Agriculture Reference
In-Depth Information
The vascular bundles are surrounded by a compact layer of elongated cells that
form a bundle sheath. The chloroplasts in this layer of cells have been shown to
contain starch (Wilson
et al.
, 1985).
In transection the garlic leaf blade shows similar tissues on both sides of
the leaf (Mann, 1952), with well-defined palisade layers. The vascular bundles
occur in two layers, one closer to the outer surface and the other closer to the
inner (adaxial) leaf surface. The central parenchyma undergoes some cell
breakdown to produce intercellular air spaces. Below the palisade layer are
numerous laticifers.
During the early stages of onion foliage leaf growth, while the leaf is within
the sheath of the next older leaf, the leaf blade elongates but the sheath does
not. When the leaf tip emerges from the pore of the older leaf the sheath begins
to elongate also (Heath and Hollies, 1965). Initially, cell division occurs
throughout the length of the onion leaf blade, but it continues for longest in the
basal parts, so these give rise to a large proportion of the fully grown blade.
During enlargement, the cells in the central region do not keep pace with the
enlargement of the outer layers, and the central cavity develops within the leaf.
Later, some of the inner parenchymal cells decay, adding to the size of the cavity
(Hayward, 1938).
The roots of the allium vegetables are comparatively thick and sparsely
branched as compared with those of most crop species. They lack root hairs
except when they are grown in moist air rather than in soil or solution culture.
Sections through onion roots show the usual series of layers from an outer
epidermis, a multicellular, thick cortex and an endodermis that surrounds the
central stele, which contains phloem and xylem vessels with their associated
parenchymatous cells (see Fig. 2.13A; de Mason, 1990, Stasovski and
Peterson, 1993).
Onions have been one of the most frequently used species in investigations
of root anatomy and physiology, probably because they can be conveniently
produced from bulbs and, being fairly thick, straight and unbranched they are
easy to handle. Hence there is a wealth of detail available on their structure and
also ultrastructure, as revealed by electron microscopy (Ma and Peterson,
2001), and how this relates to both nutrient uptake (Cholewa and Peterson,
2004) and water uptake (Barrowclough
et al.
, 2000). As the root develops,
various cell layers change and mature (see Fig. 2.14 ).
Of particular interest is the development of the exodermis immediately
below the outermost epidermal layer of cells (see Fig. 2.13). The exodermis
consists of long and short cells (see Fig. 2.13C) and, somewhat later than the
endodermis (see Fig. 2.14), these cells develop a water-impermeable, suberized
Casparian strip in their radial walls. The long cells and some of the short cells
