Biological Diversity Classified

Kingdom Plantae

Plants are eukaryotic and are mostly multi-cellular autotrophs, which possess chlorophyll and cell walls of cellulose. Note that this definition does not exclude the three groups of algae that contain multi-cellular forms. At least some of these algae are almost certainly not closely related to terrestrial plants, so it is fairly common now to include the algae with the protists. Since they look and function more like plants than protists, we will consider them as plants. You should also note that botanists traditionally use the category "division" where we are using "phylum" (but why use two names for the same category level?).

Phylum Rhodophyta

These are the most unusual algae, possessing chlorophylls a and d and simple, primitive chloroplasts, and never possessing flagella. The reddish color typical of these algae is due to an accessory pigment, phycoerythrin. Most species are marine and tropical. An example is: Polysiphonia This is a filamentous alga with distinctly different male (antheridia – A) and female (cystocarps – C) reproductive structures during the sexual phase of their lives. An asexual stage includes tetraspores (T). All reproductive structures occur with vegetative cells (V).
Phylum Phaeophyta Some of these are the largest algae (or protists, if you group the Phaeophyta there), reaching lengths of nearly 100 meters. They possess chlorophylls a and c, like some of the unicellular algae, and store energy as the carbohydrate laminarin (named for a genus of kelp). Their frequently large size has required that some kind of internal conduction of dissolved substances occur, although they are never truly vascular. Most species are marine and are found in cool temperate regions. Examples are: Fucus A common intertidal brown alga on rocky coasts in regions of cool water. The sexes are separate in this form which spends most of its life as a diploid gametophyte. Female conceptacles (F) are located on the blades of female plants; they contain archegonia (O), which produce eggs. Male conceptacles (M) are located on the blades of male plants; they contain antheridia (A), which produce sperm. These are cross sections. Laminaria Another common brown alga, but found attached in deeper waters offshore. This genus includes some of the larger species of kelps. Here we see the relatively simple structure of the leaf-like blade in cross section. It has numerous zoospores (Z) on its surface.

Phylum Chlorophyta This group of algae is mostly fresh-water, and is fairly diverse; it is the only group of algae that is not a dead-end, evolutionarily (i.e., it, alone, apparently gave rise to terrestrial plants). All members possess chlorophylls a and b, and store energy as starch; typically, they are not as large or as complex as many of the Phaeophyta. Examples include the following forms.
One evolutionary trend seen among the green algae is that of increasingly larger colonies of simple, flagella-bearing cells. Gonium is a little raft of typically 16 cells, and represents the smaller colony end of this coloniality series. Volvox is a huge colony of hundreds of cells. By means of coordinated flagellar action, it swims with a tumbling action. Volvox reproduces both asexually (note internally budded daughter colonies - D) and sexually (note macrogametes (L) and microgametes (S) inside some colonies; these stain darker).

Filamentous algae consist of chains of cells. An example is Spirogyra, named for its large, spiral-shaped chloroplast (C). This alga reproduces sexually by a type of conjugation during which haploid filaments align and adjacent cells (gametes – G) combine to form diploid zygotes (Z).

Chara This is a fairly simple green alga of fresh water, with distinct reproductive structures (R), attached to the vegetative (V), or non-reproductive, cells. Both antheridia (A) and oogonia (O) can be seen. Ulva This is a very simple green alga of sea coasts. It consists of a couple of layers of very simple photosynthetic cells, in both the haploid and diploid portions of its life cycle.

Phylum Bryophyta The bryophytes are now usually considered to be three separate phyla of non-vascular plants. For our purposes we are sticking with the old group name for all of them, since at our level of consideration, they are more alike than different. Compared to algae, bryophytes are terrestrial; compared to the vast majority of truly terrestrial, vascular plants, the bryophytes do not possess real vascular tissue, although some, like some Phaeophyta, do have some cells which inefficiently conduct water and dissolved materials. True mosses, liverworts and hornworts comprise the three recognizable groups of bryophytes. We will consider the mosses; some examples of moss structure include: moss capsule (l.s.) The spore capsule is the major portion of the sporophyte stage in the moss life cycle. It grows from a fertilized egg, on a stalk on top of the female moss gametophyte. A large sporangium (S) containing spores (P) surrounds the central core of the capsule. A cap on top opens to allow dispersal of the spores. moss antheridium (l.s.) The antheridium (M) is the reproductive structure of the male gametophyte stage of the moss life cycle. Antheridia, which produce many motile sperm, are located at the tip of the individual male plant. moss archegonium (l.s.) The archegonium (F) is the reproductive structure of the female gametophyte stage of the moss life cycle. Archegonia, each of which produces an egg (E), are located at the tip of the individual female plant.

Phylum Lycophyta The lycopods include the club mosses. Though vascular, modern club mosses are not terribly significant plants (in terms of either species diversity, abundance or size); during the coal-forming period, several hundred million years ago, however, there were tree-like lycopods. The dominant stage of the life cycle is the sporophyte, and the strobilus is a vaguely cone-like structure that contains the sporangia. The stems have vascular tissue, but are otherwise rather simply constructed; the leaves are not very well developed (microphylls). Examples of club moss structure are: Lycopodium stem (c.s.) The stem contains bundles of vascular tissue (xylem cells (X) are larger than the phloem cells (P)) surrounded mainly by pith-like parenchyma cells, and wrapped in epidermis. The structure resembles that of the ferns, though the arrangement of tissues is different. Lycopodium strobilus (l.s.) A strobilus is a sort of cone-like structure. Spores (E) are produced in the sporangia (G), the chambers attached to top of the fertile (modified) leaves arranged around the vascular core (C).

