Sexual Reproduction

Sexual reproduction in mosses begins with the formation of multicellular gametangia, usually at the apices of the "leafy" shoots of gametophytes (see Fig. 20.13), although they frequently form on special separate branches. Both male and female gametangia are often produced on the same plant, but in some species, they occur on separate plants. The archegonia (female gametangia) are somewhat cylindrical and project upward from the base of the expanded gametophyte tip (Fig. 20.11). When certain cells break down in the swollen base of the archegonium (called the venter), a cavity develops in which a single egg cell is produced. The part of the archego-nium above the venter is called the neck. The neck may taper toward the tip and contains a narrow canal. The canal is at first plugged with cells, but these break down as the archegonium matures, leaving an opening to the outside at the top. Several archegonia usually are produced at the same time, with sterile hairlike, multicellular filaments called paraphyses (singular: paraphysis) scattered among them.

Chapter 20

thickenings pore chlorophyll-bearing living cells

Chapter 20

thickenings pore chlorophyll-bearing living cells

Polytrichum Commune
pore

Figure 20.10 An enlargement of a portion of a peat moss (Sphagnum) leaf. A. Surface view. B. A further-enlarged cross section of living and dead cells.

Figure 20.10 An enlargement of a portion of a peat moss (Sphagnum) leaf. A. Surface view. B. A further-enlarged cross section of living and dead cells.

Moss Antheridia Neck Stalk Cells

Male gametangia also have paraphyses among them and are sausage shaped to roundish, with walls that are one cell thick. These antheridia (Fig. 20.12) are borne on short stalks. A mass of tissue inside each antheridium develops into numerous coiled or comma-shaped sperm cells. This mass of sperms is forced out of the top of the antheridium when it absorbs water and swells. After release, the sperm mass breaks up into individual cells, each with a pair of flagella. It is believed the breakup of the sperm mass is aided, in some cases, by fats produced by the moss, while in other instances, rain splash is responsible.

Archegonia release sugars, proteins, acids, or other substances that attract the sperm, and eventually a sperm, after swimming down the neck of an archegonium, unites with the egg, forming a diploid zygote (Fig. 20.13). The zygote usually grows rapidly into a spindle-shaped embryo. The embryo breaks down the cells at the base of the archegonium and becomes firmly established in the tissues of the stem by means of a swollen knob called a foot. As the embryo grows, cells around the venter divide, thereby accommodating its increasing size. The length of the embryo soon exceeds the length of the cavity in the venter. The top of the venter is split off and is left sitting like a pixie cap on top of the embryo. By this time, the embryo is a developing sporophyte. The pixie cap, called a calyptra, remains until the sporophyte is mature. In one genus with the common name of "extinguisher mosses," the calyptra looks just like a little candlesnuffer, and in the hairy cap mosses, it resembles a miniature, pointed, goatskin cap such as might be worn by a Shakespearean actor.

The cells of the sporophyte become photosynthetic as it develops, remaining so until maturity. The sporophyte, however, depends to varying degrees on the gametophyte for some of its carbohydrate needs as well as for at least a part of its water and minerals, which are absorbed through the foot.

Introduction to the Plant Kingd om: Bryophytes 391

^sporangium with sporocytes operculum peristome

^sporangium with sporocytes operculum peristome

Water Transport

seta antheridium rhizoids-

antheridium seta rhizoids-

Figure 20.13 Life cycle of a r

The mature sporophyte is at first green and photosyn-thetic; it consists of a capsule located at the tip of a slender stalk called the seta. Depending on the species, the seta may be less than 1 millimeter (0.04 inch) long, or it may be up to 15 centimeters (6 inches) long. Most, however, are less than 5 centimeters (2 inches) long. The capsule may resemble a tiny apple, a pear, an urn, a box, or a wingtip fuel tank of an airplane and usually has from 3 or 4 to over 200 stomata at or near its surface. Unless extremely dry conditions prevail, the stomata normally remain open until the capsule begins to age, and then they close permanently. The free end of the capsule is usually protected by a little rimmed lid, the operculum, which falls off at maturity.

As the capsule matures, sporocytes inside it undergo meiosis, producing haploid spores. These spores, often numbering in the millions, are released from the capsule, usually through a structure called a peristome, after the operculum falls off. Most peristomes consist of a circular row or two of narrowly triangular and membranous teeth arranged around the rim of the capsule, each row having 16 teeth. The teeth are frequently colored orange or red and are often beautifully sculptured with bars and fringes. They open or close in response to changes in humidity. In a few species of mosses, the peristome is a cone-shaped structure with pores through which the spores are released.

Some rock mosses have neither a peristome nor an oper-culum. The spores in these mosses are released when the capsule splits lengthwise along four lines. In the dung mosses, a putrid odor is given off when the spores are ready for release. Some of the spores adhere to the legs and bodies of flies, which are attracted by the odor, and are disseminated as the insects clean themselves. Most moss spores are, however, simply blown away by the wind, and if they fall in a suitable damp location, they usually germinate relatively quickly.

