Plant Primer

You might be tempted to think that the presence of green pigments, called chlorophylls, defines the plant kingdom. Most plants do have these chemicals to use in the food production process called photosynthesis. But some don’t.

A low-lying woodland plant called Indian Pipe, for example, is a pale ivory color for lack of chlorophyll. Its roots are saprophytic, meaning they secrete enzymes to dissolve the organic material around them for absorption by the plant. This is how it feeds.

Another way plants are set apart from the other kingdoms is the way they reproduce. Unlike the Monera and Protoctista kingdoms, which reproduce through simple cell division, plants have special tissues devoted to reproduction. Animals do, too. Both plants and animals reproduce with others of their kind by releasing sex cells, which combine to form an embryo—the earliest stage of the offspring’s life.

Understanding the plant life cycle teaches the how’s and the why’s of many wonderful aspects of plant life, like why they flower, why they release pollen spores, and how seeds develop.

Reproduction Recap
When an animal is ready to reproduce, its reproductive cells go through two cell divisions. The first (mitosis) results in two cells, each having a complete set of genetic information (called diploid cells). The second division (meiosis) doubles the number of cells, but leaves each with a half-set of genetic information (called haploid cells). These are sex cells, also called gametes.

Each animal has reproductive cells that go through these two divisions in order to produce sex cells, which can combine with the sex cells of another animal to produce a new generation.

An embryo is formed when a haploid male cell and a haploid female cell from the same species unite to combine their genetic information into a diploid cell.

The same process is at work in plants with this exception—the two cells divisions occur in alternativing generations. This is what makes plants different from all the other organisms in the other kingdoms. Plants have a diploid generation, which produces a haploid generation. The haploid generation then produces the diploid generation.

It works like this:
The Sporophyte
Let’s start with the embryo. It is the diploid generation, since it resulted from the combination two haploid cells. It is also called the sporophyte, since it releases spores, which will grow into an alternate generation.

The Gametophyte
The alternate generation grows out of the spores. It has the name gametophyte, since it is this generation that will release the male and female gametes (sex cells).

So, these generations alternate:

• The diploid sporophyte generation releases the spores
• The haploid gametophyte generation releases the gametes

The cycle of these generations takes on varied forms among major groups of plants.

In mosses, the gametophyte generation dominates. This means the gametophyte generation is the most obvious, and you hardly notice the sporophyte.

The gametophyte is represented by male and female plants, each bearing a reproductive organ that houses sperm or eggs. Raindrops disperse the sperm onto the eggs of nearby female plants. Fertilization results in a sporophyte, which grows from the female plant and is relatively small. When mature it releases spores, which germinate into either female or male plants.

The emphasis is different in seed-producing plants. The sporophyte generation dominates. The flower of a fruit bearing plant is the sporophyte. You hardly notice the gametophyte. It is reduced to teeny, tiny structures within a blooming flower.

Flowers produce:

• Microspores, which turn into pollen grains housing the male gametophytes. In the rosebud image above, these are housed in the stamens growing up in the center of the flower blossom.

• Macrospores that produce ovules housing the female gametophytes. In the rosebud above, the ovules are are When these gametes get together, an embryo is the result.

You can see how small and brief the gametophyte generation is. It amounts to a series of cell divisions, which prepare the gametes to fuse with one another. They create the embryo of the next sporophyte generation. The sporophyte is also called dominant because the gametophyte totally depends on it for its production and survival.

Some plants are also able to reproduce by sending out extensions. These plants are independent clones of the parent plant. Eelgrass, cordgrass and saltmarsh hay are some plants that multiply through this vegetative reproduction. This gives them an advantage over solitary plants producing only by seed. Vegetatively reproduced clones can extend into harsher conditions and draw nutrients and moisture from the parent plant to get them established.

Roots, Stems & Leaves?
Plants are divided into two major groups: vascular or nonvascular. These groups share certain reproductive strategies.

Vascular plants have roots, stems and leaves for circulating water, nutrients, and food made by photosynthesis. A series of tubule tissues in the roots and stems, called the xylem, carries water and nutrients from the ground out to the leaves. The release of moisture through pores in the leaves maintains the pressure necessary to draw more molecules up through the xylem. Other tubule tissues, called the phloem, convey the carbohydrates produced through photosynthesis in the leaves back down through the plant. The vascular tissues also provide support for these plants, allowing them to grow up from ground.

