Relatives Objectives

After studying this chapter, the reader should be able to:

• Explain the difference between nonseed and seed vascular plants.

• Name the major divisions of nonseed vascular plants.

• Describe the life cycles of whisk ferns, club mosses, horsetails, and ferns.

• List the uses of nonseed vascular plants.

I. Vascular Plants

A. General information

1. Vascular plants contain conducting tissue consisting of xylem and phloem

a. Vascular tissue allows plants to inhabit drier habitats

b. The ability to conduct water through the plant body permits some vascular

plants to grow extremely large (such as giant redwood trees)

2. Unlike the bryophytes, vascular plants possess true roots, stems, or leaves

3. Many vascular plants can simultaneously control water loss and permit gas ex-

change because of a waxy outer covering (cuticle) and small porelike openings on the leaf (stomata)

4. The diploid sporophyte generation is the predominant generation in vascular

plants; the gametophyte is small, short-lived, and often inconspicuous

B. Taxonomic classification

1. Vascular plants are classified according to the type of propagule produced

2. Nonseed vascular plants reproduce using spores; this group includes ferns and

their relatives

3. Seed vascular plants reproduce using seeds; this group includes gymnosperms

and angiosperms

II. Nonseed Vascular Plants

A. General information

1. Nonseed vascular plants are categorized according to the type of spores that they produce

2. Homosporous plants produce only one kind of spore in a single sporangium

a. The spore undergoes cell division to produce bisexual gametophytes that

have both antheridia and archegonia

b. The gametophytes develop outside the spore wall

c. Homospory is characteristic of whisk ferns, horsetails, some club mosses,

and most ferns

3. Heterosporous plants produce two types of spores in two different sporangia

a. Heterospory is characteristic of some club mosses, a few ferns, and all seed

plants

b. The two types of spores, called microspores and megaspores, are much

larger than those produced by homosporous vascular plants

(1) Microspores, which may or may not be smaller than megaspores, give

rise to male gametophytes

(2) Megaspores give rise to female gametophytes

c. The free-living, nutritionally independent gametophytes develop within the

spore wall

d. The gametophytes require water for fertilization so that the flagellated sperm

can reach the egg

B. Taxonomic groups

1. The four divisions of nonseed vascular plants are Psilophyta, Lycophyta, Spheno-

phyta, and Pterophyta

2. With the exception of Pterophyta (ferns), most of these plants are uncommon, un-

familiar species

III. Division Psilophyta (Whisk Ferns)

A. General information

1. Psilophyta are tropical and subtropical plants that consist of only one existent

family, Psilotaceae, with just two genera: Psilotum and Tmesipteris

2. In the United States, whisk ferns grow naturally in Florida, Louisiana, Texas, Ari-

zona, and Hawaii

3. They have no economic importance, although florists occasionally use them in ar-

rangements

B. Structure

1. Whisk ferns do not have true leaves or roots; photosynthesis occurs in the outer

epidermal cells of the stem

2. The dichotomously forking stems develop from rhizomes that grow horizontally

just under the soil surface

3. The stem and branches contain a central core of vascular tissue, with phloem sur-

rounding a star-shaped core of xylem

4. Surrounding the stele is an area of cortex comprised of parenchyma and scler-

enchyma tissue

5. These homosporous plants produce spores within sporangia, which are clustered

on the stem and branches

C. Reproduction

1. The life cycle of the whisk fern reflects the alternation of generations found in all

plants

2. Spores released from the sporangia of the sporophyte germinate and grow to

form a gametophyte within the soil, on the bark of trees, or on other surfaces

a. The colorless, saprophytic gametophyte lacks chlorophyll and obtains nutri-

ents by association with fungi

b. The gametophyte is approximately 2 mm in diameter and 6 mm long

3. Archegonia and antheridia are produced randomly on the surface of the gameto-

phyte

4. Fertilization requires water so that the flagellated sperm can swim to the archego-

nia

5. The fertilized egg, now a diploid zygote, develops into an embryo within the arche-

gonium

6. As the embryo continues to grow, it forms a foot and a shoot apex

a. The foot permits the growing sporophyte to obtain nourishment from the

gametophyte

b. The shoot apex undergoes repeated cell division to form the mature sporo-

phyte

7. The sporophyte ultimately detaches from its foot and lives independently of the

gametophyte

a. The sporophyte consists of an underground rhizome, which performs the

same functions as a root, and a series of upright aerial branches that arise from the rhizome

b. As the rhizome grows, it becomes infected with mycorrhizae, which help the

rhizome anchor the plant and absorb water and nutrients from the soil

8. Sporangia develop in clusters on the upright, dichotomously forked branches of

the sporophyte

9. Spores are released from the sporangia, and the cycle is repeated