I. Shoot = the above ground portion of the plant
A. Includes stems, leaves, buds, flowers and fruits
B. Leaves are sites of food production and stems function in conduction and
support
C. Stems may also serve as important storage organs, e.g. the Irish potato.
II. Shoot development
A. Shoot growth is initiated in the embryo from the epicotyl, which may or
may not have one or more leaf primordia
B. As the stem elongates it differentiates into nodes and internodes
1. Nodes = areas where leaves arise
2. Internodes = the intervals between the nodes
C. The growing terminal bud usually exhibits apical dominance, i.e.
it secretes hormones which prevent the initiation of lateral buds
1. As the distance from the terminal bud increases the hormonal effect
decreases, causing more lateral branching
D. The apical meristem of the shoot is more complex than that of the root
1. In addition to adding cells to the primary plant body it is also
involved in forming leaf and bud primordia
E. As in the root the apical meristem gives gives rise to three
primary meristems:
1. Protoderm - gives rise to the outer, protective coating
of the plant
2. Procambium - gives rise to the vascular tissue
3. Ground meristem - gives rise to ground
tissue
III. Arrangement of primary vascular tissues
A. There are three basic arrangements of the primary
vascular tissue and ground tissues within stems:
1. The primary vascular tissues appear as a more or less continuous
hollow cylinder which divides the ground tissue into cortex and pith
a. The outer region of the ground tissue is called cortex and
the inner region is called pith
b. In some conifers and dicots that undergo secondary growth a single
layer between the primary xylem and primary phloem remains meristematic
giving rise to the vascular cambium
2. The primary vascular tissues develop as a cylinder of discrete
vascular bundles separated from each other by ground tissue and
surrounding a central pith
a. The ground tissue separating the strands is called interfascicular
parenchyma (between the bundles)
b. In plants that do not undergo secondary growth (herbs) a complete
bundle sheath of sclerenchyma cells form around the vascular strands and the procambium loses its ability to divide
once the primary vascular tissues are formed. Therefore the potential for
secondary growth within the bundle is lost. Such vascular bundles are
called closed
c. Vascular bundles that lack complete bundle sheaths are open.
Their procambia retain the ability to divide and produce secondary
vascular tissue. An interfascicular cambium will develop to "round
out the stem"
3. The primary vascular tissues develop a complex
arrangement with the vascular bundles scattered throughout the ground tissue
a. This arrangement is characteristics of most monocots
b. Corn (Zea) is a prime example. As in all monocots the
vascular bundles are closed so no secondary growth can occur.