BS/PLS 210



Gerald A. Rosenthal

Laboratory of Biochemical Ecology, University of Kentucky, Lexington, KY 40506,


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.


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.


When the ovules, housed within the ovary, of the flower are fertilized by the mature pollen grain, they grow into the seeds while the ovary matures into the fruit. Thus, a fruit is a matured ovary that may or may not retain aged floral remnants, that protects the internal seeds. It can be confusing. Remember that the carpel houses the ovary which in turn supports the ovules.

A fruit is considered a simple fruit if it forms from a single ovary consisting of one carpel or the fusion of numerous carpels.

A compound fruit results when many separate carpels function in creating the fruit.


In an aggregate fruit, multiple carpels contained within a single flower, aggregate together to form a discrete fruit. Thus, an aggregate fruit is an aggregation of carpels (but from only one flower).

A multiple fruit is one where multiple carpels, derived from a number of flowers, fuse to form the fruit. Arizona sycamore, Platanus wrightii is an example of a tree that produces a multiple fruit.


Simple fruits are best placed into one of two groups: fleshy or dry fruits. It is best to think about a fleshy fruit as being divisible into 3 sections:



When the fruit has a thin exocarp and a fleshy mesocarp that covers a hard and stony pit (endocarp) that protects the seed. Prune, cherry, peach and plum are examples of edible drupes. Do not confuse the stony pit (endocarp) with the seed that is housed within. Usually only one carpel containing one ovule.


When the fruit has both a soft and fleshy mesocarp and endocarp and the seeds are dispersed throughout the fruit.

A drupe is a simple fruit (derived from a single pistil) with one seed. When a berry has but one seed, it can be difficult to distinguish between a fruit in which the seed is protected by hard, external tissues (endocarp) that are not part of the seed (drupe) and a fruit where all of the hard, internal tissues are of the seed (berry).


In this fruit, the mesocarp is fleshy but the exocarp surrounding the seeds is papery or cartilaginous. An apple or a pear are examples. A pome is interesting in that part of the tissues that support the ovary (receptacle) are carried over into and becomes part of the mature, edible fruit.


Most fruits are dry but they can be grouped into two assemblages: those that split open at maturity (dehiscent) and those that do not (indehiscent). Once again, this can be problematic since an immature fruit will not open, and thereby appears indehiscent, until it is fully developed.



Many important Arizonan trees such as the palo verdes are legumes. A legume is formed from a simple carpel ( no chambers or sections within the fruit) that opens along two sutures that form in the wall of the fruit. Many ovules exist in the ovary that forms a legume, so there are numerous seeds in this fruit.


Botanically, a follicle is similar to a legume in that it forms from a single carpel, and contains multipe seeds, but the fruit opens along a single suture


A capsule is created from a compound carpel. It dehisces or opens in many different ways..

Most dry, dehiscent fruit are capsules. If more than one section or chamber exists within the fruit and it is dry and dehiscent-it must be a capsule.



A single seed exists within the fruit and it rests mostly free within the fruit cavity except for a small appendage that attaches it to the fruit wall.


Modification of an achene in which the ovary wall is expanded to create a wing. This is the fruit of ash (Fraxinus) and elm (Ulmus).


These fruit superficially resemble achenes but they have a hard, stony fruit wall and they are derived from a compound ovary.

A nut can be difficult to identify. Typically, a nut is enclosed with a husk which may be papery, leafy, woody, or spiny. The outer wall of the nut is hard as it is made of fibrous sclerids. What makes matters worse, is that abortion can lead to the development of only a single ovule-even though many ovules originally existed in the ovary.

Arizona walnut has a fruit that looks very much like a nut and is called a nut, but the seed is actually enclosed in a stony pit and the outer, green husk, while not fleshy is the exocarp of a drupe.

A horsechestnut also superficially appears to be a nut, but this is not the case. The fruit opens at maturity and no nut is dehiscent. What appears to be a husk is actually the outer ovary wall of a capsule.

Recognizing individual fruit groupings is not easy; it takes time and patience to acquire skill and accuracy. H

Presented below is another key to the fruit and an opportunity to examine this material from another viewpoint

Simple Fruits

A simple fruit consists of single ripened ovary. The fruits of most angiosperms are simple fruits. The major types of simple fruits are:

A. Fleshy fruits, in which all or most of the pericarp is soft and fleshy at maturity. Seeds escape from fleshy fruits as a result of the decomposition of the fleshy tissues.

1. Berry, in which the pericarp is fleshy throughout or nearly so. Examples: grape, banana, tomato, watermelon, orange, cucumber, and currant.

