NATSCI II HANDOUT
I. Overview
Plant biologists study two levels of plant architecture: morphology and anatomy.
• Plant morphology - study of the external structure of plants
• Plant anatomy - study of the internal structure of plants
The focus of the discussion is the angiosperm plant (will not include plants from other divisions such as the moss, etc)
• Angiosperms (flowering plants) are the most diverse, widespread and ‘advanced’ among all plants.
- Characterized by flowers and fruits.
- Taxonomists divide angiosperms into two major taxonomic classes: monocots and dicots which possess either one or two seed leaves, respectively, in combination with other characteristics.
A plant can be divided into two basic systems, a subterranean root system and an aerial shoot system (stems, leaves, flowers).
Each system depends on the other for survival of the whole plant.
- Roots depend on shoots for sugar and other organic nutrients
- Shoots depend on roots for minerals, water and support.
Materials are transported throughout the plant by vascular tissues (continuous throughout the plant)
- Xylem transports water and dissolved minerals to the shoots.
- Phloem transports food from shoots to roots and other non-photosynthetic parts, and from storage roots to
actively growing shoots.
II. Roots
Root structure is well adapted to: (a) anchor plants, (b) absorb and conduct water and nutrients, and (c) store food.
2 major types of root systems:
• Taproot system (seen in many dicots) – has one large, vertical root (the taproot) that produces many smaller secondary roots; provides firm anchorage
Some taproots, such as carrots, and sweet potatoes are modified to store a large amount of reserve food.
• Fibrous root system (found primarily in monocots) – has a mat of threadlike roots that spreads out below the soil surface; provides extensive exposure to soil water and minerals
Roots are concentrated in the upper few centimeters of soil, preventing soil erosion.
Root parts
1. Primary root- thickest part; grows downward
2. Secondary roots- arise from primary root; thinner; grows sidewards
3. Root cap- protects root; secretes mucilage that lubricates root for movement in soil; designed to drill soil guides root growth by perceiving gravity
4. Root hairs- small hair-like extensions; increase root’s surface area, increasing capacity to absorb water and minerals
1. Epidermis- outermost layer; for water and mineral absorption
2. Cortex- for water and mineral transportation to central cylinder of root thru diffusion; may also be used for food storage
3. Endodermis- develops root pressure for water movement
4. Pericycle- from where secondary roots branch
Symbiotic associations:
• Mycorrhizae – symbiotic associations between roots and fungi wherein water & mineral absorption are enhanced by the fungi
• Root nodules – contain symbiotic bacteria that converts nitrogen to plant fertilizer
Fungi and bacteria get their food from roots in return.
III. Stems
Stems are generally upright and above ground: (a) plant support and for elevation of leaves, flowers and fruits (b) place where buds and shoots generally develop (e) for transportation of fluids between the roots and the shoots in the xylem and phloem (f) storage of nutrients (g) production of new living tissue.
Stem parts
1. Nodes- points where leaves are attached to stems
2. Internodes- stem segments between the nodes
3. Axillary bud- embryonic side shoot found in the angle formed by each leaf and stem; usually dormant
4. Terminal bud- bud on the tip of a shoot / stem; usually has developing leaves and a compact series of nodes and internodes
1. Epidermis- for protection from water loss, gas regulation and secretion of secondary metabolic compounds
2. Fibers and sclereids- supporting tissues
3. Pith- center of the stem and may function as food storage; transports nutrients to outer tissues in the plant
4. Apical meristem- area of cell division responsible for growth at tip; composed of tunica cells and corpus cells
5. Lateral meristems- areas of cell division that give rise to growth in girth
a. vascular cambium- give rise to secondary xylem and phloem
b. cork cambium- replaces the epidermis of roots and stems
6. Endodermis- store food; allows solutions to pass from vascular bundles to cortex
7. Pericycle- strengthens the stem; protects vascular bundles
Stem modifications
Some plants have modified stems which are often mistaken for roots. There are several types of modified stems, each of which performs a specific function
a. Stolons/ Runners – horizontal stems where new plants grow; creeps above ground (eg. strawberry plant runners)
b. Rhizomes – horizontal stems growing underground; stores food (eg. ginger)
c. Tuber - usually a swollen rhizome; grows underground (eg. potato)
d. Corms – upright swollen stem covered with papery scales; grows underground; stores food (eg. gladiolus)
e. Bulbs – like corms but has leaf bases that are the ones that store food (eg. onions)
The trunk of a tree is a modified stem. There are other types of stem modifications aside from the ones listed above.
III. Leaves
Leaves are commonly flat, broad and green to maximize its function in absorbing sunlight and transforming it into food
Plant taxonomists use a variety of leaf characteristics to classify plants:
a. Basic leaf types
Simple leaf – Compound leaf –
has one blade attached to stem has a blade divided
into leaflets
b. Leaf shape- broadly lance-shaped, narrowly lance-shaped, obovate, ovate, etc
c. Leaf arrangement on stems- alternate (left), opposite (center), or whorled (right)
d. Leaf vein pattern- pinnate (left) or palmate (right)
e. Leaf margin- serrated, smooth with pointed lobes, smooth/ entire, smooth with rounded lobes
Leaf parts
• External parts:
1. Blade- the entire leaf unit
2. Vein- vascular bundles (made up of xylem and phloem)
3. Apex- tip of the leaf
4. Base- part of the blade that is closest to the stem
5. Margin- term used to describe the edge of a leaf
6. Petiole/ stalk- connects the leaf blade to the stem
7. Stipule- tiny leaf-like structures that may or may not be present at the base of the petiole
1. Cuticle – waxy layer on the epidermis which reduces water loss and prevents disease-causing organisms from entering the leaf
2. Palisade layer- filled with chloroplasts and carry on most of the photosynthetic activity
3. Spongy mesophyll layer- temporary storage of sugars and amino acids; aids in exchange of gasses between leaf and environment
4. Stoma (plural-stomata)- pores at the lower epidermis; for gas exchange
5. Guard cells- regulate opening and closing of stomata thereby controlling gas exchange
Leaf modifications
Leaves can be modified into several forms:
a. Spines – function to protect plant (eg. barberry)
b. Tendrils – support stems of climbing plants (eg. garden pea)
c. Bracts –may be green or brightly colored (mistaken for flowers) to attract pollinators (eg. bougainvillea)
d. Scale leaves – provide protection for buds and can be further modified for food storage (eg eastern redcedar)
IV. Flowers
Flowers are the plants’ reproductive adaptation.
• Perfect flower – flower that has both male and female reproductive parts
• Imperfect flower – flower with either male or female parts
• Complete flower – flower with petals, sepals, stamen and pistil
• Incomplete flower – flower that lacks at least one of the parts of a complete flower
Floral parts
Sterile parts (not involved in reproduction):
1. Sepals- modified leaves at the base of flower; usually green; enclose the flower before it opens
Calyx- made up of 2 or more sepals
2. Petals- lie inside ring of sepals; often brightly colored in animal- pollinated plants; lack bright coloration in wind-pollinated plants
Corolla- made up of 2 or more petals
Reproductive parts:
3. Stamen- male reproductive organ; consists of
a. Filament- stalk
b. Anther- terminal sac where pollen is produced
4. Pistil- female reproductive part; consists of
a. Stigma- receives pollen
b. Style- leads to the ovary at the base of the pistil
c. Ovary- protects the ovule
As ovules develop into seeds, ovary develops into a fruit.
