Seeds and Seedlings - Seed Testing - page 25

SEEd MorPHoLoGy
To understand seeds and seed formation, one must have a basic knowledge of fruit development and mor-
phology. Many of the structures which are called seeds by analysts and the seed industry are actually fruits,
while others are true seeds. The botanical deinition of fruit is much broader than that conveyed by popular
usage of the term. Botanically, a fruit is a mature or ripened ovary that usually contains one or more ovules
which develop into true seeds. The ovary wall (also called pericarp ) may contain two to three layers. The
exocarp is the outermost layer, often consisting of just the epidermis. The middle layer, or mesocarp , varies
in thickness and is often leshy. The endocarp is the innermost layer of the pericarp and varies greatly
among species. Legume pods, peppers, and cereal grains are fruits, as are apples, oranges, and peaches.
While a thorough presentation of fruit development is beyond the scope of this discussion, it will be useful
to present a broad overview of types of fruit recognized in plant morphology and taxonomy and how struc-
tures commonly called “seeds” relate to this.
fruit Types
There are four main types of fruits:
Pseudocarpic fruit consists of one or more ripened ovaries attached or fused to modiied bracts or
other nonloral structures. Examples: Xanthium (cocklebur), Mirabalis (four o'clock), Fragaria (straw-
berry), Ananas (pineapple).
Multiple fruit is composed of the ovaries of more than one lower. Each unit of these fruits may be
berries, drupes, or nutlets. Examples: Ficus (ig), Morus (mulberry).
Aggregate fruit is composed of several ovaries of a single lower. Each unit of these fruits may be a
berry, drupe, or nutlet. Examples: Rubus (raspberry), Ranunculus (buttercup).
Simple fruit is derived from a single pistil (i.e., a single ovary in a single lower). It includes the fol-
lowing types:
A. Fleshy fruits have a leshy or leathery pericarp at maturity (none of which are referred to as “seeds”).
They may be one of these types.
1. Berry - has a leshy pericarp. Examples: grape, tomato, gooseberry, huckleberry.
2. Pepo - has a hard rind but no internal separations, or septa. Examples: watermelon, cantaloupe,
squash, cucumber.
3. Pome - has a loral cup that forms a thick outer leshy layer and a papery inner pericarp (endocarp)
forming a multiseeded core. Examples: apple, pear, quince.
4. Drupe - is also called stone fruit, and has a stony endocarp, a thick, leathery, or leshy mesocarp,
and a thin exocarp. The pit is usually one-seeded, but occasionally several one-seeded pits are
present. Examples: cherry, coconut, peach, plum, olive.
5. Hesperidia - are berrylike fruits with papery internal separations, or septa, and a leathery, sepa-
rable rind. Examples: orange, lemon, lime, grapefruit.
B. Dry fruit has a thin pericarp that is dry at maturity. They may be dehiscent or indehiscent.
1. Dehiscent fruits - split open at maturity and release mature seed.
a. Legume - has a simple (single) pistil that splits open at maturity along two sutures. Examples:
bean, pea, soybean, alfalfa, clover.
b. Follicle - has a simple (single) pistil that splits open at maturity along one suture. Example:
milkweed.
c. Capsule - has a compound pistil that splits open at maturity in several ways:
Loculicidal—splitting open through the midrib of the carpel into the locules. Examples: Iris ,
Tulipa , Hibiscus