these cases, the air is contained within the xylem elements. 

 Aeschynomene aspera, Willd. 2 is a Leguminous shrub, frequent 

 in India on the margins of fresh waters, in which a pith-like 

 tissue, white and homogeneous, occupies the greater part of the 

 stem. This substance is, in fact, secondary xylem. It is so 

 extremely light in weight that it is collected to make toys, 

 floats for fishermen's nets, and 'pith' helmets. Another 

 member of the same genus which grows in Venezuela, Ae. his- 

 pidula, H. B. K. 3 , has remarkable swellings on the submerged 



1 Scott, D. H. and Wager, H. (1888). * Moeller, J. (1879). 



3 Ernst, A. (i8;2 2 ). 