Phylum Pterophyta The pterophytes include the ferns, truly vascular plants with leaves of the most highly developed type (megaphylls). The fern life cycle, with its non-vascular gametophyte stage, and the complex leaves, usually arising from an underground stem (or rhizome), are characteristic of the group. Ferns are the most complex non-seed-bearing plants. Examples of fern structure include: fern leaflet with sorus Although some fern sporophytes consist of really obvious modified leaves for producing their spores, most species have inconspicuous structures, sori (U), on the undersides of their typical leaves. Often the shape or location of these structures is useful in identifying fern species. Here, one can observe the sporangia (G) full of spores (E) within the sorus. fern "stem" The underground stem (or rhizome) of ferns has a reasonably complex structure. Vascular bundles (V) of xylem and phloem and strands of sclerenchyma cells (S), both with much thickened cell walls, function in support in some ferns. They are scattered among simpler, pith cells (P). fern gametophyte (prothallium) The non-vascular gametophyte (G) stage of the fern life cycle produces antheridia (M) and archegonia, which produce motile sperm cells and non-motile egg cells, respectively. A fertilized egg, or zygote, produces a young sporophyte (S), which grows out of the gametophyte. At higher magnification, vascular tissue (V) can be seen in the young sporophyte.

Phylum Coniferophyta Species in this group of seed-bearing vascular plants possess "naked" seeds (i.e., without protection from the ovary or fruit), and scale-like or needle-like leaves. Many, but not all, are evergreen; many, but not all, are found in habitats that are at least occasionally quite cold or quite dry. Examples of conifer structures are: leaf (needle) cross-section Although conifer needles look much different than flowering plant leaves, they do have generally the same structcure. An epidermis (E) covers the surface of the needle and a vascular bundle (V) lies in the center. Much of the volume of the leaf is the photosynthetic region, the mesophyll (M), which usually also contains one or more pitch ducts (P). female strobilus (ovulate cone) The female reproductive structures take more than one year to mature in most conifers. By the second year in a pine tree, the cone is small but clearly visible, and microscopic examination reveals the following cone structure: cone scale (S), ovule (O) and vascular core (V). male strobilus (pollen cone) The male reproductive structures develop in a single year. The cone is small and contains several sporangia (S) or pollen chambers, where the copious pollen (P), typical of wind-pollinated plants, is produced. A vascular core (V) is present here, too.

Phylum Anthophyta Species in this group of seed-bearing vascular plants possess seeds protected by the modified ovary or fruit. Their most obvious unique trait is the involvement of flowers in sexual reproduction. Other characteristics of flowering plants tend to be more applicable to the two classes of this group, rather than to all members of the phylum. For example, each class has a characteristic arrangement of tissues in its roots and stems. The two classes are:

Class Monocotyledones Grasses, lilies and orchids are three of the more familiar families belonging to this group. Characteristics of monocots include having the flower parts in threes; leaves generally elongate with parallel veins (vascular tissue); and, vascular bundles scattered throughout the stem. Tissue samples include: Allium root tip (l.s.) Even though onions are monocots, the root tip is like that of dicots in its basic structure. Notice that there are three zones of cells (division (D), elongation (G), and maturation (M)) behind a protective cap of thick-walled cells (R). Zea mays root (c.s.) As a typical monocot root, the corn root has a more organized arrangement of tissues than does its stem. An epidermis (E) covers a thick layer of cortex cells (C), used for storage; inside this, bounded by an endodermis (N), is a vascular core of xylem (X) and phloem (L), with pith at the center (P). Compare to the dicot root. Zea mays stem (c.s.) As a typical monocot stem, the corn stem does not have a very organized arrangement of tissues (especially compared to dicots). Most of the cells in the corn stem are thin-walled pith cells (P), with the vascular bundles, containing xylem (X) and phloem (L), scattered throughout the stem; epidermis (E) covers the outside of the stem. Compare to the dicot stem.

Class Dicotyledones

Tilia two-year old stem (c.s.)

Basswood, a deciduous tree, has a woody stem, which results from secondary growth produced by the lateral meristem or cambium (U). Like any meristem, this is a region of dividing cells, which is how the secondary tissues, both xylem (X) and phloem (L), are produced. The most recent year's growth of xylem and phloem is adjacent to the cambium, and the concentric rings of xylem (tree rings) in seasonal ecosystems allow one to age a tree. In such a young stem, one can see an outer layer of epidermis (E) and pith (P) cells at the stem's center. Compare to the monocot stem. Ranunculus root (c.s.) Buttercups have a root structure typical of herbaceous dicots. The xylem (X) and phloem (L), form a central, vascular core, surrounded by the endodermis (N). The largest portion of the root lies outside this core, and consists of cortex (C) cells, which store starch in the plastids seen filling the cells. An epidermis (E) covers the outside. Compare to the monocot root.
Coleus stem tip (l.s.) A plant grown for its decorative foliage, the tip of a Coleus stem shows the apical meristem (A), bud meristems (B) and leaf primordia (F). Although none of the cells near the stem tip have matured, they will become the primary tissues seen in stem cross sections. leaf The basic structure of a leaf, seen in cross section, includes both upper (U) and lower (W) epidermis, as well as the mesophyll comprising the bulk of the leaf. The epidermal cells appear empty, and are not photosynthetic; you may see stomata and guard cells in the lower. The mesophyll has an upper, regularly arranged palisade (S) layer and an irregular spongy (Y) layer, and includes veins (V). Veins contain xylem (X) and phloem (L) surrounded by sheath (H) cells. A surface view of the epidermis shows the irregular-shaped epidermal cells (E), with their breathing structures, stoma (O), protected by guard cells (Z).