Agriculture Reference
In-Depth Information
Fig. 2.4. A diagrammatic representation of the development of the stem, leaves and
roots in a young onion plant. Leaf 1 is the youngest and 4 the oldest shown. As the
stem grows it broadens, as shown by the divergent arrows below the shoot apex.
New roots continually arise in the upper part of the stem. In this figure the space
between adjacent leaf sheaths is much exaggerated (from Jones and Mann, 1963,
Fig. 5. Courtesy of John Wiley & Sons Inc., New York).
which leaves are initiated oppositely and alternately, so that the leaves emerge
in two ranks at 180 degrees to each other (see Fig. 2.1F). Each leaf consists of
blade and a sheath. The sheath develops to completely encircle the growing
point, and ultimately forms a tube that encloses younger leaves and the shoot
apex. What at first sight is the stem of the plant is in fact a 'false' or 'pseudo'
stem formed from concentric leaf sheaths and young leaf blades growing up
the centre of the older encircling sheaths (see Fig. 2.4). At the junction of the
blade and sheath there is an opening or pore where the tip of the blade of the
next youngermost leaf can be seen. Ultimately, the blade of the younger leaf will
elongate and emerge through this pore. On a fully elongated leaf the pore is
visible at the top of the pseudostem, with younger green blades elongating
through it. As new leaves are initiated and expand near the shoot apex, older
sheath bases get pushed further away from the apex by continuing lateral
expansion of the disc-like stem (see Fig. 2.5).
Roots are initiated in the stem near the base of young leaves and push
downwards through the outer layers of the stem disc to emerge (see Fig. 2.5).
The exception is the primary root, which emerges from the seed, but this
normally lives only a few weeks. As the stem disc expands, the bases of older
Search WWH ::




Custom Search