2. Drupe, in which the exocarp is a thin skin, the mesocarp is thick and fleshy, and the endocarp hard and stony. The endocarp ("stone" or "pit") encloses one, rarely two or three seeds. Examples: peach, plum, olive, cherry, apricot and coconut.

B. Dry fruits, in which the entire pericarp becomes dry and often brittle or hard at maturity.

1. DEHISCENT FRUITS, which split open along definite seams or at definite points at maturity. Contain several to many seeds.

a. Legume, consisting of one carpel, which usually splits open along two seams. Examples: pea, bean, and locust

b. Follicle, consisting of one Propel, which splits open along one seam. Examples | larkspur, columbine, peony, and milkweed.

c. Capsule, consisting of two or more fused carpels and splitting open in various ways. Examples: lily, snapdragon, tulip, and violet

2. INDEHISCENT FRUITS, which do not split open along definite seams or at definite points at maturity. Usually contain only one or two seeds.

a. Achene, bearing only one seed which is separable from ovary wall, except at point of attachment of seed to inside of pericarp. Examples: sunflower, buttercup, dandelion.

b. Caryopsis, or grain, bearing only one seed, the coat of which is completely fused to the inner surface of the pericarp. Examples: corn, wheat and oats.

c. Samara, a one- or two-seeded, achene-like fruit, the pericarp of which bears a flattened, winglike outgrowth. Examples: elm, maple, ash.

d. Nut, a one-seeded fruit, much like an achene, but usually larger and with a thickened, very hard pericarp. Examples: acorn (oak), hazelnut, and chestnut.

Aggregate Fruits

An aggregate fruit is a cluster of several to many ripened ovaries produced by a single flower and borne on the same receptacle. The individual, ripened ovaries may be drupes (as in raspberries and blackberries), achenes (as in buttercups), and so forth.

Multiple (Compound) Fruits

A multiple fruit is a cluster of several to many ripened ovaries produced by several flowers crowded on the same inflorescence. As in aggregate fruits, the fruitlets of a compound fruit may be drupes, berries, or nutlets. Examples: mulberry, Osage orange, and pineapple.

Accessory Fruits

Accessory fruits are structures that consist of one or more ripened ovaries together with tissues of some other floral part, such as calyx or receptacle. In an accessory fruit, these additional tissues are often extensively developed to the point of constituting the major part of the 'structure popularly called the "fruit." Among familiar accessory fruits are strawberries, in which the individual fruits are achenes, borne upon an extensively developed, sweet, red, succulent receptacle.

The strawberry flower (left) and the resulting accessory fruit-the strawberry (right)

Another common type of accessory fruit is a pome, exemplified by apples and pears, in which the matured ovaries (sections of the core) are surrounded by enlarged receptacle and calyx tissues in which large amounts of food and water are shred.

The pericarp (fruit wall) and its protected seeds are only a small part of the mass of an apple. Most of the edible portions of the fruit that one eats are receptacle and associated calyx tissues.

Thus, in strawberries, apples, and pears, edible portions are not true fruits; that is, they are not the matured ovaries, but they are stem and calyx tissues in which or on which the matured ovaries, or true fruits, are embedded.

The concept of fruit as a botanical entity is not simple and required dedicated, focused attention. It is complicated by the numerous errors in our everyday preception of fruit. For examples, blackberries and raspberries are not berries, but rather aggregates of tiny drupes, mulberry is not a berry but a multiple fruit composed of tiny nutlets that are surrounded by fleshy sepals, the strawberry is also not a berry but rather a fruit with a greatly enlarged receptacle bearing a number of dry achenes on its surface. An apple is not merely a fruit but is a complex fruit with a core plus a thin layer of fleshy tissues immediately surrounding it. It is embedded in an extensive mass of edible fleshy, succulent tissue, which is the product of the fused bases of the other floral organs.

The popular concept of a "nut" is anything but correct. A peanut is not a nut but rather a legume whose flowers penetrate the soil so that the furit develops subterraneously. A walnut is not a nut but a drupe. The fleshy husk of the walnut is the outer part of the pericarp (exocarp), and the "shell" is the inner part of the pericarp (endocarp). The "meat" of the walnut is the seed with two large, greatly convoluted cotyledons, a minute epicotyl and hypocotyl and a thin, papery testa. A brazil nut is a seed borne with other seeds in a large, thick-walled capsule. A coconut is a drupe and an almond shell is the hardened endocarp of a drupe with a single seed, rarely more. BE CAREFUL, EVERYTHING IS NOT WHAT IT SEEMS TO BE.