Fig. 8.

Sandsheath formation by roots growing in contact with mesh screens in the field (Fig. 7). (A) Sheath on the upper side of a root growing downward over a nylon screen (38-μm pores) beyond where the root tip first contacted the mesh (arrow) the sheath was firmly attached to both root and mesh and was somewhat wider than in the unattached portion. (B) Underside of the same piece of mesh. A broad sheath was firmly attached to the mesh beyond the point (arrow) where the root tip first contacted the upper side of the mesh. Some senescent fine roots of unknown origin and other debris were incorporated into the sheaths. (C) Detail of sand grains and tangled root hairs on the underside of the mesh (PAS reaction). (D) Transverse section of root with sheath development on each side of nylon mesh (1-μm pores). Note the coherence of the residual epidermis/cortex complex to the sheath, and the shrunken stele, due to dehydration. (E) Small sheaths formed on the underside of mesh of 1-μm mesh, as root hairs were able to displace the fibres slightly and squeeze through to the lower side. CSEM. Scale bars: (A, B) = 7 mm; (C) = 680 µm; (D) = 1·25 mm; (E) = 52 µm.