Unit 2 Evolution of Leaves

Question 1

Define Leaf

Answer 1

They are lateral appendages of the vascular plant (sporophyte)

Question 2

What is a stele?

Answer 2

It is the central cylinder of roots and stems of vascular plants, inside the cortex

Question 3

Protostele

Answer 3

-simplest/most primitive type of stele in stems -consists of a solid core of vascular tissue, no pith

Question 4

What are the three types of protosteles?

Answer 4

1) Haplostele 2) Actinostele 3) Plectostele

Question 5

Haplostele

Answer 5

earliest type of stele, with a column of xylem surrounded by phloem

Question 6

Acinostele

Answer 6

-xylem is lobed or star-shaped -found in Psilotum and some species of Lycopodium

Question 7

Plectostele

Answer 7

xylem seems to consist of separate plates of tissue surrounded by phloem

Question 8

Siphonostele

Answer 8

-has a central pith surrounded by the vascular tissue -found in stems of most seedless vascular plants -can have presence of a leaf gap

Question 9

Eustele

Answer 9

-vascular cylinder consists of a system of discrete strands around a pith -found in most seed plants

Question 10

Leaf trace

Answer 10

The part of a vascular bundle extending from the base of the leaf to its connection with a vascular bundle in the stem

Question 11

Leaf gap

Answer 11

The region of parenchyma in the primary vascular cylinder above the point of departure for the leaf

Question 12

What happened in the major split in vascular plants?

Answer 12

1) Appearance of lycophyte and euphyllophyte clades 2) The leaves of lycophytes are microphylls and euphyllophytes have megaphylls 3) The split came from the zosterophylls and trimerophytes, which are related to lycophytes and euphyllophytes, respectively

Question 13

Microphylls

Answer 13

-are relatively small leaves that contain only a single strand of vascular tissue -there is usually only a single vein in each leaf -typically associated with stems that have protosteles

Question 14

What are the theories to the origin of microphylls?

Answer 14

1) Enation theory 2) Sterilization of sporangia 3) Telome theory -None of these theories cannot be rejected even though enation has the most support.

Question 15

Enation Theory (microphylls)

Answer 15

Enations are scale-like or spine-like lateral outgrowths of the stem -lack vascular tissue; gradually leaf traces developed

Question 16

What are the two generas according to the enation theory?

Answer 16

1) Asteroxylon 2) Huperzia (relative of Lycopodium)

Question 17

Asteroxylon

Answer 17

-Had “leaf-like” appendages (“leaf” traces ended at the base of each appendage)

Question 18

Huperzia

Answer 18

Microphyll is complete

Question 19

Sterilization of Sporangia Theory

Answer 19

Naked stem with vascular tissue (time—>) sporangia (time–>) microphyll

Question 20

Telome Theory (Microphylls)

Answer 20

Process of reduction of flattened, lateral branches

Question 21

Euphyllophytes (Megaphylls)

Answer 21

-Likely evolved multiple times by the Carboniferous -the common ancestor was leafless and these leaves did not evolve from microphylls

Question 22

Telome Theory (Megaphylls)

Answer 22

1) Earliest plants had a leafless, dichotomously branching axis -unequal branching resulted in more aggressive branches overtopping the weak ones 2) The subordinate branches represented the beginnings of leaves -the more aggressive portions became stem-like axes 3) Flattening out (planation) of lateral branches 4) Fusion or webbing of the separate lateral branches to form a blade

Question 23

Megaphylls

Answer 23

-Complex system of branching veins -associated with stems that have either siphonosteles or eusteles

Question 24

Leaf gaps

Answer 24

The presence/absence of these gaps does not always distinguish between micro/megaphyll -exceptions to size and vein complexity to differentiate between the two groups