192 AERATING SYSTEM [CH. 



parts of its stem, said to be due to aerenchyma. A third Legu- 

 minous plant, which has been described under the name of 

 Herminiera elaphroxylon, G. and P. 1 but which is perhaps better 

 regarded as another member of the genus Aeschynomene^ also 

 has aerenchyma 2 . The floating wood of this plant, which is 

 known as the "Ambatsch," is employed on the Blue Nile to 

 make rafts. The pieces used are as thick as a man's arm, and 

 show under the bark a shining white woody mass, penetrated 

 by numerous rays. The wood is exceedingly light; a segment 

 of stem 2 1 feet long and about 4 inches in diameter, is described 

 as weighing less than \\ ounces. It has been shown that the 

 pits of the xylem are real perforations with no pit-closing mem- 

 branes, so that there is free passage for gases 3 . 



The chief function served by the lacunar system of sub- 

 merged stems seems to be aeration 4 , but there are also instances 

 in which it plays a very important part in adding to the buoy- 

 ancy of the plant. In Trapa natans^ for instance, the aquatic 

 stem is formed exclusively of soft tissue, and would be unable, 

 if it depended on its own stiffness, to rear itself to the surface 

 of the water. It is entirely due to the increase of lacunae in the 

 upper part of the stem, and the swelling of the petioles of the 

 upper leaves, that the axis is enabled to raise the flowers into 

 the air. In the deeper regions, the pith is a compact tissue, and 

 there are only two circles of lacunae in the cortex, but in the 

 upper part of the stem the pith is lacunate and the number of 

 circles of air spaces increases to four or five 5 . 



The secondary lacunar tissues were always assumed by the 

 earlier writers to serve for flotation alone ; in certain cases (e.g. 

 some of the Leguminosae already mentioned) it is quite pos- 

 sible that they were correct. Martins 6 , who long ago described 

 and figured the air roots of Jussiaea, regarded them merely 



1 Also called Aedemone mtrabilis^ Kotschy. 



2 Kotschy, T. (1858), Hallier, E. (1859), Jaensch, T. (1884!) and 

 (i88 4 2 ), Klebahn, H. (1891). See also Hope, C. W. (1902). 

 3 Goebel, K. (1891-1893). 4 Schenck, H. (1889). 



5 Costantin, J. (1884). 6 Martins, C. (1866). 



