xiv] JUSSIAEA AND NEPTUNIA 189 



water 1 . It has been suggested 1 that the stimulus that causes 

 the phellogen to develop aerenchyma in lieu of cork, is the lack 

 of oxygen in the inner tissues. The present writer would prefer, 

 however, to express the same idea somewhat differently, and to 

 say that the presence of some minimum of oxygen is possibly 

 a necessary condition for the process of suberisation, which is 

 inhibited when the oxygen-content of the cell-sap falls below a 

 certain point. 



Some remarkable cases of aerenchyma development are found 

 in the tropical Onagraceous genus Jussiaea^\ in J. peruviana 

 (Fig. 1 22, p. 1 90), the submerged parts of the shoots are clothed 

 with this tissue, which is also developed on the normal roots 

 which enter the mud (m.r.'), and in certain erect roots which 

 seem to serve entirely for aeration (a.r.*). Fig. 1 22 B exhibits the 

 origin of the stem aerenchyma (a) from a phellogen (pg). Special 

 breathing roots also occur in the case of Jussiaea repens. They 

 show, in transverse section, a tiny stele, surrounded by a volu- 

 minous aerenchyma. That the modification of these roots is 

 directly related to the aquatic environment, is indicated by the 

 fact that Jussiaea grandiflora^ when cultivated for some years 

 in the botanical garden at Marburg as a land plant, produced 

 only normal adventitious roots, but when it was transferred to 

 water it developed roots with aerenchyma 3 . 



The aerenchyma of certain members of the Leguminosae has 

 been recognised for many years. Humboldt and Bonpland 4 , for 

 instance, more than a hundred years ago, recorded that in "Mi- 

 mosa lacustris" (Neptunia oleracea^ Lour.), the Floating Sensitive 

 Plant (Fig. 1 23, p. 19 1), the stems and branches were covered by 

 "une substance spongieuse, blanchatre." They made the mis- 

 take, however, of supposing that this tissue was a foreign body, 

 and not an integral part of the plant. More recent observations 5 



1 Schenck,H.(i889). 2 Martins, C.( 1866). 



3 Goebel,K. (1891-1893). 



4 Humboldt, A. de, and Bonpland, A. (1808). 



5 Rosanoff, S. (i 87 1 ). This author uses the name " Desmanthus natans " 

 for the plant now called Neptunia oleracea. 



