Plant Structure, Growth, and Angiosperm Reproduction

Plant organs

Roots (part of the root system)

Stems (part of shoot system)

Leaves (part of the shoot system)

The roots are an organ that anchor a vascular plant into the soil and also function to absorb water and minerals, also storing carbohydrates

Taproot system

Fibrous root system

Single main vertical root which penetrates deep into the soil and prevents the plant from falling over

Lateral roots responsible for absorption

Helps plant grow tall

Mat of thin roots that spread below surface of the soil

Many small roots spread out from stem

May be adventitious: growing in an unusual location

Root hairs- finger-like extensions of root epidermal cells that increase surface area for absorption

The organ to which leaves are attached whose main function is elongation and orientation of the shoot in a way to maximize photosynthetic capability of the leaves

Also elevates reproductive structures to maximize dispersal range

Has a system of alternating nodes and internodes

Nodes: points where leaves are attached

Internodes: stem segments in between nodes

Apical bud: bud at the tip of a plant stem (terminal bud) that is composed of developing leaves and compact series of nodes and internodes (grows vertically)

Axillary bud: structure that has the potential to form a lateral shoot of branch, which forms in the angle formed between a leaf and a stem (grows laterally)

This is the main photosynthetic organ of the plant- responsible for photosynthesis, gas exchange, heat dissipation, and defense against pathogens and herbivores

Consists of a flattened blade and a stalk (petiole) that joins the leaf to the stem at a node

Leaves have vascular tissue (veins) which transport supplies for photosynthesis toward the leaf and the products away to the rest of the plant

Great variation exists in leaf morphology

Vital structures

Stomata- allow for gas exchange

Guard cells- regulate opening and closing of stomata (impact water loss)

Mesophyll- contains specialized cells for photosynthesis

Vascular tissue- transport nutrients and products of photosynthesis

Major tissues in plants

Vascular tissue system

Ground tissue system

Dermal tissue system

This is the plants outer protective covering that is its defense against pathogens and physical damage

Non-woody plants: have epidermis (tightly packed cells) and cuticle (waxy outer covering that prevents water loss)

Woody plants: have periderm instead of epidermis in older regions of stems and roots which provides more protection

Facilitates transport (water and minerals) through plant as well as provide mechanical support

2 tissue types

Xylem: conduct water and dissolved minerals from root to shoot

Phloem: transport sugar to roots and growth sites

Stele: vascular tissue of a root or stem

Vascular cylinder in root (solid central bundle of tissue)

Vascular bundles in stems and leaves (separate xylem and phloem)

Includes tissues that are neither dermal nor vascular

Pith: ground tissue internal to vascular tissue

Cortex: ground tissue external to vascular tissue

Contains cells that function in photosynthesis, short distance transport, storage, and support

Primary growth

This type of growth occurs in all plants (woody and non-woody)

Primary growth occurs in the apical meristems at the tip of shoots and shoots and axillary buds of shoots

Enable growth in length- producing almost all of the plant body in non-woody plants

Apical dominance occurs when an active apical bud inhibits growth from nearby axillary buds through plant hormones

If an apical bud is removed, an axillary bud will grow a lateral shoot

If an apical bud is removed and plant hormones are applied at the site of removal, the axillary bud will not grow

Secondary growth

This type of growth only occurs in woody plants

It occurs in the lateral meristems that enable growth in thickness

The vascular cambium adds layers of vascular tissue (secondary xylem and phloem)

The cork cambium replaces the epidermis with thicker, tougher periderm

Consists of the tissues produced by vascular and cork cambium

Increases diameter of stems and roots- mostly occurs in eudicots and gymnosperms but rarely monocots

Primary and secondary growth occur simultaneously

Pollination: act of placing pollen (has male gametophyte) on the stigma of the carpel

During this process, the male gametophyte then travels to the gametophyte (inside embryo sacs in the ovule)

Plants tend to transfer pollen to a different flower

3 types of pollination:

Abiotic pollination (ex. wind)

Biotic pollination: living organisms pollinate other plants (insects, bees, bats, birds, etc.)

Self-fertilizing: in some crop plants (ensures every ovule develops into a seed)

Combine self pollen and ovule to form a zygote (potentially detrimental because it does not increase genetic diversity)

Some angiosperms have evolved mechanisms to prevent self-fertilization

Angiosperm reproduction

Forming the male gametophyte

Process of pollination

Forming the female gametophyte

Double fertilization

In the megasporangium of each ovule (in the ovary), the megasporocyte undergoes meiosis to produce 4 megaspores

One megaspore survives to become the female gametophyte (embryo sac)

Embryo sac consists of: antipodal cells, polar nuclei, synergids, egg (haploid)

On the anther, each microsporangium contains microsporocytes that undergo meiosis each to produce 4 haploid microspores

The microspore undergoes mitosis to produce the male gametophyte (pollen grain, generative cell and tube cell)

The generative cell divides to form 2 sperm which pass into the tube cell during gametophyte maturation

The pollen grain is transferred to the surface of the stigma

Tube cell produces pollen tube which delivers sperm to the female gametophyte

1 sperm fertilizes the egg to form the zygote

1 sperm combines with 2 polar nuclei to form the 3n endosperm

After double fertilization

The zygote develops into the embryo (which is packages with food into seed)

The seed may be enclosed by fruit

Asexual vs. sexual reproduction in plants

Many angiosperm species can do both

Asexual reproduction- the formation of offspring from a single parent without the fusion of gametes

Occurs by budding, division of a single cell, or division of a whole organism into multiple parts

Offspring generally genetically identical to the parent

Also called vegetative reproduction

Predominant mode of reproduction for some species of angiosperms

Sexual reproduction- formation of offspring from two parents by fusion of gametes

2 forms:

Fragmentation- detached fragment from the plant that can become a whole new plant

Apomixis- plants may produce seed without pollination or fertilization

Ex. Aspen tree in Utah

Ex. dandelions

Increases genetic diversity

Pollen grain (sperm) fertilizes the egg to produce the zygote

Offspring not identical genetically to the parents