Stems are that part of a plant which grow above the cotyledon of a seed. Most stem are located
above the ground; however, some stems such as tubers (potato) and rhizomes (underground stems)
are located below the soil surface. In most plants the stem functions as a structure to display the
leaves to maximize the photosynthetic yield. In some plants the stem serves as a storage structure
for food and water. Water is moved from the root to the leaves and photosynthates are
translocated from the leaves to the root by vascular tissue located in the stem.

 The stem of the cacti in the main photosynthetic structure for the plant. . Stems are different
from roots in that they contain nodes and internodes. The nodes are the locations on a stem
where leaves are attached Axillary buds may form between the leaf axis and the stem. These
axillary buds may develop into new vegetative branches or reproductive structures (flowers).
In your notes draw a figure showing the external parts of a stem and identify the following
structures: node, internode, buds ( buds may be classified as axillary or terminal depending on
location and vegetative or reproductive depending into what they develop).

Stems also may be classified as herbaceous or woody.

Herbaceous stems are soft and flexible. Plants which survive for only one growing season have
herbaceous stems. The terminal buds and axillary buds do not have scales covering them for
protection. Examples of plants with herbaceous stems are annual plants (plants with live for one
season) such has most vegetables (corn, bean).

Woody stems are hard and stiff. Plants which can grow many seasons have woody stems. With
time the stem becomes very hard a rigid due to secondary growth which will be studied later. The
terminal buds and axillary buds have hard productive scales covering them. Examples of woody
plants are trees and shrubs. Because woody plants drop their leaves during certain seasons the
stems will have leaf scars.  Above each leaf scar are axillary buds.  The terminal bud will lose its
bud scales each spring when the terminal buds grows in length. Thus terminal buds scale scars
are left of the stem. The age of a woody twig can be determined by the number of terminal buds
scars present on a stem. Also the length between the terminal bud scars can be used to predict the
conditions of previous seasons. As the twig grows in diameter gas exchange must be allowed to
occur between the outside and internal cells. This is accomplished by lenticles (openings) along
the length of the stem.

Longitudinal Section of Stem Tip

To learn the structures of a stem you must learn the various tissues located in a cross section and
a longitudinal of a stem. By examining the longitudinal section of a stem you can see the
relationship between the four tissue areas of an embryonic stem tip, three primary meristems and
the leaf primordia. In your notes draw a longitudinal section of a stem and identifying leaf primordia,
leaf protoderm,  procambium and ground meristem. There is no cap protecting the stem apex as in
the root apex (root cap). The leaf primordia develops into a leaf. In woody stems the stem apex has
bud scales covering the tip. These scales drop off when the buds are stimulated to grow.
 

                Refer to figures handed out in class.

meristematic region - mitosis -

Subapical Meristems
 

Three Primary Meristems and the Leaf Primordia
 

  protoderm - epidermis

  procambium - xylem and phloem (vascular tissue)

  ground meristem - pith and cortex

  no cap protecting the stem apex as in the root apex (root cap)

  leaf primordia develops into a leaf
 

Refer to figures handed out in class.

Fill in the table below indicating which tissues (epidermis, cortex, pith, vascular bundles) that
develop from the various primary meristems.
 
 

Primary Meristems	Tissues
Protoderm
Procambium
Ground Meristem

 
 

Cross  Section of Herbaceous  Stem Tip in Region of Maturation

Draw a figure showing the cross section at the region of maturation of a stem showing the
following tissues: epidermis, cortex, vascular bundles ( primary phloem, primary xylem, vascular
cambium), and pith .

 epidermis - outside of stem

  cortex - parenchyma cells, storage

  vascular bundles  ( primary phloem, primary xylem, vascular
                                   cambium)
  vascular cambium produces new xylem and phloem called secondary
     xylem/phloem

  pith - parenchyma cells, storage
 

Refer to figures handed out in class.
 

Tissue	Cell Make-up	Function
epidermis
cortex
primary xylem
primary phloem
pith
vascular cambium

Click to see cross section of stem.
 http://www.mancol.edu/science/biology/plants_new/anatomy/mainmenu.html

Angiosperms are divided into two classes - Monocots and Dicots

Monocots are the flowering plants whose embryo has one cotyledon. The cotyledon is the large
storage organ of a seed.  If a seed is can not be separated into two parts when the seed coat is
removed than it is a monocot. The white heart of a corn seed is the cotyledon.  Examples are corn,
grasses, and palms.

Dicots are the flowering plants whose embryo has two cotyledons.  If a seed is easily separated
into two parts when the seed coat is removed than it is a dicot. The two large structures which make up
a bean seed are the cotyledons. Examples are beans, peas, nuts.

Difference between monocot and dicot stems

Refer to figures handed out in class.

 The arrangement of vascular bundles in the dicot stem are
different than the arrangement of vascular bundles in the monocot stem. In your notes write a
paragraph comparing the arrangement of vascular tissue between these two plants. Monocot stems
do not develop a vascular cambium between the primary xylem and primary phloem. The vascular
bundles are scattered throughout the pith in a monocot stem.  The ground tissue is not divided into
cortex and pith  in the monocot. Using the text draw a figure showing the different arrangements of
tissues between a dicot stem and monocot stem. Include the following: epidermis, pith, cortex in phloem,
vascular bundles, xylem and phloem.

No vascular cambium in the monocot fibrovascular bundle.
 

Review xylem – consists of four cells types

Water conducting cells: tracheids and vessel elements
                                          Angiosperms have mostly vessel elements and a
                                          few tracheids

                                          Gymnosperms and lower vascular plants have
                                          Tracheids

Tracheids have secondary cell walls and are dead at maturity (sclerenchyma)
 

Different types of thickenings – annular, helical, scalariform

Annular thickenings  -  rings of secondary cell wall (thin) on the interior face of the primary wall.
 
 
 
 
 

Helical thickenings  - one or two helices  interior to primary cell wall.
 
 
 
 
 

Scalariform thickenings – most of the interior cell wall is covered with secondary cell wall.
 

Reticulate thickenings - secondary cell dposited in the shape of net-like.
 

Refer to figures in text book pp 136- 144.

Pits – circular and bordered (rim of extra cellulose)

Pit pairs

Refer to figures in text book pp 134.
 
 

Vessel elements are shorter and wider with one or two perforations
 

Compare and contrast between plasmadesmata, pits (circular or bordered), sieve plate, perforations.
 
 
 
 

Phloem consists of four cells types including the sieve tube member and the companion cells (phloem loading)

Plasmodesmata are cytoplasmic bridges which allow the passage of photosynthates freom one seive tube cell to another.
 

Refer to figures in text book.
 

Primary xylem and phloem are directly derived from the procambium.
In an older root or stem the procambium will form a vascular cambium between the xylem and phloem which will form secondary xylem and phloem.