You Gotta Know These Distinctions Among Types of Plants
Bryophytes vs. pterophytes: Not all plants produce seeds. Seedless plants are divided into bryophytes (mosses, liverworts, and hornworts) and pterophytes (ferns, club mosses, quillworts, and horsetails). Both of these groups, like all other plants, reproduce by producing sperm and eggs on a structure called the gametophyte. The gametes fuse to form another structure called the sporophyte, which produces spores that disperse and grow into new gametophytes. Both groups produce flagellated sperm that require water for fertilization. Note that these uses of “bryophyte” and “pterophyte” here are informal, and should not be confused with the actual phyla Bryophyta (true mosses) and Pteridophyta (true ferns).
Bryophytes are small enough that water and nutrients can diffuse to all parts of the plant without any specialized vascular tissue. They lack true leaves and roots, instead fastening themselves to the ground with rhizoids. Unlike other land plants, bryophytes have a prominent gametophyte stage that is usually dioicous, meaning that an individual plant produces only one type of gamete (either sperm or egg). The short-lived sporophyte grows from the female gametophyte.
The more complex pterophytes can grow taller thanks to vascular tissues (see number 4 below) that provide structural support and transport water and other materials throughout the plant. Many of them do have true leaves and roots. Pterophytes have a prominent sporophyte stage that grows from a small, short-lived gametophyte. Pterophyte gametophytes may be dioicous or monoicous, producing both sperm and egg on the same plant.
Angiosperms vs. gymnosperms: Seed-producing plants can be divided into gymnosperms (cycads, ginkgos, conifers, and gnetophytes) and angiosperms (phylum Anthophyta, or flowering plants). Most of these plants produce male gametophytes that grow into the female, allowing fertilization to take place in relatively dry conditions. Many of them also exhibit secondary growth of woody tissues, allowing them to grow even taller than the pterophytes.
The word gymnosperm means “naked seed,” referring to the fact that their gametophytes develop on the surface of leaves or on the scales of cones. In contrast, angiosperm means “receptacle seed.” Their gametophytes develop enclosed within flowers. Angiosperms are further classified based on their seed structure, described below.
Monocots vs. dicots: Most (but not all) angiosperms fall into one of two classes based on the number of cotyledons, or embryonic seed-leaves, in the plant embryo. Monocots, or Monocotyledonae, have one cotyledon, while dicots, or Dicotyledonae, have two. While there are no other hard-and-fast distinguishing characteristics between the two groups, plants in each category tend to share other characteristics:
Monocots produce pollen grains that have a single furrow (monosulcate); flower parts in multiples of three; numerous, fibrous roots; parallel leaf veins; and stems with scattered vascular bundles. They also lack secondary growth, remaining herbaceous throughout their lives. Dicots, on the other hand, tend to have pollen with three furrows (tricolpate); flower parts in multiples of four or greater; taproot systems; stems with rings of vascular tissue; and branching leaf veins. Many of them exhibit secondary growth that produces wood.
Xylem vs. phloem: There are two types of vascular tissue in plants. Xylem transports water and soluble nutrients from the roots to the leaves. Phloem, on the other hand, carries nutrients like sucrose from their origin of synthesis or absorption to all parts of the plant. Both tissues originate in the procambium of the apical meristems of both the stems and roots. In woody plants, secondary vascular tissues arise in the vascular cambium.
Xylem contains distinct elongated cells called tracheids that have lignified cell walls and help provide structural support. Vessel elements are also reinforced by lignin, but they are open at each end at perforation plates and connect to form long tubes for water transport. Xylem functions via transpirational pull and osmosis. Cell types in phloem include companion cells, fibers, and sclereids. In trees, it is usually the innermost layer of the bark.
- Flower parts:
- The calyx is composed of sepals, specialized green leaves that protect the flower as a bud and provide support for the fully bloomed flower
- The stem supporting a flower is called the peduncle. If multiple flowers bloom from a peduncle, the stems supporting each flower are called pedicels. The torus is the swelling at the top of the pedicel or peduncle, just below the calyx.
- Petals are specialized leaves, often brightly colored to attract pollinating species. Collectively, they are called a corolla.
- The pollen-producing reproductive organ of the flower is called the stamen. The stamen consists of a thin filament topped by an anther, which actually contains the pollen.
- The pistil, or female reproductive organ of the flower, is composed of leaf-like carpels. The ovary-containing ovules are at the base of the pistil, while a tube called a style topped by a sticky, pollen-receptive stigma rises from the ovary. There may be one or many pistils in each flower.
This article was contributed by NAQT editors Hannah Kirsch and Farrah Bilimoria.