FALL SEMESTER, 1997

Laboratory of Biochemical Ecology, University of Kentucky, Lexington, KY 40506, garose@pop.uky.edu

INTRODUCTION

The trees of Kentucky can be divided into two groups: gymnosperms and angiosperms. The Gymnosperms produce "naked seeds" borne on an exposed surfaces of a seed-bearing cone. In a pine, for example, the seeds rest on the exposed scales of the mature cone. These plants do not produce showy flowers nor seeds that are protected, enclosed within fruit. Also known as conifers, they are taken to be "evergreen" because nearly all members retain their foliage for more than one year.

PLANT NAMES

How do trees get their name? For the purpose of identification, every plant needs to have a unique name, but problems results from name changes and the not-so-surprising fact that the same plant is known by differenct names from different locations. The potential for confusion and error is real and too often realized.

To bring some order to this chaotic situation, a system of naming has been established that is based on Latin and employs two elements to create a system known as binomial nomenclature. The final term designates the name of the species. A species is a fundamental unit that refers to a group that act as an independent, interbreed population. Species with similar characteristics, resulting from the fact that they share many common genes, are grouped together into a genus (genera is the plural).

There are a number of palo verdes that grow in Arizona. This group of trees share many characteristics and they are grouped together into a genus known as Cercidium. Close scrutiny of these trees reveals populations with distinct characteristics-genes that they do no share with other palo verdes. This uniqueness creates a separate species for these trees. Thus, the foothill palo verde whose latin binomial is Cercidium microphyllum (the species name denotes the minute foliage) is a distinct group within the genus of palo verde plants. No matter how many common names this tree may have (and it will have more than one), everyone knows what plant is being talked about when you use the name Cercidium microphyllum. Plant samples that are submitted that do not follow the rules of a correctly written latin binomial will not be graded.

The angiosperms of Kentucky comprise the largest assemblage of trees. These plants are characterized by their showy, often fragrant flowers and the protection of their seeds by enclosure within substantial structures that are the fruit. The foliage is typically deciduous for it is lost at the end of each growing season. Gymnosperms, by contrast, have much more persistent foliage.

The leaf of an angiosperm is its food-producing organ. Tiny bodies within the leaf called chloroplasts, are storehouses of chlorophyll molecules that trap the energy of the sun for use in synthesizing carbohydrates for the plant. The bulk of the leaf is the broad, flat portion or blade which is attached to the stem by the petiole. The petiole attaches to the twig at the node.

The underside of the leaf blade has numerous small openings for gas exchange known as stomata (stomates).

The nodal area can also contain stipules, small appendages that are modified leaves and occur paired and attached to the petiole.

ATTACHMENT

Leaves can be attached to the branch in three different ways:

• opposite-nodes appear directly opposite each other at the same position on the stem.
• alternate-nodes alternate onopposite sides of the stem.
• whorled-three or more leaves grow at each node.

ORGANIZATION

Angiosperms typically possess a simple leaf consisting of a single leaf blade. In some instances, however, they have a compound leaf which is made of leaflets.

Compound leaves can be:

palmately compound-multiple leaflets arise from a common place on the petiole.

pinnately compound-the leaflets arise from multiple points along the petiole. Typically, the leaflet is sessile, it arises directly from the petiole. This leaf arrangement occurs in Arizona walnut, Juglans major.

Arizona walnut, Juglans major

bi-pinnately compound-occasionally, the petiole is subdivided by a structure called a rachis which is actually an extension of the petiole. Individual leaflets are attached directly to the rachis which, in turn, is connect to a common petiole. This leaf arrangement is found in various Arizona mesquites, Prosopis.

Arizona mesquite, Prosopis julifloria

Leaves can be distinguished from leaflets in two ways:

• a bud is found at the node where the petiole of the leaf is attached to the stem, but never when the leaflet attaches to the rachis.
• leaves protrude from the stem in many planes, but the leaflets of a single leaf all reside in a common plane.

SHAPE, BASE, APEX, AND MARGIN

SHAPE:

1-needle-like, 2-scale-like, 3-linear (long and narrow, with parallel sides), 4-oblong (approximately 3x longer than wide with sides that are largely parallel), 5-lanceolate (shaped like a lance), 6-inverted lanceolate, 7-ovate (shaped like a longitudinal section through an egg), 8-obovate, 9-elliptical, 10-oval, 11-circular, and 12-crenate (kidney-shaped).

13-cordate (heart shaped), 14-triangular, 15-diamond-shaped, and 16-spalulate (shaped like a spatula).

BASE

First row (left to right): cuneate (wedged-shaped), acute, cordate (heart-shaped), inequilateral

Second row (left to right): obtuse, rounded, flattened, auriculate (ear-like).

APEX:

First row (left to right) acuminate, acute, mucronate (bristle-tipped), cuspidate (spine-tipped)

Second row (left to right): obtuse, rounded, truncate (flattened), and emarginate tched).

MARGIN:

(left to right) rovolute, entire, repand (undulates), sinuate (strongly undulate)

(left to right): crenate, serrate, dentate, doubly serrate

(left to right): doubly crenate, lobed, cleft, parted

A twig is that part of a tree which supports the buds and leaves and is an extension of a branch. The leaves, arranged on the twig as previously described, are supported by the petiole which is attached to the twig at the node.

Buds are formed in the angular zone formed by the petiole and the stem. Since this is an axil, such buds are known as axillary buds. These buds contain the following season's branches or flowers. A twig also has a terminal bud at the apex which generates tissues responsible for twig increase in length.

Angiosperms are largely a deciduous assemblage. They lose their foliage annually. When the leaf and petiole fall from the twig in the autumn, a leaf scar is created at the node. Careful examination of this scar reveals small bundle scars which form from tissues that connect into the leaf from the branch to provide water and move food materials from the leaf.

The size and shape of the leaf and bundle scars are distinct for each tree and a vaulable clue for their identification when the leaves have fallen.