|
BI 102 Spring 2003. Thursday April 17, Davison Lecture Notes. Ch 36 Plant Structure: Leaves, Stems & Roots. LeavesGeneralized forms, fig. 36.19: compound vs. simple
leaves (based on whether the blade is divided into leaflets
or not; the blade is the expanded, photosynthetic portion of the leaf), petiole
(leaf stalk). Leaf Anatomy, fig. 36.18: cuticle (waxy deposit, extracellular), epidermal
layers, mesophyll (the ground tissue and vascular tissue
between the dermal layers), palisade parenchyma (tightly packed,
columnar cells, maximum photosynthetic ability), spongy parenchyma
(loose arrangement allows for rapid gaseous diffusion; photosynthetic but
w/fewer chloroplasts), vein (a.k.a. vascular bundle), stoma
(pl. stomata; stoma is a term for the opening only), guard cell
(two guard cells bound each stoma). Modified Leaves:
tendrils for grasping while climbing (young, straight
tendrils coil and contract upon contact with solid support, the coiling
shortens the tendril’s length and the tendril may then resemble a
tightly coiled spring), spines for protection from herbivory, bracts
for botanists (ha ha), bracts are typically reduced leaves associated with
flowers and they have great importance in plant identification. StemsStem Anatomy (of young stems),:
the stem apex is a dome of meristematic cells (fig. 36.12) that
comprises the stem’s apical meristem.
The mature tissues of a young stem (fig. 36.15a) consist of vascular
bundles running lengthwise through the stem, and ground tissue
comprises the rest of the stem tissue within the outer single layer of
epidermal cells. Vascular
bundles are arranged in a ring in dicot stems while they are
scattered in monocot stems. The
center of the stem is soft and spongy ground tissue and is called the pith;
sometimes the center becomes a hollow pith. Modified Stems, fig. 36.17 :
thorns for protection; bulbs contain a short stem
from which a series of fleshy leaves arise, bulbs function for
overwintering and asexual reproduction; tubers function as a
storage organ, tubers can shrink as they give up food reserves to the
above ground shoot; stolons are horizontal above ground stems (some
plants are said to be “stoloniferous” if the have a creeping above
ground stem like the strawberry plant that aids in vegetative spread and
asexual reproduction); rhizomes are horizontal below ground stems
that also aid in vegetative spread, asexual reproduction, and as
overwintering structures. Roots Roots as thick and thicker than your pinky fingers
function in transport and as anchorage for the above ground shoot.
Much smaller diameter roots, the feeder roots, are active in
absorption of water and mineral nutrition. Root Anatomy (of young feeder root), fig.
36.8: the root apex is capped by protective cells that form the root
cap, the root cap protects the apical meristem where continuous
cell divisions produce new cells and constant growth (unlike stems, most
roots continue to grow during winter).
The mature tissues of the young dicot stem consist of a central
strand of vascular tissue: xylem
occupies the center of the central strand and forms a cross-like central
core, pholoem is found between the arms of the “cross” formed by
xylem; surrounding this vascular core is a single cylinder of cells called
the endodermis (more about the endodermis in next chapter).
Ground tissue surrounds the stele and the outer root surface
consists of dermal tissue the cells of which have long, thin
extensions called root hairs (a root hair is an extension of a
dermal cell, functions to increase the absorptive surface area). Adventitious roots:
above ground roots (aerial roots); roots arising from above
ground stems or leaves. (e.g. prop roots at base of corn stalk; aerial
roots hanging from a grapevine; roots from English Ivy vine that anchor
plant to rock/brick/wood walls of houses). BI 102 Spring 2003. Tuesday, April 22, 2003 Davison Lecture Notes. Ch 36 Plant Structure cont.:
Plant Growth. Life span (for how long does an individual plant grow?) Annuals – live ca. one year then die (ex. Morning Glory vines, some Geranium species) Biennials – live ca. two years (1st year grows vegetatively, 2nd year blooms and dies, ex. Some members of the mustard family) Perennials – live for many years; includes
both herbaceous and woody plant species; the perennial body may be
underground only or both above and below ground. Plants grow by action of meristems and regions of
cell elongation. Compared to meristematic cells, most mature cells are
greatly elongated. The zone
of elongation is adjacent to the apical meristem. The elongating cells thrust the stem or root tip forward. Types of Meristems: Apical Meristems – a dome of cells at the tips of stems and roots (leads to primary growth). Lateral Meristems (cambia)–cylinder of cells near outer perimeter encircling the long axis of stems/roots. Vascular Cambium – produces secondary vascular tissue.
