Presented via Trans Texas Videoconference Network

Cross listed as:

PLSS 6390 (Texas A&M University-Kingsville)
PLSS 4390 (Texas A&M University-Kingsville)
HORT 422 (Texas A&M University-College Station)

Fall 2004


OBJECTIVE: to learn to distinguish between various members of the Citrus and Persea spp. by the difference in leaf morphological characteristics.

I. Leaf Morphology

A. Citrus spp. belong to the Rutaceae, which contains trees or shrubs, rarely herbs, with aromatic alternate or opposite, usually compound leaves, dotted with translucent glands containing an essential oil. Plants mostly of tropical or subtropical origin, largely in the Old World.

1. Poncirus trifoliata Raf., [Trifoliate orange] is a densely branched tree of rather upright habit, 12 to 15 feet (3.7 to 4.6 meters) in height; younger branches smooth, dark green, angled, older branches rounded, thorny, with the thorns stout, stiff, sharp, 1-1.5 inches (2.5-3.8 cm) long, flattened at the base; leaves deciduous, trifoliate, leaflets thin, more or less elliptical, crenate, borne singly or in tufts.

2. Fortunella spp., [Kumquats] are small evergreen trees or shrubs, densely branched; branchlets angular or rounded; spines small, axillary, at one side of bud, or lacking; leaves unifoliate, blunt pointed, thick, dark green above, paler beneath, thickly dotted with glands on lower surface; petioles narrowly winged, articulated or not articulated with the blade.

3. Severinia buxifolia Poir. Ten., [Box Orange] is a spiny shrub or small tree; leaves obovate, emarginate, and dark green in color; with a single spine at each leaf.

4. C. limon L. Burm., [Lemon] is a small tree, 10 to 20 feet (3-5 meters) in height, with a rather open head of short, round or angular branches; thorny; bark grayish; young shoots purplish, smooth; leaves evergreen, alternate, 2 to 3 inches (5-7.6 cm) in length, ovate-oval, sharp, pointed, light green, margin serrate; petioles entirely wingless.

5. C. medica L., [Citron] is a small tree or shrub, about 10 feet (3.05 meters) high, with a short indistinct trunk and short, thick irregular, straggling, thorny branches; bark light gray; thorns short, sharp, rather stout; young shoots smooth, violet colored or purplish, stiff; leaves large, not articulated, 4-6 inches (10-15 cm) long, oval-oblong, serrate or somewhat crenate, dark green above, lighter beneath; wingless petioles.

6. C. aurantifolia Christm. Swingle, [Lime} is a small tree or shrub of straggling habit, with small, stiff, interlocking or drooping thorny branches, the thorns small, sharp, numerous; bark grayish brown; young branchlets light green, becoming darker with age; leaves elliptic-oval, glossy green, margins slightly indented; petioles margined.

7. C. grandis L. Osbeck, [Pomelo] is a tree 15-30 feet (4.6-9 m) high, with compact, rounded, or slightly flattened head; branchlets pubescent when young; leaves large, 4-8 inches (10-20 cm) long, ovate-oblong to elliptical; somewhat pubescent on the underside, midribs, veins, and on the margin especially when young; apex rounded or somewhat acuminate; base rounded to the broadly winged petiole; veins depressed, distinctly defined; margins more or less crenulate or entire.

8. C. paradisi Macf., [Grapefruit] is a tree 30-50 feet (9-15 m) in height, with a rounded or conical head and a trunk 1 1/2 to 2 1/2 feet (30.5-76 cm) in diameter; bark smooth, grayish brown; young leaves and shoots smooth, light green; leaves ovate, blunt, pointed or rounded, emarginate, smooth, dark glossy green, leathery margin crenate; petioles articulated, broadly winged.

9. C. aurantium L., [Sour Orange] is a small to medium sized tree, 20-30 feet (5-9 m) in height, with a dense, compact, rounded head; young shoots light green, thorny; thorns alternate, slender, long, sharp pointed on older wood, strong, stiff; leaves alternate, articulated, evergreen, ovate, pointed, strongly scented, somewhat larger than C. senensis.

10. C. sinensis L., Osbeck [Sweet Orange] is a tree 25 to 40 feet (7.6-12 m) high, with a compact, conical head; bark grayish brown; thorns generally present, 1/2-2 inches (1.3-5 cm) long, sharp, stout; leaves oval or ovate oblong, 3-4 inches (6.8-10 cm) long, smooth, shiny, somewhat lighter below than above, margins entire, or very slightly serrate, petiole 0.5-1 inch (1.3-2.5 cm) long, slightly winged (occasionally broadly winged).

11. C. reticulate Blanco, [Mandarin] is a small spiny tree with a dense top of slender branches; leaves long and narrow to ovate pointed; petioles slightly margined, articulated; leaves turn up slightly.

12. C. paradisi X reticulata , [Tangelo] presents an appearance like that of C. reticulata. Leaf curling is found in cv. Orlando.

B. The avocado, Persea americana , belongs to the Lauraceae along with the Lauris nobilis [bay laural}, Sassafras spp., Cinnanmonum camphorum [camphor], and C. zeylanicum [cinnamon].

1. Persea americana Mill., is a tree 30-40 feet (9-12 m) in height, with bark grayish-brown to brown, large entire leaves, elliptical 3-10 inches (7.6-25 cm) in length, placed opposite to each other, pubescent and reddish when young, smooth, leathery, and dark green when mature. Leaves of the Mexican race have a scent of anise which is absent in the other races.

2. Persea drymifolia, is similar to P. americana with exception that the leaves of P. drymifolia possess an aromatic odor that may detected when the leaves are crushed, resembling that of anise or sassifras. Leaves of P. americana have no such odor. The flowers of P. drymifolia are typically more pubescent, with the under surface of the leaves more glaucos, and the fruit peel thinner than those of P. americana.

II. Exercise

1. Draw and identify the plant material displayed in the lab.

2. Describe the leaves using botanical names for the various shapes.


1. Chandler, W.H.. 1958. Evergreen orchards. Lea & Febiger, Philadelphia.

2. Hume, H.H.. 1957. Citrus fruits. MacMillan Co., New York.

3. Malo, S.W. and C.W. Campbell.( no date).: The avocado. Fruit Crops Fact Sheet. Florida Cooperative Extension Service, Gainsville.

4. Reuther, Walter, Herbert J. Webber, and Leon D. Batchelor. 1967. The citrus industry, Volume I History, World Distribution, Botany, and Varieties. A Centennial Publication of the University of California.