Review   No vascular cambium in the monocot stem cross section.

LEAVES -  

In most plants leaves are the main organ containing  chloroplast which directs the photosynthetic process. However in some plants such as the cacti the leaves have been modified into spines.
 
 
 
 

See picture in text - extrnal structure of leaves.
 
 
 

External Structure of  Typical leaves (example ? oak leaf)

See picture in text - extrnal structure of leaves.

Large blade and a petiole which is connected to the stem at the node.
 

When a leaf naturally dies an abscission layer of tissue will form at the base of the petiole.  This layer of dead, thick cells allows the leaf to separate from the plant.  The cells on the stem will have callused over preventing water loss. This process is called abscission.

Deciduous leaves are dropped seasonally.

Leaves also contain openings or stoma for gas exchange. Carbon dioxide from the atmosphere enters the leaf and is used for photosynthesis and oxygen gas which is produced by photosynthesis is released through these stoma.
 

See picture in text - extrnal structure of leaves.
 

The opening and closing of the stoma are controlled by two guard cells.
 

Cuticle covers the upper and lower epidermis.

Internal  Structure

The internal part of leaves contain zones of photosynthesizing tissue called mesophyll.

Mesophyll may be divided into the

 palisade mesophyll - top of leaf - columnar  shape - many chloroplast
 spongy mesophyll - bottom of leaf - irregular shape - fewer chloroplast
 

See figure from handout  - internal structure of leaves
 

 The vascular tissue, xylem and phloem are contained in the veins.
 

 xylem -

 phloem-

 bundle sheath-

See figure from handout  - internal structure of leaves

Does a leaf need a vascular cambium between the primary xylem and primary phloem?

Explain.
 
 

Leaf Types:

Monocot and dicot leaves are morphologically different . Monocot leaves have parallel venation and dicot leaves have net like venation.

Some leaves are lacking a petiole between the blade and stem. The blade is directly attached to the stem. These leaves are said to be sessile (example : corn or grass).
 

Leaves are classified as to the adaptations they acquired to conserve water.

Xerophytic leaf: plants that are adapted to arid habitats, example aloe or jade plant
 

Hydrophytic leaf: plants that require a large supply of water or grow wholly or partly submerged in water, example water lily
 
Mesophytic leaf: plants require an environment that is neither too wet nor too dry, example coleus
 
 
 
 
 

Mesophytic Leaf Characteristics

1. Typical as described above
2. Stoma located on both the upper and lower side of leaf

See figure from handout  - internal structure of leaves

Hydrophytic Leaf Characteristics

1. Thin cuticle
2. Stoma located on upper surface of the leaf
3. Poorly developed xylem tissue
4. Spongy intercellular spaces contain air

See figure from handout  - internal structure of leaves

Xerophytic Leaf Characteristics

1. Thick cuticle
2. Multiple layers of epidermal cells
3. Sunken stomata ? stomata located in crypt or chamber with trichomes to prevent water       loss
4. Spongy parenchyma may have water stored in intercellular spaces
5. Stoma located on the lower surface of the leaf
 

See figure from handout  - internal structure of leaves

 
The types of leaves above refer to angiosperms.  Gymnosperms, such as pines, have needles in place of leaves.

Pine needles have   the following structures: epidermis, mesophyll, resin ducts, endodermis, xylem, phloem.
 
See figure from handout  - internal structure of leaves
 

Initiation and Development of Leaves
 

Leaf Primordia

See figure from text  - Leaf Initiation 
 
 
 
 
 
 
 
 
 
 

Click to see cross sections of leaves.

 http://biog-101-104.bio.cornell.edu/BioG101_104/tutorials/botany/Ligustrum1.html
 http://biog-101-104.bio.cornell.edu/BioG101_104/tutorials/botany/Zea_leaf1.html
 http://biog-101-104.bio.cornell.edu/BioG101_104/tutorials/botany/stomata2.html
 

In your notes draw a figure of a leaf labeling the structure indicated above.

From the readings list the functions for the structures listed below.
 

Structure	Function
xylem
phloem
bundle sheath
palisade mesophyll
spongy mesophyll
epidermis
guard cells
stomata