Cork Cambium–produces cork cells containing suberin
(a waxy material) in cell wall; cork is dead. Laterial Meristems give rise to secondary growth (growth in girth): See Figure 36.15 and know: pith, 1o xylem,
2o xylem, vascular cambium, 2o phloem 1o
phloem, cork cambium, cork, bark, wood.
Know that cork and cork cambium (and even older 2o
phloem) eventually become part of dead outer bark having been pushed
inexorably outward as the stem/root grows in girth by accumulating 2o
xylem (i.e. wood). The
living cork cambium is not lost however, for new cork cambium periodically
forms from parenchyma in 2o phloem.
Bark is a technical term for all tissues beyond the outer
regions of the vascular cambium (includes 2o phloem, cork
cambium, & cork). Bark
replaces dermal tissue as a protective barrier. Lenticels, spongy regions in bark with
taxonomic value, may form in outer bark as an adaptation for gas exchange.
Lenticels help overcome the problems produced by the many layers of dead
cells in outer bark, that is, the layers outside the cambia can be an
effective barrier to O2 and CO2 diffusion (the
metabolically active cambia must exchange these respiratory gases in the
same direction as animals—O2 inward and CO2
outward from the body). Lenticels
provide air spaces for gaseous diffusion. The Vascular Cambium produces an accumulating cylinder of wood to the inside and an ever expanding, yet thin band of phloem to the outside. See figure 36.16, Anatomy of a Tree Trunk. Heartwood is dead 2o xylem often
darker in color due to accumulating chemicals produced as the xylem tissue
completely dies and becomes nonfunctional for water transport, functioning
only for structural support. Sapwood
is living 2o xylem often lighter in color (still contains dead
tracheids and vessel elements) and functions for transport of water and
mineral nutrition (Chapter 37). Tree Rings. In many tree species the character of xylem tissue produced by the vascular cambium changes with the growing season. During spring and early summer early wood is produced in which the tracheids and vessels are much wider in diameter and typically lighter in color. Late in the growing season late wood is produced with smaller diameter tracheids and vessels typically darker in color. The width of the bands is indicative of growth rate of the tree, if water and light are plentiful the bands are wide, during times of drought or suppression the bands are narrow. Dendrochronology is “the science of dating events and variations in environment” by study of tree rings. BI
102 Spring 2003. Tuesday,
April 29, Davison Lecture Notes Ch 37 Nutrition &Transport in Plants Plant Nutrition Most plants take in nutrients in an inorganic
form. Unless they are
parasitic or carnivorous, plant normally can’t absorb from the
environment organic molecules such as carbohydrates, proteins, etc.
Most plants uptake inorganic chemicals to acquire the elements
needed to construct organic molecules.
Plant nutritional uptake includes simple inorganic molecules (e.g.
CO2, H2O, NO3- [nitrate], NH4+
[ammonium], H2PO4- [phosphate], SO42-
[sulfate]) and simple ions (e.g. K+, Ca2+, Mg2+).
All of the essential macronutrients plants require are found in the
above selection of chemicals. Recall
our mnemonic device: CHOPKNS
CaFe Mg
B Mn CuZn Mo, Cl & Ni Table 37.1 lists these nutrients and their major
physiological functions. The macronutrients
(needed in relatively large amounts) are double underlined above.
Those not underlined are micronutrients (needed in small
amounts). Those elements
obtained primarily from atmospheric uptake are in italics [C & O
only]. All others are
obtained from the soil. SoilWe can begin to think of the nonliving aspects of soil as consisting of two components:
The fertility of the soil depends in part on
the presence of essential nutrients in proper inorganic form.
As the inorganic nutrients dissolve in the water between soil
particles, they become available for uptake by feeder roots. Humus, while rich in elemental nutrients, supplies no
nutrients to plants unless the elements within are decomposed to an
inorganic state. Humus does
help condition mineral soil by improving aeration and moisture retention. Negatively Charged Clay Surfaces Affect Nutrient Availability Inorganic nutrients that are cations (e.g. K+
& Mg++) are bound to the negative charge on the surface of
clay particles. Plants
actively participate in a phenomenon called cation exchange (see
fig. 37.3) by secreting H+ into the soil solution.
