Casparian bands

The absorption of elements by roots can be both active (metabolic) and passive (nonmetabolic). It was known from early experiments that a number of metal ions are able to diffuse passively in and out of the root up to the Casparian band although uptake in some cases, such as K+ and Na+, by epidermal and cortex cells seems to be by active transport. There is evidence that ions cross the root by two general pathways ... 

Suberin is laid down in various internal locations in order to seal off specific regions of the plant (234). In the primary development of plant roots, suberin is deposited in the Casparian band of endodermal cells (45, 103, 162, 317, 364, 367, 369, 474, 483). The presence of suberin in the Casparian band of the endodermis of Sorghum bicolor was shown chemically by depolymerization (244). Suberized layers are also found in the mestome sheath and bundle sheath cell walls of grasses (52, 61, 108, 118, 171, 172, 331, 368), and these layers have been chemically characterized in Zea mays and Secale cereale (114, 146). 

Perumalla C J, Peterson C A 1986 Deposition of Casparian bands and suberin lamellae in the exodermis and endodermis of young corn and onion roots. Can J Bot 64 1873-1878... 

Peterson C A, Emanuel M E, Wilson C 1982 Identification of a Casparian band in the hypoder-mis of onion and corn roots. Can J Bot 60 1529-1535... 

Peterson C A, Perumalla C A 1984 Development of the hypodermal Casparian band in corn and onion roots. J Exp Bot 35 51-57... 

---