Leaf Anatomy

Leaf Anatomy

Parts of the leaf

One of the most obvious features of plants are their leaves. Leaves are particularly useful for identification because they are usually readily available, unlike flowers and fruit which are seasonal. When you identify trees in Washington Park for the next part of this course, you will be using leaf characteristics extensively.

We will start with the parts of the leaf then focus on characteristics of the leaf such as shape, margin, leaf type and the arrangement of leaves on the stem.

Axillary bud: this is where the leaf starts! It is a bud that is capable of growing into a new shoot, and it indicates where the leaf begins. This point on the stem is also called a node. 

Petiole: the stalk that holds the leaf off the stem. If there is no petiole, the leaf is said to be "sessile" (see illustration below) 
Secondary veins: veins usually branching off the primary vein, are noticeably narrower than the primary vein.

Note: some leaves are divided into smaller units called leaflets. Margin terms apply weather you are looking at a leaf or leaflet.               

Primary vein: is the main vein starting at the base of the leaf. It is noticeably wide than the other veins. 

Leaf shapes

Leaf shapes can be very useful for identifying plants. It is important to remember though that shape can be somewhat variable even on a single plant. When determining leaf shape, look at many leaves to get a feel for the overall leaf shape represented.

A number of common leaf shapes are illustrated below, but be aware that intermediate shapes also exist. For example leaves may be ovate-lanceolate; not quite ovate or lanceolate, but somewhere in between. In addition, leaves that grow in the shade can have a different shape (large and thin) compared to leaves growing in the sun (smaller and thicker).

There is another consideration when looking at leaf shapes, and that is something called leaf heteroblasty. Leaf heteroblasty refers to different leaf shapes between juvenile and adult growth. It is noticeable in Eucalyptus and other species such as Acacia melanoxylon.

           

 

 

Leaf margins

The leaf margin (edge of the leaf blade) can be very useful when identifying plants. Like leaf shapes, you want to look at more than just one leaf to determine its margin type. Some plants, like the tupelo tree illustrated below, will have the majority of leaves on the plant without any lobes (leaf on the left). If you check the plant carefully, you will find a few leaves with lobes (leaves on the right).

BASIC LEAF or LEAFLET MARGINS

         

Like leaf shapes, there are also leaf margins that don't fit into a single category. For example there are plants with crenate-serrate margins which are intermediate between crenate and serrate. In general though, margins are fairly consistent on a plant.

Leaf Base And Apex

The shape of the leaf base and apex (tip) of the leaf can be useful for identifying plants. Below are some basic shapes associated with those areas

Leaf venation

Leaf venation refers to the pattern of veins on the leaf. The veins supply the leaf with water and minerals from the roots and transport materials from the leaf to the rest of the plant. The vein pattern can be useful for plant identification when the pattern is distinct and obvious, so only a few basic patterns will be used in this course.

LEAF VENATION PATTERNS

The primary vein is like the trunk on a tree. It is the widest vein on the leaf and starts at the base of the leaf.

Secondary veins are like the main branches on a tree, they are smaller than the primary vein.

There are two ways the secondaries may come off the primary. In one pattern, illustrated on the left below, the secondaries come off of the primary vein all along the length of the primary. In the second pattern, illustrated on the right below, secondaries originate from at or near the base of the primary. 

The secondary veins can also create a pattern that can be helpful for identifying certain plants. Again, the pattern must be distinctive and obvious to be of use. Here are three patterns:

 

    Leaf Types

                                                                LEAF TYPES

Leaf Arrangements On The Stem