The H+ from the plant may be exchanged with the nutrient
cation bound to the clay particle. The
plant root and the clay particles exchange cations, the plant swaps H+’s
for K+ & Mg++. Inorganic nutrients that are anions (e.g. NO3-,
H2PO4-, & SO42-)
are repelled by negatively charged clay particles.
Anions tend to leach out (or dissolve out, or wash out) of
the soil as rainwater percolates down through the soil. Symbiotic Relationships as Nutritional AdaptationsGiven
the potential difficulties in obtaining mineral nutrients (nutrients bound
tightly to negatively charged clay particles or they may be in limited
supply due to the leaching), it is no wonder that plants have enlisted the
help of organisms that are more efficient is securing these nutrients. Mycorrhizae Mycorrhizae
are a fungus-root symbiotic combination found in almost all plants.
The microscopic, filamentous form of a fungus (the filaments of
fungus are called hyphae) is more efficient in acquiring mineral
nutrients from the soil. The
membrane transport mechanisms of the fungus are more powerful than that of
the plant root, plus the hyphae are much longer extending through much
more soil than possible for any root hair.
In addition to delivering mineral nutrients to the root the fungus
also delivers water to the root. The
region where fungus and root cells intertwine is the mycorrhiza (singular). For its trouble the fungus is feed an abundant supply
of carbohydrates by the plant root. Many
of the larger mushrooms you see on the forest floor are the fruiting
bodies of mycorrhizal fungi. Nitrogen fixersNitrogen
containing molecules are not made available to plants from the breakdown
of parent rock as are many other chemicals (e.g. K+, Mg++,
H2PO4-, SO42- ).
The element N is plentiful in the air we breath but it is not in a
form plants can use. All
plants rely on the activity of certain soil bacteria that convert nitrogen
into forms suitable for plant absorption.
There are two points that I will emphasize.
Nitrogen fixing bacteria that convert N2 into NH4+ do so with their enzyme nitrogenase. Nitrogenase, which converts N2 into NH4+ is inhibited by O2 levels found in normal, aerobic conditions. The nitrogen fixing bacterial genus Rhizobium has entered into a symbiotic relationship with plants in the legume or bean family. Part of this symbiotic relationship in which the bacteria are intracellular symbionts housed within the cytoplasmic contents of living plant cells in root structures called root nodules (see fig. 37.6), involves the synthesis of leghemoglobin, an iron containing protein very similar to our own hemoglobin. Leghemoglobin binds with O2 thus eliminating the interference O2 has on the enzyme nitrogenase. The symbiotic relationship ensures Rhizobium an adequate supply of carbohydrates and the plant is ensured an abundant supply of ammonium. Thus, I offer one way to understand why beans are the most protein rich food we obtain directly from plants--with nitrogen readily available, beans (legumes) synthesize more proteins than the average plant. It should be acknowledged, that some of the ammonium from root nodules will leak into the surrounding soil (especially following death of the bean plant), thereby other species of plants also enjoy some of the benefits of this global, ecologically important relationship between Rhizobium and legumes. Plants with Nutritional Specializations (see p.
671 in Mader) Carnivorous Plants Some
plants have greatly modified their leaves into animal traps.
Once trapped, the animal is slowly digested and its
nitrogen-containing proteins are absorbed by the plant.
These so-called carnivorous plants typically inhabit environments
that are high in peaty organic matter and very poor in mineral nutrition,
especially nitrogen. Some
carnivorous plants you should recognize are:
Venus flytrap, native only to a small region of North and
South Carolina; Pitcher plants found widely, and Sundews
found widely. How does each
of these trap insects? Do
carnivorous plants make their own food, i.e. are they photosynthetic? Parasitic Plants Some
plants have lost the ability to photosynthesize and feed on other plants
for all of their nutritional needs. Dodder
is a locally common plant parasite easily recognized by the slender
orange vine intertwined amongst the green plants it feeds upon.
Indian Pipe, also common locally, is a colorless plant
parasitic on tree roots by means of a mycorrhizal connection.
Perhaps the most familiar plant parasite is the mistletoe.
Mistletoe, while parasitic is photosynthetic.
Mistletoe taps into the xylem tissue of its host for its water and
mineral nutrition needs. Mistletoe
is a parasitic epiphyte. The
term epiphyte (epi =upon, phyte=plant) describes a plant that is
rooted upon and lives perched on another plant.
Usually, epiphytes such as Spanish moss and other “air
plants” are not parasitic. Rafflesia,
of Borneo, is the most dramatic plant parasite having the largest single
flower or any plant, yet its threadlike body is found as an internal
parasite in woody vines of the jungle.
