Agriculture Reference
In-Depth Information
Fig. 2.8.
Diagrammatic representations of the structure of edible allium bulbs of
progressively increasing complexity. (a), rakkyo; (b), onion; (c), garlic.
The vessels (xylem and phloem) from the roots branch at the root base and
interconnect with vessels from other roots to form a networked layer of vascular
tissue that parallels the outer surface of the stem (see Fig. 2.5). Immediately
external to this vascular layer is a single-celled endodermis and, external to that,
a multicellular cortex. Vessels from the leaves pass perpendicularly through the
vascular network formed by the root traces, but then loop backwards, divide and
interconnect with the vascular network layer from its inner side. In the centre of
the stem is the pith, a region devoid of vessels.
At the top of the stem, encircling the apical meristem but separate from it, is
a region of cell division called the primary thickening meristem (PTM). Roots are
initiated in this meristem and the cells responsible for the growth in width of the
stem proliferate here. The cells of the PTM are unusual for onion in containing
prominent starch granules, although these are absent during bulb dormancy
(Ernst and Bufler, 1994). Young roots and their associated vascular network
