Imhof, S. (2003):

A dorsiventral root: unusual structures of the subterraneous system of Sciaphila polygyna (Triuridaceae).

In: Bayer, C., Dressler, S., Schneider, J. & Zizka, G: 16th International Symposium "Biodiversity and Evolutionary Biologogy" of the DBG - Frankfurt, September 21-27, 2003, p. 48.

Abstract:

The Triuridaceae, 45 to 73 species in 6 or 7 genera, are exclusively characterized by achlorophylly and morphological reductions. Their systematic position, therefore, has been difficult to ascertain, and the family was placed in an own order, superorder, or even subclass. Only recently the Angiosperm Phylogeny Group, using molecular data, assigned it to the Pandanales. All descriptions available so far on their subterranean organs reveal thin (less than 0,5 mm) filiform roots, emerging from scale leaf axils of vertically growing subterranean shoots. Hence, it was a surprise to see the collection of Mori et al. 1987 (No. 18631), doubtlessly assigned to Sciaphila, having the deviating root system introduced here.
Roots of the investigated specimen of Sciaphila aff. polygyna were brittle, up to 1.4 mm thick and not longer than 1 cm. They form a star-like root system, the roots seemingly radiating from a single origin. In fact, as it could be seen on serial sections, the roots develop endogenously in the axils of scale leaves of the shoot. These scale leaves may also bear side shoots which again have scale leaves where further roots emerge. All these organs develop very close to each other, resulting in the star-like appearance.
Not surprisingly the root tissues were largely colonized by intracellular, aseptate hyphae. However, in contrast to all other mycorrhizal pattern known so far the mycorrhiza in S. polygyna attains a bilateral symmetry when viewed in cross section. Most prominent features therein are the 'giant cells' (up to 320 x 130 µm) of the heteromorphic fourth root cell layer. Furthermore, rather loose hyphal coils including vesicle like swellings colonize the dorsal part of the third root layer, but extremely dense coils of thin hyphae without swellings are seen in its ventral part. The differing hyphal structures are linked by transitions, although these are rare and inconspicuous. In longitudinal direction the dorsal side of the third layer is colonized for almost the entire length of the root, whereas the colonization of the ventral side, when viewed from root tip to root base, starts late and fades earlier. Only in the fourth layer the hyphae degenerate to amorphous clumps, most readily in the giant cells, whereas the hyphae in the third layer keep their definite structure.
The root morphology fits well in a proposed progression line from filiform to stout, abbreviated roots, already seen in many other families of myco-heterotrophic plants. The mycorrhizal pattern in S. polygyna is interpreted as an adaptation in order to attain a sustained use of the indispensable fungus by compartmentation of root tissue: keeping the fungus alive in one part and digesting it in another.