Evolution of Flowering Plants

Question 1

Angiosperms

Answer 1

• Fossil evidence indicates that the angiosperms (Magnoliophyta) originated during the early Cretaceous Period, about 130-135 mya, and became dominant by the end of this period.
• They now account for about 95% of all land plants alive today, and they range in size from Wolffia (Lemnaceae) at less than 0.5 mm across to trees as large as Eucalyptus (Myrtaceae) at about 100 m tall.

Question 2

Angiosperm Apomorphies

Answer 2

• Flowers (generally w/ perianth)
• Stamens w/ 2 laterial thecae
• Male gametophyte 3-nucleate
• Carpel and fruit
• Ovules w/ 2 integuments
• Female gametophyte 8-nucleate
• Endosperm and double fertilization
• Sieve tube members w/ companion cells

Question 3

Flower Parts

Answer 3

Question 4

Stamen w/ 2 lateral thecae

Answer 4

Question 5

Male Gametophyte

3-Nucleate

Answer 5

Question 6

Carpel and Fruit

Answer 6

Question 7

Carpel and Fruit

Answer 7

Question 8

Ovules w/ 2 Integuments

(Bitegmic)

Answer 8

Question 9

Female Gametophyte

8-Nucleate

Answer 9

Question 10

Endosperm

and

Double Fertilization

Answer 10

Question 11

Sieve tube members

w/ companion cells

Answer 11

• Albuminous cells are found with sieve cells, whilecompanion cells are foundwith sieve tube members.

Question 12

Vessel Members

Answer 12

• Vessels may not represent atrue apomorphy, as a fewgroups of angiosperms donot possess them.

Question 13

Angiosperm Life-Cycle

Answer 13

Question 14

Origin of Angiosperms

Answer 14

• To Charles Darwin, the “sudden” origin of the angiosperms during the Cretaceous Period was an “abominable mystery.”
• And, the details of angiosperm evolution from a gymnosperm precursor are still not clear. However, certain advances have been noted during the Cretaceous and Tertiary Periods…

Question 15

Geologic Time Scale

Answer 15

Question 16

Barremian Stage

Answer 16

• The first monosulcate angiosperm pollen grains, characteristic of the primitive dicots and the monocots.

Question 17

Aptian Stage

Answer 17

• The first tricolpate pollen grains, which are found in the more advanced dicots.

Question 18

Turonian Stage

Answer 18

• Angiosperm pollen more abundant than fern spores and gymnosperm pollen.

Question 19

Maestrichtian Stage

Answer 19

• A number of modern angiosperm orders such as Magnoliales, Hamamelidales, Ranunculales, and some monocots represented by fossil pollen and leaves.

Question 20

Oligocene Stage

Answer 20

• Pollen of the Asteraceae, an advanced dicot family, appears at the end of this stage.

Question 21

Pteridosperms - Seed Ferns

Answer 21

• Angiosperms form a monophyletic group. Although it is unreasonable to believe that they evolved from a single species, there is little doubt that they evolved from a single group ofgymnosperms. Most systematists believe that the ancestral group was most likely a family that included several genera and species.
• Seed ferns (pteridosperms), especially those in the orderCaytoniales, are probably the group from which the angiosperms arose.

Question 22

Seeds ferns had:

Answer 22

• Ovules semi-enclosed in small pouches (cupules), which might represent the beginning of the carpel.
• Leaves with a netlike pattern of venation.
• Pollen tubes leading to the micropyle of the ovule.
• Microsporangia that superficially resembled anthers.
• At some point in the evolution of this taxon, double fertilizationappeared. This must have had some selective advantage.
• Thus it was maintained in the population, and a group of plants appeared that were outwardly gymnospermous, but in this most important feature were angiospermous.

Question 23

Most Primitive

Living Angiosperm

Answer 23

Amborella trichopoda is considered the most basal angiosperm living today.

It has the following “primitive” features:

• No vessels
• Unisexual flowers with spiral perianth
• Laminar stamens
• Separate carpels