Plants are eukaryotic and
mostly multi-cellular autotrophs, which possess chlorophyll and cell
of cellulose. Life cycle includes alternation fo generations, with a
dependent embryo. You should also note that botanists
traditionally use the category "division"
where we are using "phylum" (but why use two names for the same
The bryophytes are now usually considered
to be three separate phyla of non-vascular plants. 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.)
(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.)
(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.
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.)
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.)
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).
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.
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
magnification, vascular tissue (V) can be seen in the young sporophyte.
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
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.
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:
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
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.
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).