Plants

Some General Characteristics

Plants, which are defined here to include vascular plants and bryophytes, share the following characteristics:

• They are multicellular organisms composed of eukaryotic cells
• They are photoautotrophs and/or evolved from an ancestor that was a photoautotroph. Most plants make their own food through the process of photosynthesis, converting carbon dioxide into an organic, bioavailable form of carbon.
• Their cells have cell walls that contain cellulose
• Other characteristic components of plant cells include vacuoles and plastids (including chloroplasts, where photosynthesis takes place).
• Plants don’t actively move from one location to another; however, some plants (i.e., bryophytes, seedless vascular plants, cycads, and Ginkgo (Ginkgo biloba) produce sperm cells that are motile and swim by means of flagella.
• Their life cycle is characterized by an alternation of generations and includes an embryonic stage.

Life cycle

All plants have two alternating multicellular stages or “generations”: a sporophyte stage, which produces spores and a gametophyte stage, which produces gametes. The suffix -phyte is derived from the Greek word for plant, so the terms sporophyte and gametophyte literally mean "spore plant" and "gamete plant".

The cells of the sporophyte have a chromosome count of 2n, or two times the number of chromosomes found in a gamete. The sporophyte contains specialized cells that undergo meiosis to produce single-celled spores, which have a chromosome count of n (same as that found in a gamete). Each spore undergoes regular cell division (mitosis) to form a multicellular gametophyte.

The cells making up the gametophyte all have a chromosome count of n. Special cells in the gametophyte undergo cell division (mitosis) to produce single-celled gametes (which also have a chromosome count of n).

Gametes of the two different types (sperm and egg) come together and fuse, forming a zygote (with 2n number of chromosomes). The zygote undergoes cell division and differentiation, developing into a sporophyte. During at least its early/embryonic stage of development, the sporophyte develops within and is dependent on the gametophyte. (Because of this, the vascular plants and bryophytes are sometimes referred to as embryophytes.)

This alternation of generations life cycle varies in different groups of plants. In bryophytes, the gametophyte is the conspicuous and dominant life stage and the sporophyte is permanently attached to the gametophyte. In the vascular plants, the sporophyte is the conspicuous and dominant life stage.

Groups of Plants

Vascular vs. Non-vascular Plants

Most plants alive today are vascular plants (tracheophytes). Vascular plants have special conducting tissues known as xylem (through which water and minerals pass) and phloem (through which food products of photosynthesis are transported). Xylem and phloem allow plants to efficiently move water, minerals and food from one part of the plant to another. A polymer known as lignin is present in xylem, which provides rigidity and strength.

The bryophytes (mosses, liverworts, and mosses) are considered to be non-vascular plants – they lack xylem and phloem (although some bryophytes contain vascular tissues with similar functions). Bryophytes also lack true roots, and their conductive tissue — if present at all — lacks lignin. Lacking strong structural support, and, in most cases, an efficient vascular system, bryophytes don’t grow very tall. Most bryophytes reach heights of 2-5cm (Shofield 2010).

Seed-forming vs. Seedless Plants

The vascular plants may be subdivided based on whether or not they produce seeds. Seeds are an important adaptation in the plant world. They enclose, protect and provide stored food for embryonic plants, enhancing their chances for survival. Seeds may allow plants to exist in a dormant state (in some cases for 50 years or more) until conditions become suitable for the plant to grow. In addition to these advantages, the seed plants do not require water for fertilization to occur (unlike the seedless vascular plants and the bryophytes).

Another difference between the seed-forming plants and the seedless plants is that they undergo dispersal at different stages in their life cycle. The seed-forming plants disperse into new locations at the seed/embryo stage, while the seedless plants spread into new areas at the spore stage. Seedless plants, in the sporophyte stage, release spores, which, upon encountering favorable conditions, germinate into new, independent, gametophyte plants. In contrast, in the seed plants, the spores never leave the mother plant (i.e., they remain within the sporophyte). The male and female spores undergo division within the tissues of the sporophyte to form male and female gametophytes. The immature male gametophytes are released from the sporophyte as pollen grains, which land on the reproductive part of a plant (near where the female gametophyte resides). A pollen grain germinates and becomes a mature gametophyte, forming a tube that extends towards the egg. The sperm carried by the pollen tube fuses with the egg, forming the embryo.

The seed plants consist of two groups: the flowering plants (angiosperms) and the gymnosperms. In the angiosperms the seeds are enclosed by an ovary (with the ovary and its seeds constituting a fruit) whereas in the gymnosperms there are no fruits or flowers and the seeds are “naked” — they are unenclosed and usually found on the scales of a cone.

In all angiosperms and almost all gymnosperms (with the exception of the cycads and Ginkgo, described below), the sperm are non-motile — they are transported to the egg by the growing pollen tube. This is in contrast to the sperm of the bryophytes and seedless vascular plants, which need to actively move (swim through water) to travel from the male structure on the gametophyte (antheridium) to the female structure (archegonium), passing through the outside environment in the process.

Groups of Gymnosperms

The gymnosperms include the cycads, the conifers, the gnetophytes, and the Ginkgo. Of these, only the conifers occur naturally in our area. The Ginkgo, which is native to China, is commonly planted in New England as a street or landscape tree. Cycads, which are found in tropical and subtropical regions, and gnetophytes, which occur in tropical forests and in deserts, would be unlikely to survive in our climate.

The cycads and the Ginkgo are the only seed plants alive today that have motile sperm. Another unusual characteristic of these taxa is that the male gametophytes grow invasively into the tissue of the female plant. Like other seed plants, the cycads and Ginkgo release pollen (immature male gametophytes), some of which end up on the female plants and enter into the ovule (the female structure that develops into the seed). Once inside the ovule, the male gametophyte behaves like a parasitic plant — slowly growing highly branched pollen tubes, which suck nutrients and water from the female tissue. The pollen tubes don't carry the sperm all the way to the egg as they do in other seed plants. Instead when the tubes eventually rupture, they release the sperm into the watery interior of the ovule, where the sperm are then able to swim to the egg. Because the sperm only swim within the fluid inside the ovule, a wet outside environment is not required for fertilization to take place.

Groups of Angiosperms

The angiosperms are the largest and most diverse group of living plants. Traditionally the angiosperms were divided into two groups: the monocots (which typically have one cotyledon or embryonic seed leaf) and the dicots (which typically have two cotyledons). Unfortunately, the dicots do not appear to be a monophyletic group. So, more recently, the dicots have been split into groups that are more likely to be monophyletic. Most of the formerly labeled dicots are now placed into a group called the eudicots or tricolpates. The other formerly labeled dicots that occur in our region have been split into three groups: the coontails, the water lilies, and a clade called the magnoliids. The monocot group, which is probably monophyletic, has been retained.

Plants Found in New England