Chap. IV] 



THE AIR 



73 



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

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

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

 Last)}', in \arious Leguminosae the wood is modified into air-containing 

 tissue and consists of thin-wallcd, air-carrj-ing tracheids, resembling 

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

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

 wellings at the bases of stems. 



In many cases, certain lateral roots are differentiated as oxygcu-puuips, 

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



-> j/> 



Fig. 47. Jussieue.T peruviana, Linn, aw pneumatophores under tlie water-level sp. One-third 

 natural size. After H. Schenck. 



uch respiratory roots or pncuinatopkores (Jost) have been studied by 

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

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

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

 nud they develop normal positively geotropic lateral rootlets which 

 )enetrate the soil, and also spongy respiratory roots, which are apparently 

 ot geotropic, but, owing to the air they contain, stand upright in the 

 ■'ater and conclude their growth in length when they reach the surface, 

 espiratory roots in contrast with terrestrial roots are simple ; less frequent!}- 



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