A waxy layer of cutin on the upper surface of their leaves prevents dehydration and water logging.

Nonvascular plants—mosses, liverworts and hornworts—have no roots, stems or leaves. They absorb nutrients and moisture directly into their tissues, for example through hair-like cells in the ground. Without the support of vascular tissues, nonvascular plants are unable to rise far above the ground and, instead, have a compact, spreading form. They also lack the waxy layer of cutin on their exterior.

These are found away from the salt water, in upland areas surrounding the intertidal habitats of the estuary. They require wet habitat for reproduction since their flagellated sperm must swim to an egg to fertilize it.

The description of the moss life cycle above is an example of nonvascular reproduction.

Seeds?
The vascular plants are further divided into two groups: seed producing and nonseed-producing.

-Nonseed-producing plants require wet habitats since, like the nonvascular plants, their sperm must swim to an egg to fertilize it. After fertilization, the embryo benefits from the nurture of that moist environment. These plants can be found in woodsy uplands surrounding the estuary.

club moss
Lycophyta
Small evergreen plants, which look like but are unrelated to pine trees. Many species have clusters of spore-bearing leaves that form club-shaped structures at the end of their stems.

horsetails
Spheophyta
Small, hollow, jointed stems topped with a spore-bearing cone said to look like a horse’s tail. Scale-like leaves appear to hug the stem.

ferns
Pterophyta
Ferns have fronds, those large leaves divided into many leaflets. Some fronds may have on the underside of their leaves spore-producing clusters called sporangia. Other fronds are sterile, with no sporangia. Spores germinate into a very small, flat plant bodies without stems and leaves. Apt to go unnoticed, this gametophyte generation has reproductive structures, which release either eggs or sperm. Depending on the species, it may produce both male and female gametes.

-Seed-producing plants produce embryos with a protective covering and food reserve to support the new plant. One of the benefits of the seed is that it is more easily dispersed than embryos of other plant groups. Seeds contained in fruit, for instance, may drop and roll away from the plant, or be eaten by an animal and deposited in feces far from the parent plant.

The male gametes of seed-producers are not outfitted with flagella for swimming to the female gametes in wet environments. More suitable to a dry environment, they are released in the form of immotile pollen, which depend on being blown by the air or carried by insects to reach female gametes. If the pollen grain comes into contact with the female reproductive structure, it develops a pollen tube that reaches into an ovum to deposit its sperm near the female gamete.

The flowering sporophyte generation dominates in all seed-bearing plants. The spores (pollen grains/ovules) produce the gametes (sperm/eggs). When the sperm fertilizes the egg, the ovule develops into a seed, which grows into the next flowering sporophyte. The embryo has one or more seed leaves, or cotyledons, which help feed it before and after the seed germinates.

Gymnosperms
(Greek: “gymnos” naked, “spemas” seeds)
Conifers [and other groups]
The ovule and seeds of the gymnosperms are called naked because they grow within the exposed scales of conifer cones. Each scale contains a spore-producing structure called sporangia. The spores go through a series of cell divisions that result in either male or female gametes.

Take pine trees as an example. They have both male and female pinecones. The smaller male cones have sporangia to produce microspores, in the form of pollen grains. When the sporangia burst in spring or early summer millions of pollen grains are let loose to fly in the wind. This is the yellow dust that coats car tops and driveways in the vicinity of pines.

The larger female pinecone is a complex of tough scales containing the megaspores. Through a series of cell divisions each megaspore becomes the female gametophyte. If a grain of pollen happens to land in the right place on the scale, a sugary liquid secreted by the ovary draws it through a tiny opening in the ovule. It forms a pollen tube and releases sperm near the ovule. Eventually the sperm and egg fuse. The ovule of the parent plant provides food for the embryo. When the seeds mature, they separate from the cone.