Long
Distance Transport Through Vascular Tissue in Plants Phloem Transport The flow of phloem sap occurs through sieve tubes
from sugar source to sugar sink at a rate of about 1 meter per hour (ca.
16 mm per minute or just over ½ inch per minute) [a point to make is that
this is much slower than xylem transport as we will see]. Sieve tubes are filled with living cytoplasm which flows as
phloem sap. Sieve tubes
extend as continuous pipes linking the leaf mesophyll with the tips of
roots. See figure 37.14. Phloem
Transports Sap from “Sugar Sources” to “Sugar Sinks.”
Sugar sources, where sugar is loaded into phloem for
transport, include photosynthesizing leaves and mature storage organs
(sprouting bulbs and tubers). Sugar
sinks, where sugar is unloaded from phloem, include meristems and
growing plant parts (newly forming leaves, stems, roots, flowers &
fruit as well as swelling tubers and bulbs). Active
and Passive Transport Across Plasma Membranes are required for long
distance transport in phloem tubes. Long
distance transport of body fluids usually occurs through tubes in both
plant and animal bodies. The
fluid in plant tubes however, is not driven by a pumping heart. The relatively slow movement of phloem sap is driven by
osmotic pressure. Osmotic
pressure is manipulated by the plant as sugar (solute) is actively
transported across membranes at both “sugar source” and “sink”
ends of the sieve tube. This
solute movement creates a hypertonic phloem sap at the “source end”.
The formation of sugar-rich, hypertonic phloem sap requires the
membrane transport proteins that concentrate sugar in the cytoplasm. Notably, companion cells at the sugar source end have
a very extensive plasma membrane surface.
This increases the amount of membrane level transport these cells
can perform. The high surface
area of plasma membrane is due in part to the many “hills and valleys”
of cell wall ingrowths that the membrane lies against.
Plant cells, such as these companion cells, that possess wall
ingrowths with increased plasma membrane surfaces, are called transfer
cells. Transfer cells are
efficient in transferring solutes across plasma membranes.
Companion cells, in transferring sucrose, help create the
hypertonic cytoplasm at source end. A
hypertonic cytoplasm will lead to a spontaneous osmotic uptake of water
into the sieve tube. The spontaneous osmotic
water uptake in the sieve tube’s “sugar source” end generates a
positive pressure forcing the sucrose laden sap to ooze through the
sieve plates towards the direction where sucrose and water are
exiting the sieve tube via membrane processes identical to the source end,
only opposite in direction. Thus
the “sugar sink” end of the sieve tube relieves the pressure generated
at the “sugar source” end. The
result is the mass flowing (bulk flow) of phloem sap from sugar
source to sugar sink. Xylem Transport Ascent of Xylem Sap Root Pressure -
Osmotic pressure generated in xylem of roots pushes xylem upwards.
The concentration of mineral ions through membrane level processes
operating in the endodermis creates a tonicity gradient between xylem and
ground tissue that favors the osmotic uptake of water in root xylem.
Xylem sap can be pushed upwards only a few feet by root pressure. Root pressure may cause guttation - the exudation of
water droplets at leaf margins (see fig. 37.10). Transpiration-Cohesion-Tension
Mechanism. Transpiration -
defined as simply the loss of water from inside leaves due to evaporation. Given that water is both
cohesive (clings to itself) and adhesive (clings to cell walls), a force
of tension (negative pressure) is generated as water molecules
evaporate from the intercellular spaces in mesophyll.
This tension creates a pull that extends through the continuous
column of water of xylem that connects leaves to roots.
As quickly as water is transpired from the leaves it is replaced by
the rise of xylem sap. This
is quite rapid, at times reaching rates of 15 meters per hour (4 mm/sec.).
Were it not for the evaporative water loss, xylem sap could not be
so rapidly transported. Solar
and wind powered, xylem is an engineering marvel able to move hundreds of
gallons of water upward each day in a large tree in complete silence. Benefits of Transpiration· Results in the transport of mineral nutrients and water to stems and leaves. · Cools leaves via evaporative heat loss Controlling TranspirationModifications that reduce transpirational water loss. · Cuticle on leaves limit water loss · Stomata on underside of leaf where evaporative stress is less ·
Leaves with epidermal hairs reduce air flow at leaf surface
(hairs trap dead air space)
|