General plant
Dr. Huda Jassim Al-Tameme
Lab -6- The Stem
The stems of a plant represent elongate axes composed of nodes and internodes.
Stems function as the principal supporting organ of the plant body, holding up and
separating the leaves to increase their exposure to sunlight for photosynthesis,
supporting the flowers for pollination, and holding up the fruits to increase their
chances for effective dispersal. Stems contain vascular tissue for the conduction of
materials throughout the plant body, and they produce new living tissue at apical
meristems. Stems also can be modified for the storage of water or food,
Modifications of the Stem
In most plants, the stem is an upright structure that provides support for other
parts of the plant body. Various types of stem modifications occur, however. You
should be familiar with the following kinds of modified stems:
Stolons. Stems that run horizontally above ground are termed stolons or runners.
Rhizomes. Stems that run horizontally below the ground surface are termed rhizomes.
Tubers. The enlarged, fleshy tips of certain rhizomes (such as those of the Irish
potato) are modifed for food storage and are temed tubers.
Bulbs. These are unusual below ground structures that consist of a shortened, upright
stem with modified, fleshy leaves attached to it.
Herbaceous Dicot Stems
The stems of most herbaceous dicots are composed of several different kinds of
simple and complex tissues and exhibit a very characteristic structure when viewed in
cross section. You should also be able to identify (and be familiar with the functions
of) the following parts of the stem:
Epidermis. The outer layer of cells surrounding the stem.
Cortex. The cells lying between the epidermis and the vascular tissues constitute the
cortex of the stem. Most of the cortical cells are parenchyma, but there may be one or
more layers of collenchyma lying immediately beneath the epidermis.
Pith. The central portion of the stem is filled with parenchyma cells that have very
distinct intercellular spaces; this region is collectively known as the pith.
Vascular Bundles. Separating the pith from the cortex is a ring-shaped arrangement of
structures known as vascular bundles. Each bundle represents a longitudinal strand
running the length of the stem and containing two different kinds of complex tissues,
the vascular, which are responsible for the transport of materials throughout the plant
body.
Xylem. A complex tissue composed of parenchyma cells, sclerenchyma fibers, and
two kinds of specialized water-transport cells, tracheids and vessel elements.
General plant
Dr. Huda Jassim Al-Tameme
Phloem. A complex tissue composed of parenchyma cells, sclerenchyma fibers, sieve
tube members, and companion cells; this tissue type is specialized for the transport of
dissolved organic materials throughout the plant.
Pith Rays. Regions of parenchyma tissue separating the individual vascular bundles.
Herbaceous Monocot Stems
The basic structure of herbaceous monocot stems is very similar to that of
herbaceous dicot stems. In cross section, you should be able to identify the epidermis,
supporting layers of collenchyma tissue, the ground tissue composed of parenchyma
cells, and the vascular bundles. How does the arrangement of vascular bundles differ
between monocot and dicot stems?
Practical parts:
1- Examine the permanent slide of Corn (Zea mays) stem in 4x magnification ,
showing the vascular bundles scattered in a ground tissue composed of
parenchyma cells.
2- Examine the permanent slide of Corn (Zea mays) stem in 10x magnification,
Showing Individual vascular bundle from a monocot stem. You should be able
to identify the vascular tissues (xylem and phloem) as well as the air space
General plant
Dr. Huda Jassim Al-Tameme
3- Examine the permanent slide of Sunflower (Helianthus) stem in 4x
magnification . showing the epidermis, cortex, pith, and the characteristic ringlike arrangement of vascular bundles.
4- Examine the permanent slide of Sunflower (Helianthus) stem in 10x
magnification. You should be able to identify the vascular tissues (xylem and
phloem), the bundle cap fiber, and the surrounding parenchyma tissue.