Chap. IV] 



THE AIR 



73 



taken from the atmosphere through large lenticels, and partly from the 

 water (Rhizophora, Bruguiera, Avicennia, and others). Less frequently 

 the air-canals are chiefly found in the secondary cortex (Laguncularia) 1 . 

 Lastly, in. various Leguminosae the wood is modified into air-containing 

 tissue and consists of thin-walled, air-carrying tracheids, resembling 

 cambium-cells in shape and size, and intercommunicating by means of 

 open pores. Such air-containing wood, if largely developed, causes large 

 swellings at the bases of stems. 



In many cases, certain lateral roots are differentiated as oxygen-pumps, 

 and in accordance with this function differ structurally from other roots. 



Fig. 47. Jussieuea peruviana, Linn, aw pneumatophores under the water-level sp. One-third 

 natural size. After H. Schenck. 



Such respiratory roots or pneumatophores (Jost) have been studied by 

 Schenck in species of Jussieuea inhabiting in numbers, as shrubs or under- 

 shrubs, the shallow waters of warmer districts (Fig. 47). These plants grow 

 in still parts of the water, and from their rhizomes creeping through the 

 mud they develop normal positively geotropic lateral rootlets which 

 penetrate the soil, and also spongy respiratory roots, which are apparently 

 not geotropic, but, owing to the air they contain, stand upright in the 

 water and conclude their growth in length when they reach the surface. 

 Respiratory roots in contrast with terrestrial roots are simple ; less frequently 



1 See, for example, H. Schenck, III ; Schimper ; Karsten. 