Angiosperms
(Greek: “angeion” receptacle, “sperma” seed)
Flowering Plants
Only angiosperms have flowers to house their reproductive structures and form protective fruits around their seeds. This largest group of plants is unique in its ability to create a food source called endosperm, which nourishes the growing embryo. Some of the starches, proteins and fats in our diet come from the endosperm of the grains we eat.

Angiosperms in the estuary include the eelgrass beneath the tides, and the fleshy herbs, succulents, grasses, sedges and rushes of the marshes.

There are two divisions of angiosperms, based on the number of their cotyledons, as well as other unique characteristics you’ll recognize.

The monocots (like saltmarsh cordgrass, eelgrass, and narrow-leaf cattail) have one seed leaf, or cotyledon. It transfers food from the endosperm to the developing embryo. Monocots also have parallel veins, flower parts in multiples of three, vascular tissue throughout the stem, and a fibrous root system.

The dicots (like bayberry, roses, and poison ivy) have two cotyledons. In some dicots, the endosperm disappears after being absorbed by the cotyledons (e.g. rose hip seeds). Dicots all have a network of veins in their leaves, flower parts in multiples of four or five, tubular vascular tissue in stem, and a taproot system.

Again, the sporophyte generation dominates: flowers produce spores, which develop into gametes. These sex cells merge to form the next flowering generation. Male and female reproductive parts of angiosperms are present either in the same flower, or in separate flowers on the same plant, or in flowers of separate plants.

Let’s use the Rugosa rose to describe flowering and fruit bearing. The lovely dark pink flowers and bright red fruits of this bush are a common sight around the estuary and along the outer coast since the bush is able to withstand salty sprays.

Both female and male organs are present in each flower. Notice in the picture at left how the Rugosa petals surround thin yellow stalks. These are male organs called stamens. On their tips are anthers filled with the pollen-producing sporangia. At their base is button-shaped patch of yellow fuzz, made up of the tops of the many female organs. The top of the stem holding the flower has opened up into an urn-shaped receptacle to hold these reproductive organs.

The female organs are called carpels, or pistils. Each has a stigma on top (poking out the top of the receptacle), to receive the pollen. At the base of the stigma is a tubule filament called the stile, which connects to an ovary. Within the ovary is an egg-bearing ovule. The styles and ovaries are within the receptacle.

Though plenty of pollen is available in the anthers hanging directly above, self-fertilization is decreased because the stamens lean away from the center. So the stamens are more apt to be cross-pollinated by pollen from other rugosas. The wind may blow the pollen over. A bird may carry it. Pollen-feeding insects also provide reliable transport. They are drawn to the colors and sweet fragrance of the flowers. When they land on the stigmas to feed, they drop pollen grains from the flower of another bush.

When a pollen grain lands on a stigma, it is stimulated to grow a tube down through the style to the ovary, where it deposits two male gametes. One sperm fuses with the egg. The other fuses with a second cell within the ovule to become the endosperm, or nutritional material for feeding the embryo. This is called double fertilization, and it is what makes angiosperms unique. The egg develops into the plant embryo and feeds on the reproducing endosperm. The outer layer of the ovule becomes the seed’s coating. Meanwhile, the wall of the ovary, called the pericarp, grows and ripens.

Angiosperm fruits have two classifications based on the texture of the ovary wall: fleshy fruit (blueberries, tomatoes, oranges, etc), or dry fruit (grains, nuts, etc.). In fleshy fruit, the pericarp grows succulent and is soft throughout. In dry fruits, the pericarp dehydrates, either whole or in part.

The flowering plants of Great Bay Estuary (excluding upland habitats) produce dry fruits: the grasses produce grain, the sedges produce nutlets, and even the bayberry is a kind of a dry fruit called a drupe. As with all drupes (cherries, peaches and plums), part of a bayberry’s pericarp dehydrates and hardens, while the outside portion becomes fleshy and succulent.

Once fertilized, each ovary of the Rugosa becomes an embryo that grows into an achene (pronounced akeen). An achene is actually a small fruit made up of a single, hard seed within (but separated from) its dry pericarp. A rose hip is actually the receptacle that has grown up from the stem into a fleshy pod holding these achene fruits. Eventually the stamens and stigmas wither and the petals fall off, leaving a bright red rose hip.

Copyright © 2006 Barbara Driscoll.