INTRODUCTION TO VASCULAR PLANTS

I. Two major types of true plants:

A. Vascular (higher) plants:

1. Have true conducting tissues (xylem and phloem), leaves, stems and roots

2. The sporophytes are the dominant phase and the gametophytes are much reduced

3. Constitutes the majority of plants

B. Non-vascular (lower) plants  

1. Lack true conducting tissues, leaves and roots. Since they lack elaborate conducting tissues they are relatively small

2. The gametophytes are the dominant phase and the sporophytes are dependent upon them

3. Require water for fertilization so they must grow in moist or seasonally moist habitats

II. Earliest land plants:

A. The invasion of land by plants probably occurred about 450 million years ago because by 430 million years ago there were many fossil land plants

B. The earliest land plants were relatively simple and undifferentiated

1. They had dichotomously branched photosynthetic axes that lacked true leaves and roots

2. Specialization led to differentiation of these simple axes into roots, stems and leaves

ORGANIZATION OF THE VASCULAR PLANT BODY

I. Vascular plants have three systems:

A. Root system - collective term for roots which anchor the plant and also absorb water and minerals from the soil

B. Shoot system - made up of the stems and leaves together. Leaves are specialized photosynthetic organs

C. Vascular system - conducts water and minerals to the leaves and the photosynthetic products away from the leaves to the rest of the plant

II. Tissue systems:

A. Different kinds of cells are arranged into tissues, and the tissues are further arranged into tissue systems, which are arranged into the organs (either roots, stems or leaves)

B. Three tissue systems occur, in different proportions, in all organs of the plant

1. Dermal - makes up the outer protective coating of the plant

2. Vascular - xylem and phloem, the conducting tissues

3. Ground - all other tissue

C. All three tissues systems occur in all organs of the plant and they are continuous from organ to organ

D. The principle differences between roots, stems and leaves lie primarily in the relative distribution of the vascular and ground tissue systems

GROWTH

I. Growth in plants is restricted primarily to meristems

A. meristem = undifferentiated tissue from which new cells arise

B. There are two types of growth:

1. Primary growth -  it occurs relatively close to the tips of roots and stems

a. It is initiated by apical meristems and it is primarily involved in the extension of the plant body

b. The tissues that arise during primary growth are called primary tissues and the plant body composed of these tissues is called the primary plant body

c. Most primitive vascular plants are entirely made up of primary tissues

2. Secondary growth - in addition to primary growth some plants undergo additional growth that thickens the stems and roots

a. Secondary growth results from the activity of lateral meristems

Lateral meristems are called cambia (cambium) and there are two types:

1) Vascular cambium - gives rise to secondary vascular tissues (secondary xylem and phloem). The vascular cambium gives rise to xylem to the inside and phloem to the outside

2) Cork cambium - which forms the periderm. The periderm replaces the epidermis in woody plants. 

b. The secondary vascular tissues and periderm make up the secondary plant body

c. Secondary growth appeared in the fossil record about 380 million years ago

VASCULAR SYSTEM

I. The vascular system is made up of:

A. Phloem - the food conducting system

1. The individual cells are called sieve elements. They have soft walls and they often collapse after they die. Therefore they are rarely preserved in fossils

B. Xylem - water conducting system. The principal conducting cells are called tracheary elements. They have rigid and persistent walls, and they are usually well preserved in the fossil record. There are two major types:

1. Tracheids - long, thin cells with imperforate end walls

a. Most primitive type of conducting cells and they are found in most of the seedless vascular plants and gymnosperms

2. Vessels (vessel members) - shorter, wider cells with perforate end walls

a. Vessels members are strung end to end to make continuous "tubes" for conducting water throughout the plant

b. They are the principle water conducting cells in the flowering plants and a few other groups

II. Arrangement of  primary vascular tissues:

A. Stele = the arrangement of the primary vascular tissues and the pith, if present. There are three types of steles:

1. Protostele - consists of a solid strand of vascular tissue in which the phloem either surrounds the xylem or is interspersed within it

a. Most primitive type and it is found in extinct seedless vascular plants as well as the Psilotophyta, Lycophyta and the roots of most extant plants

2. Siphonostele - consists of a central column of ground tissue called the pith, which is surrounded by the vascular tissue. The phloem may form outside the cylinder of xylem or on both sides of it

a. Found mostly in ferns, the Pterophyta

3. Eustele - consists of a system of discrete vascular strands around a central pith

a. Eusteles are found in Sphenophyta, and both the gymnosperms and angiosperms

ORIGIN OF ROOTS

I. Roots are relatively simple structures and they probably are just derived from subterranean stems

A. The roots of nearly all extant plants are protosteles, indicating how little they have changed during the course of evolution

ORIGIN OF LEAVES

I. Two very different kinds of leaves occur in the vascular plants, suggesting there were probably two different ways they evolved

A. Microphylls

1. Have only one vascular strand and they appear to have originated as outgrowths of the stem

2. Found in groups that have protosteles

3. They were probably first just scale like growths in which leaf traces developed

B. Megaphylls

1. Have more than one vascular trace and they appear be derived from webbing and fusion of several branches

2. They are associated with siphonosteles and eusteles

VASCULAR PLANT REPRODUCTION

I. All vascular plants are oogamous and they have alternation of generations in which most of the gametophytes are reduced and nutritionally dependent upon the dominant sporophyte

A. Alternation of generations =  a reproductive cycle in which a haploid organism (or tissue) gives rise to a diploid organism (or tissue) which later undergoes meiosis, the products of which later grow into a haploid organism (or tissue)

1. The haploid, gamete-producing phase is called the gametophyte

2. The diploid spore-producing phase is called the sporophyte

II. The earliest vascular plants produced one kind of spore so they are called homosporous

A. Following meiosis the spore germinates into a bisexual gametophyte which gives rise to antheridia and archegonia, which produce sperm and egg, respectively

B. Homospory is common in most of the extinct, primitive vascular plants as well as the Psilotophyta, Sphenophyta, some Lycophyta, and most ferns, the Pterophyta

III. More advanced (recent) plants are heterosporous

 A. Plants produce two types of spores in two different kinds of sporangia

B. The spores are called microspores (male) and megaspores (female), and they are produced in microsporangia and megasporangia

C. Microspores give rise to microgametophytes (male) and megaspores give rise to megagametophytes (female)

D. Heterospory occurs in some Lycophyta, a few ferns and all seed plants

E. Heterospory evolved by at least by 370 million years ago  

You should know the heterosporous life cycle.

[ Home | Lecture Notes | Text | Syllabus ]