ACIDITY: AERATION 43 



one of the important factors affecting the development of 

 the root system. 



5. Aeration. — Many plants grow in stations where the 

 supply of oxygen to the roots is very restricted. This is 

 the case with the aquatics of still water and with plants of 

 bogs and marshes. The root systems of such plants we 

 have already seen to be relatively poorly developed, though 

 in some cases they are extensive. Such roots show, in 

 general, a well-developed aerating system of cortical spaces, 

 intercellular or lysigenous (formed by the breaking down of 

 cells). In secondary tissues an aerating system is developed 

 by the action of the cam.bium or of a special phellogen, or 

 cork-forming meristem (Arber). The system is in com- 

 munication with the air spaces of the shoot, these too being 

 very prominent in aquatic plants. In some plants there is 

 considerable variation in regard to the extent to which the 

 aerating system develops under different conditions. 

 Batten (19 18) has shown that the roots of Epilohium hirsutum 

 when grown in clay have more extensive air spaces than 

 when grown in sand. The total extent of the root system 

 is much less in the former case although the shoot growth 

 is rather better. 



We may here refer to the pneumatophores, or special 

 aerating roots, possessed notably by various mangroves. 

 According to Schimper, these are simplest in Carapa 

 obovata, where the upper edge of creeping roots projects 

 above the surface of the mud or water. In Bruguiera, the 

 horizontal roots bend out of the mud in knee-shaped 

 structures. In Avicennia, special negatively geotropic 

 lateral roots grow vertically above the surface for a few 

 inches, while in Sonneratia they may be a few feet long. 

 In the tropical shrubby Jussietia repeiis, there are normal 

 laterals growing into the mud and others which rise to the 

 surface, evidently floated by the air they contain in a remark- 

 able cortical aerenchyma. All these roots possess very 

 marked aerenchyma, or aerating tissue, developed in the 

 primary cortex, and, while that cortex is retained, abundant 

 lenticels (openings to the atmosphere). 



