Distichlis blade with salt crystals, Hopkins
Marine Station (site A)
Salt Gland Mechanics:
As should be clear by this point, Distichlis owes
its amazing halophytic characteristics to the activity of specially
evolved salt glands. These appear as spherical protrusions along all
surfaces of its blades and stem. Current research indicates that these
glands extrude an extremely briny solution of mostly sodium and potassium
ions dissolved in water .
Regarding design, through the use of transmission electron
microscopy, the salt gland has been found to contain two cells, a
basal cell and cap cell. Though the mechanics of salt extrusion in Distichlis have
yet to be definitively determined, the hypothesis of Hansen and Dayanandan
et. al. provide a reasonable explanation (see Hansen, D.J. et. al.  for
a full explanation) - this hypothesis is summarized in diagrams and
Above the cap cell there exists a cuticular cavity,
a space between the cuticle of the blade surface and the cap cell.
The cap cell initially contains a high concentration of sodium
ions in water.
Salt ions and water are actively transported
(involving use of ATP) 
of the cap cell and into the cuticular cavity.
As the pressure in this space increases as a
result of this ion pumping, the turgor pressure causes the cuticle
to stretch to the point that the salts in solution are able to
flow out of the cuticular cavity through microscopic pores in
As the cuticular cavity drains, the pressure
decreases, returning the cuticle to its previous, unstretched
state. The briny solution left on the blade surface dries to form
the classic large salt crystals found on the plant.
Salt glands can be identified in electron microscope photographs by their
spherical shape and their encrustation with salt, as contrasted with the
much mor common papillae which generally display a longer cylindrical shape.
Two scanning electron microscope photographs, shown below, were taken of
the outside of a Distichlis blade collected at site
E at Hopkins Marine Station. Unfortunately, due to the drying and coating
process used to prepare the sample, along with the relatively high accelerating
voltage used (15 KeV), many of the salt glands in this sample appeared to
Scanning electron microscope photos, 15KeV, 1200x magnification
The photograph on the left shows a large, cubic salt crystal likely
formed by the briny extrusions of a number of salt glands clustered
closely together. Smaller cubic crystals can be seen towards the
top of the photo as well.
The second photograph shows what appears to be a large collapsed
salt gland towards the center of the photo - the salt "flakes" which
once covered the spherical surface can still be seen clustered above
the collapsed top of the gland. A smaller, uncollapsed gland, with
its bulbous surface still covered in a solid layer of salt, can
be seen in the upper left corner of the photo.
Information about the interactions between salt glands, salt extrusion
rates, soil salinity, and growth can be found on the salinity
and morphology page.
Last updated March 2003, Justin Kitzes.
Copyright and reproduction information can be found here.