THE STRUCTURE OF EDIBLE ALLIUMS - Onions and other Vegetable Alliums - page 42

Fig. 2.14. Diagram of an onion root (not to scale) showing positions of early
metaxylem (central ladder-hatched structure), Casparian bands (dashed lines),
Casparian bands plus suberin lamellae (solid lines) in the endodermis (at edge of
central stele) and exodermis (near root surface). Segment A, immature endodermis
and exodermis; segment B, endodermis with Casparian bands, immature
exodermis; segment C, developing exodermis with Casparian bands and sometimes
also suberin lamellae; segment D, endodermis with Casparian bands, exodermis
with Casparian bands and suberin lamellae; segment E, endodermis and exodermis
with both Casparian bands and suberin lamellae. The water conductivities of Zones
1,2 and 3 were measured in experiments (from Barrowclough et al ., 2000. Courtesy
of Journal of Experimental Botany ).
the exodermis. These cells, along with the remaining unsuberized exodermal
short cells, which expose a living plasmalemma surface representing just
0.47% of the root surface-area (Kamula et al. , 1994), can remain alive for up to
200 days (see Fig. 2.13E).
In addition to controlling the pathway of nutrient flow into the root cortex,
there are other roles that the exodermis may play in the functioning of roots.
The short cells may provide entry points for arbuscular mycorrhizae (AM) (see