Plant Bio Quiz 2 Flashcards
- similar in leaves and stems
- water tight seal provided by a tight layer
- thick external layer of waxy cutin
- stoma allow for exchange of gases
epidermis
- thick external layer on epidermis
- virtually transparent so as to not block light from penetrating to photosynthetic cells below
cutin
allow for exchange of gases
stoma
across angiosperms, there is great diversity in what?
stoma
across angiosperms, there is a great diversity in stoma in terms of:
shape, size, number, density, position on leaf (upper vs lower surface), and structure
- e.g. some species, leaf stoma are sunken into ‘stomal crypts’; these are cavities that create a region of nonmoving air
Water molecules that bound out may easily re-enter the what just by random molecular motions?
stomata
leaf epidermis is often hairy due to the presence of what?
trichomes
- provide shade on the upper surface of the leaf, deflecting rays of sun
- slow air movement, decreasing water loss from stomata
- make walking or chewing difficult for insects
- if glandular, produce sticky chemical compounds that deter herbivores
trichomes
the tissue interior to the epidermis are the
mesophyll
for most species, mesophyll is divided into:
- palisade layer
- spongy layer
- upper region of the leaf mesophyll, with vertically oriented cells just under the epidermis
- often just one cell layer thick; may be multiple layers thick
- these cells are packed with chloroplasts and are the main photosynthetic tissue of most plants
- loosely packed, exposing surface area for gas exchange
Palisade layer
- lower portion of the leaf mesophyll
- these cells are open, very loose, forming an ‘aerenchyma’ [parenchyma with abundant air spaces] that permits CO2 to diffuse rapidly from stomata (often on bottom of leaf) into the leaf’s interior
spongy layer
Vascular bundles occur where?
between the palisade and spongy layers
A typical eudicot leaf usually has:
- one large midrip (or midvein)
- numerous lateral veins that then branch into narrow minor veins
The veins contain what?
xylem on the upper side and phloem on the lower side
the larger veins of a leaf typically also include:
- a bundle sheath
- a mass of supportive fibers (above, below, or both) that form a bundle sheath extension
- around vascular tissues
- may include parenchyma, collenchyma, and fibers
- provides structural support for the leaf
- also has a regulatory function for transfer of water and sugars between mesophyll cells and vascular cells
bundle sheath
- mass of supportive fibers (above, below, or both)
- these are often more pronounced on the underside of a leaf, leaving the upper surface +/- smooth for light interception
- these provide protection of the bundle and support for the leaf
bundle sheath extension
very small and may include single vessel elements above a sieve cell/companion cell pair
minor, terminal veins
most leaves have a what that connects the leaf blade to the stem?
petioles
one, three, five , or more vascular bundles branch from the stem and diverge toward the petiole
- collenchyma may be present to provide support
leaf traces
where the leaf attaches to the stem is a what?
abscission zone
Cells in the abscission zone are involved in cutting off the leaf in these conditions:
- at the end of the summer (if deciduous)
- if it becomes damaged by weather or herbivores
- if attacked by pathogens
What happens after abscission?
the leaf falls away and remaining undamaged cells become corky, forming a protective leaf scar
distinctive for each species, allowing identification of trees in winter
leaf scars
Which trees do not abscise their dead leaves in the fall?
- these dead leaves stay attached to the tree until bud break the next spring
- known as marcescence
oaks, beeches, and chestnuts
retention of dead plant organs that normally are shed
marcescence
why would a tree hold on to dead leaves?
- buds are present in the axils of leaves
- in winter, stem twigs and buds are a readily available source of nutrition
- trees that retained their old dead leaves may deter herbivores, who don’t want to chew on old dead leaves
Most current tree species evolved when?
the herbivores present were very different than today
- many of these animals were members of the pleistocene megafauna including, mastodon, mammoth and giant ground sloth
advantages of shedding leaves in fall:
- physiologically costly to maintain over winter
- conserves water
- provides opportunity to shed leaf pathogens
- can decrease limb damage from ice
- decreases threat of winter herbivory
- strategy used in junction with early spring flowering (before new leaves appear)
What is the advantage of retaining leaves over winter in temperate zones? (i.e. being evergreen)
- avoids the cost of building entirely new leaves
- especially important in nutrient stressed habitats (including acidic soils):
- evergreens don’t shed these valuable resource
- deciduous trees would have to reacquire these nutrients from environment
Examples of evergreen trees
e.g. laurel cherry, live oak, big-flower magnolia
leaves are initiated by what?
shoot apical meristems
where are younger leaves located? Older ones?
younger: near the top
older: further down
During development, the older developing leaves what?
closely surround and protect the delicate meristem
At the base of the shoot apical meristem, cells interior to the what bulge outward?
protoderm (immature epidermis)
- leaf protoderm
- leaf ground meristems
leaf primordium
In the leaf primordium, a strand of cells in the center of the primordium differentiation into what?
pro-vascular tissues, then into primary xylem and phloem, forming a direct connection with the vascular bundles in the stem
The young leaf consists of what?
a midrib and two small, then wings of lamina tissue
All cells in the wings are what?
meristematic (capable of dividing)
What enlarges the lamina rapidly?
division of leaf wings
- stomata, trichomes, and vascular bundles differentiate
- the petiole becomes distinct from the midrib
- when all the structures are in place, the leaf is still very small, much less than one-tenth is mature size
- no new mitotic division will ever occur
Lamina expansion
Full expansion to mature size, typically in spring with bud break, occurs as what?
water is taken up and expands the cells
Leaf structure in what are highly varied in composition?
angiosperms
Leaves with a single blade are what?
simple
leaves divided into many separate leaflets are what?
compound
For the development of compound leaves, the leaf primordium consists of what?
lateral regions that expand into separate leaflet lamina
In many perennial plants, leaves and shoots for _ initiate development in summer or autumn
next year
What becomes dormant within the resting terminal or axillary bud until the growing season begins?
leaves and shoot for the next year
what surround and protects the embryonic shoot?
bud scales
What happens when the bud opens?
the young shoot (and often flowers) rapidly expands
In _, meristem cells adjacent to the leaf primordium grow upward along with it, encircling the apical meristem
monocots
the primordium has what shape in monocots?
hood-like shape
The leaf primordium becomes tubular and forms a _ leaf base, while the upper portion forms the _ (blade)
sheathing; lamina
Linear, strap-shaped leaves (like grasses) grow continuously from a unique what?
intercalary meristem
Where is the intercalary meristem located?
between the sheath and blade
- allows for continuous lengthening of the blade
- remains active so that if lamina is consumed, it can grow
- likely an adaption in response to herbivory
intercalary meristem
- leaves are thick and fleshy
- adaptations to survive in desert habitats
- reduced surface-to-volume ratios, favoring water conservation
- mesophyll contains few air spaces, and is more transparent so photosynthesis may occur deeper in the leaf
- plants also frequently employ modified photosynthesis
succulent
- tough often spiny leaves
- leaves have a sclerenchyma layer below the epidermis and in the bundle sheets
- their structure provides a high level of protection
- common in harsh dry climates, such as the California chaparral, and a few other biomes
sclerophyllous
- small modified leaves that protect young buds in the winter from extremes in weather and herbivores
- often numerous and tightly overlapping
- frequently pubescent, and also with a thin, corky bark-like layer
bud scales
What are common adaptions in plants to prevent herbivory?
sharp pointy structures: thorns & spines
a modified, sharp-pointed stem. It occurs in the axil of a leaf where an axillary bud and shoot would normally develop
thorns