In plant anatomy, the Casparian strip is a band of cell wall material deposited on the radial and transverse walls of the endodermis, and is chemically different from the rest of the cell wall, being made of suberin and lignin. It blocks the passive flow of materials such as water and solutes into the stele of a plant. The band was first recognized as a wall structure by Robert Caspary (1818–1887).
The strip forms during the early ontogeny of the cell and is a part of the primary wall. It varies in width and is often much narrower than the wall in which it occurs. It is typically located closer to the inner tangential wall than the outer.
The chemistry of the Casparian strip has been variously described as composed of suberin. According to some studies, the Casparian strip is initiated as a localized deposition of phenolic and unsaturated fatty substances in the middle lamella between the radial walls, as partly oxidized films. The primary wall becomes encrusted with and later thickened by deposits of similar substances on the inside of the wall. The encrustation of the cell wall by the material constituting the Casparian strip presumably blocks the submicroscopic capillaries in the wall. The cytoplasm of the endodermal cell is firmly attached to the Casparian strip so that it does not readily separate from the strip when the cells are subjected to the effects of plasmolytic or other agents normally causing a contraction of protoplasts. Thus, the Casparian strip appears to form a barrier at which the apoplastic flow is forced to pass through the selectively permeable plasma membrane into the cytoplasm (the symplast).
Casparian strips differentiate after the centripetal growth of the cortex is completed. At this level of the root, primary xylem development in the vascular cylinder may be more or less advanced. In gymnosperms and angiosperms having secondary growth, the roots commonly develop no other kind of endodermis than that with Casparian strips. In many of these plants the endodermis is later discarded, together with the cortex, when the periderm develops from the pericycle. If the pericycle is superficial and the cortex is retained either the endodermis is stretched or crushed or it keeps pace with the expansion of the vascular cylinder by radial anticlinal divisions, the new walls developing Casparian strips in continuity with the old ones.
In the absence of secondary growth (most monocotyledons and a few eudicots), the endodermis commonly undergoes certain wall modifications. Workers distinguish two developmental stages in addition to the first stage when only the Casparian strip is present. In the second stage a suberin (or endodermin) lamella covers the entire wall on the inside of the cell, so the Casparian strip is separated from the cytoplasm and the connection between the two ceases to be evident. In the third stage, a thick cellulose layer is deposited over the suberin lamella, sometimes mainly on the inner tangential walls. The thick wall, as well as the original wall in which the Casparian strip is located, may become lignified. The Casparian strip may or may not be identifiable after the thickening of the endodermal wall has occurred. The thick endodermal wall, a secondary cell wall, may have pits. The successive development of endodermal walls is clearly expressed in monocotyledons. In dicotyledons, the distinction between the second and third stages of endodermal development may not be sharp (Guttenberg, 1943), and in the seedless vascular plants the differentiation is terminated with the deposition of the suberin lamella. An endodermis with Casparian strips and later wall modifications occurs in aerial roots).
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