PBIO 100 Lecture Notes
Undergraduate Program in Plant Biology, University of Maryland
LECTURE 22 FLOWERING PLANTS (FRUITS AND SEEDS)
I. Seeds
A. Composed of plant embryo, food storage tissue, and seed coat (from
mature ovule); high concentration of food, little water. Seeds
possess:
1. A well-formed multicellular young embryo (germ) - diploid
2. Nutritive tissue (food)
-
The stored food provides enough energy for the embryo to grow through the
soil when the young seedling is not yet able to photosynthesize. In contrast,
spores are single cells that contain little nutritive tissue and require
much growth before attaining the size and development of embryos in seeds.
3. A thick protective coat (the seed integument) formed from the megasporangium.
4. Some seeds are modified to aid in their dispersal. For example,
pine seeds possess papery wings that catch in air currents and help to
disperse the new generation.
B. Gymnosperm
seeds (see Lecture
19-Gymnosperms)
1. The single
fertilization produces the diploid embryo.
2. The food
source is the haploid megagametophyte.
C. Flowering plant seeds
1. Double fertilization
produces both the diploid embryo and the triploid endosperm
food source
(see Lecture
20-Flowering plants).
2. Two
classes based on number of cotyledons (modified seed leaves):
a. monocot "seed" with a single cotyledon; actually
a fruit containing a seed
i. primary food source endosperm tissue
ii. cotyledon is the absorptive organ that takes in nutrients from the
digested
endosperm
iii. example: corn (fruit,
cross-section)
b. dicot seed with two cotyledons
i. primary food source fleshy food-storing cotyledons which have absorbed
the
endosperm
ii. example: bean
D. Most important family of seeds used for food: bean or pea family
1. Seeds of
bean or pea family high in protein and oil
E. Many seeds pressed for oil: sunflower, sesame, peanut, soybean
II. The fruit
A. Fruit
(a mature, ripened ovary; contains seeds)
B. Pericarp-
the ovary wall (exocarp, mesocarp, and endocarp)
C. Purpose to disperse seeds
III. Types
of dispersal
A. Self-dispersal
1. Disperse their seeds by forceful ejection
2. Examples: touch-me-not, witch
hazel
B. Wind dispersal
1. Wings - example: maple
and ash fruits
2. Plumes - examples: dandelion
and milkweed fruits
C. Water dispersal - air spaces and corky floats- example: coconut
with fibrous husk
D. Dispersal by animals
1. Non-fleshy fruits clinging to animal
a. spines, hooks, and barbs - example: cockleburs
b. covered with sticky substances - example: mistletoe
(fruit
sticking to host)
2. Fleshy fruits eaten and seeds dispersed with feces
IV. Types of fruits
A. Simple fruit- develops from a single ovary of a single flower; may
be either dry or fleshy when mature
1. Simple fleshy fruits
a. Berry-
entire fruit wall is soft and fleshy at maturity (ex- grapes and tomatoes)
b. Drupe- outer part of fruit wall is soft and fleshy,
inner part is hard and stony
(ex- cherry, peach)
c. Capsule-
dry at maturity and splits open along several seams (ex- cotton)
2. Simple dry fruits
a. Legume-
dry at maturity and splits open along two seams (ex- peas, beans)
b. Grain-
dry at maturity; contains single seed; seed coat is fused to fruit wall
(ex- corn, wheat, rice)
c. Nut - single-seeded fruit in which the ovary
wall and seed coat remain
separate (ex- acorn,
chestnut,
walnut)
B. Aggregate fruit develops from one flower with many ovaries (ex-blackberry,
raspberry)
C. Multiple
fruit- develops from ovaries of several flowers borne together on same
stalk (ex- pineapple)
D. Accessory fruit- fruit in which the edible part is primarily tissue
other than ovary tissue
(ex- apple
and strawberry
- edible flesh from the receptacle)
V. Illustrations of various fruits and fruit types (from Texas A&M):
