The genus Persea, to which the avocado belongs, is a member of the laurel family (Lauraceae); hence it is related to the cinnamon tree, camphor, and sassafras. The avocados cultivated in the United States usually have been considered to represent a single species, Persea americana, but careful study shows that they are derived from two species, as follows:
P. americana, Mill. (P. gratissima, Gaertn.). All of the varieties classified horticulturally as belonging to the West Indian and Guatemalan races are of this species. It is the common avocado of the tropical American lowlands, and the one which has been most widely disseminated throughout the tropics.
P. drymifolia, Cham. & Schlecht. (P. americana var. drymi-folia, Mez). This includes the small avocados of the Mexican highlands, now grown in California, Chile, and to a very limited extent in southern France, Italy, and Algeria. Horticulturists in the United States use the term "Mexican race" to indicate avocados of this species.
In addition to these two species, a third is well known in southern Mexico and Guatemala, and has recently been introduced into the United States. This is the coyo or chinini, P. Schiedeana, Nees. The yas of Costa Rica (probably P. Pittieri, Mez) is another species which is likely, when known in this country, to be classed popularly as an avocado.
The two species from which the cultivated avocados are derived are closely alike in many respects. It is easy to distinguish them by the smell of the crushed leaves; those of P. drymifolia possess an aromatic odor, resembling that of anise or sassafras, which those of P. americana entirely lack. The flowers of P. drymifolia are typically more pubescent, and the under surfaces of the leaves more glaucous, than those of P. americana. The fruits also are distinct, having a thin, almost membranous skin in the former species, and a thick leathery or brittle skin in the latter. The horticultural differences are of more interest here than the botanical; they will be referred to later, in the discussion of the horticultural races.
Seedling avocados of both species vary in habit of growth, being sometimes short and spreading, but more commonly erect, even slender. On shallow soils they may not reach more than 30 feet in height, while on deep moist clay-loams they sometimes reach 60 feet. Budded trees are usually more compact in habit than seedlings, and probably will not attain such great ultimate dimensions.
While the avocado is classed as an evergreen, trees of some varieties cast their foliage at the time of flowering, the new leaves making their appearance almost immediately. The leaf-blades are multiform, some of the commonest shapes being lanceolate, elliptic-lanceolate, elliptic, oblong-elliptic, oval, ovate, and obovate. The apex differs from almost blunt to acuminate, while the base is usually acute or truncate. The length of the blades ranges between 3 or 4 inches and as much as 16 inches. P. drymifolia usually has smaller leaves than P. americana, both species exhibiting a wide diversity in leaf form.
In the United States the flowers appear from November to May, according to locality and variety. Occasionally some of the Mexican avocados (P. drymifolia) bloom in November, while the Guatemalan varieties (P. americana) may not begin flowering until March or April. The flowers (Fig. 1) are produced in racemes near the ends of the branches, and are furnished with both stamens and pistils, all of them being inherently capable of developing into fruits. From their immense number, however, it is easy to see that only a minute percentage can actually do so. They are small and pale green or yellowish green in color. At first glance they appear to have six lanceolate or ovate petals, but on closer examination these are seen to be perianth-lobes; the usual differentiation into two whorls or series, calyx and corolla, does not occur in the avocado. The perianth-lobes are of nearly equal length in most varieties, the inner three occasionally being longer than the outer; they are more or less pubescent, heavily so in P. drymifolia, sometimes almost glabrous in P. americana. The nine stamens are arranged in three series; the anthers are 4-celled, the cells opening by small valves hinged at the upper end. At the base of each stamen of the inner series are two large orange-colored glands which secrete nectar, presumably for the attraction of insects. Inside the stamens are three staminodes or vestigial stamens. The ovary is 1-celled, and contains a single ovule; the style is slender, usually hairy, with a simple stigma.
The fruit is exceedingly variable in both species. The smallest fruits of P. drymifolia are no larger than plums, while the largest of P. americana weigh more than three pounds. The form in both species is commonly pear-shaped, oval, or obo-void, but ranges from round and oblate at one extreme to long and slender, almost the shape of a cucumber, at the other. The color varies from yellow-green or almost yellow through many shades of green to crimson, maroon, brown, purple, and almost black. The skin is as thin as that of an apple in many varieties of P. drymifolia; in P. americana it is occasionally a quarter of an inch thick, and hard and woody in texture. The fleshy edible part which lies between the skin and the seed is of buttery consistency, yellow or greenish yellow in color, of a peculiarly rich nutty flavor in the best varieties, and contains a high percentage of oil. The flesh is traversed from the stem to the base of the seed by streaks or fine fibers (invisible in the ripe fruit of many varieties) which represent the vascular system. The single large seed is oblate, spherical, conical, or slender, inverted so that the young shoot develops from the end which lies toward the stem of the fruit. It is covered by two seed-coats, varying in thickness, often adhering closely to one another. The cotyledons are normally two, occasionally three in P. drymifolia, white or greenish white in color, smooth or roughened on the surface.
Fig. 1. Flowers of Fuerte avocado. (X 1/2)