Sexual Reproduction Flowering Plants

w arenes s

Hibernating Mosses

Mosses are the "amphibians" of the plant world, so-called because they are at home in either semiaquatic environments, such as moist stream banks, or drier habitats, such as rock surfaces or the arctic tundra. Unlike their towering cousins, the flowering plants, mosses are dependent on a watery landscape to reproduce. Because moss sperm are flagellated, a film of water is needed for the sperm to swim to the egg. But life on land is not easy, especially when water becomes limited or nonexistent for months. The genetic diversity of mosses is exceptional, and they inhabit some of the most inhospitable habitats on earth. Dry heaths, rock faces, tree trunks, and even deserts are home to these remarkable plants. Distinctive adaptations allow moss species this wide range of habitats, including the ability to tolerate drying out, a process known as desiccation.

Vascular plants have structural mechanisms that maintain an adequate water supply within plant tissues. These include an impermeable waxy cuticle on leaf surfaces, an internal water transport system (xylem), water-absorbing organs (roots), and leaf pores (stomata) that can close, conserving water. Parts of the life cycle, such as seeds, are especially tolerant of desiccation. Mosses do not possess these adaptations to living in a dry land environment. This means the internal water balance in mosses is in equilibrium with the atmosphere. When the air is dry, mosses are dry. When it rains, mosses quickly absorb water and become rehydrated. They are "opportunists" in this regard.

Some species of mosses are incapable of withstanding this desiccation-rehydration cycle, but those that have this ability can "return from the dead" with each cycle. It is as if they have been hibernating. There are several mechanisms that slow water loss from mosses that make the cycle less drastic. Many mosses form dense mats or tufts that create a moist microatmosphere within the tufts and over the plants. Polytrichum commune is a common moss that has moderate desiccation tolerance. It lives in habitats such as bogs or temperate, moist forests and can grow up to 40 centimeters tall. It grows luxuriantly during rainy periods but then twists into rusty red mats upon drying in the sun. These mosses have a thin, waxy cuticle covering their tiny "leaves" (8 to 10 millimeters in length) that retards water loss. They also have primitive water-conducting cells (hydroids) that move water through the plant when water is lost to the atmosphere by evaporation. Instead of root hairs, they have a rhizoid system that absorbs water from the soil. However, if dry conditions persist, all available water is lost to the atmosphere.

Additional mechanisms by which mosses are able to survive these desiccation-rehydration cycles are being discov-

Box Figure 20.1 Polytrichum commune (hairy cap moss) growing in a deciduous forest of the northeastern United States. (Photo by Daniel Scheirer)

ered. Tortula ruralis is one of the most desiccation-tolerant mosses known, and research has centered on its ability to recover after prolonged or repeated desiccation events. Hydrated T. ruralis cells are similar to mesophyll cells in the leaves of vascular plants. There are chloroplasts with stacks of grana and a prominent nucleus. Mitochondria are numerous and similar in size and shape to those of higher plants, with internal membranes folded into cristae.

During desiccation, T. ruralis dries out, and its "leaves" fold up around the stem. Cells of desiccated plants are damaged. There is extensive plasmolysis as water is lost from the protoplast. Chloroplasts become smaller and more spherical, and starch is not present. Internal thylakoid membranes are collapsed and disorganized. Mitochondria in the hydrated cell are elongated with numerous cristae membranes. In the desiccated state, mitochondria are smaller and rounded, with few internal cristae membranes. The nucleoplasm of nuclei becomes dense, and chromatin is condensed. Compact-appearing nucleoli are prominent. The plasma membrane is damaged, and electrolytes leak out. In all desiccated mosses, photosynthesis stops, and respiration slows or ceases.

When the plants are rehydrated, dried "leaves" unfold and return to the normal hydrated state within 2 minutes. Most of the internal damage is repaired within a few more minutes. Activity of a drought-repair gene increases in the minutes after rehydration, and repair proteins are quickly manufactured and mobilized for action. Respiration resumes after a few minutes. Photosynthesis resumes up to 24 hours later.

Introduction to the Plant Kingd om: Bryophytes

Awareness Box Continued

Scientists at the USDA's Agricultural Research Service are attempting to locate the genes responsible for drought repair in Tortula species in the hope of transferring them to crop plants. With crops having more drought tolerance, rain shortage would be less of a problem than it is now. Arid lands currently unsuited for agriculture could blossom with crop plants genetically engineered with drought-tolerant genes. World population increases of 1.6% yearly means that an additional 78,000 metric tons of grain per day are required just to maintain current consumption levels. A lowly moss might someday be responsible for a revolution in food production.

D.C. Scheirer

In most mosses, fine, green tubular threads, consisting of single rows of cells with chloroplasts, first emerge from the spores. These soon branch and grow, forming an algalike protonema. The protonema can be distinguished from a filamentous green alga by the oblique crosswalls of its cells and by the lens-shaped chloroplasts. If light and other conditions are favorable, tiny "leafy" buds appear at intervals along the protonemal filaments after about 2 to 4 weeks of growth. These "leafy" buds develop rhizoids at the base and grow into new "leafy" gametophytes, completing the cycle.

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    What are the egg cells in the cavity of a flower called?
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