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LAB 11 Seed-Free Vascular Plants by kxq14559

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									General Botany – BOT 105

LAB 11
Seed-Free Vascular Plants

Today’s laboratory will examine three phyla of living seed-free vascular plants. You will look at both
macroscopic and microscopic features of these plants and you will become familiar with terms used to
describe their morphology.
Seed-free vascular plants form a paraphyletic assemblage basal to the seed plants. They comprise the
following extant phyla: Psilotophyta (whisk ferns, not covered in our lab), Lycopodiophyta (spike-mosses
and club-mosses), Pteridophyta (or Pterophyta, ferns), and Sphenophyta (horsetails, scouring rushes).
Seed-free vascular plants have sporic meiosis-type life cycles similar to those of the bryophytes. Other
characters shared by seed-free vascular plants and bryophytes include:
        1. The gametophytes produce gametangia: archegonia (female) and antheridia (male).
        2. The gametangia are multicellular, consisting of a sterile outer layer that encloses the gametes.
        3. Antheridia produce many motile gametes: flagellated sperm cells; these require liquid water to
          swim and reach the eggs for fertilization.
        4. Each archegonium produces one non-motile egg; thus, fertilization is oogamous.
Major differences between seed-free vascular plants and bryophytes include:
        1. The sporophytes of seed-free vascular plants always have specialized conducting tissues (vascular
          tissues), hence the name of the group:
          – lignified xylem that carries water and dissolved minerals from the roots into the leaves.
          – phloem that carries the photosynthates produced in the leaves back into the rest of the plant
              body.
        2. Although both groups have life cycles with multicellular haploid and diploid phases, the seed-free
          vascular plant life cycle is dominated by the sporophyte (diploid), whereas the life cycle of
          bryophyte is dominated by the gametophyte phase (haploid).

OBJECTIVES
   Learn the life cycles and key reproductive structures of the three phyla: Lycopodiophyta, Pteridophyta,
    and Sphenophyta.
   Learn the vegetative morphology of these groups so that you can quickly distinguish them in the field.
   Understand the differences between the life cycles of homosporous and heterosporous plants.

EXERCISES
1. Early Vascular Plants
   Examine the fossil compression of Sawdonia, a representative of the extinct zosterophylls. The
sporophytes of these plants did not have stems, leaves, and roots; instead, the whole sporophyte body
consisted of a system of undifferentiated branching axes covered in spiny protrusions.

2. Phylum Lycopodiophyta
This once diverse phylum contains many extinct orders, including the large forest trees that produced most of the
Carboniferous coal seams still mined out as an energy source at many places around the world. Lycopodiophytes
are homosporous or heterosporous vascular plants characterized by the presence of small leaves with a single
vein (sometimes referred to aas microphylls). Living lycopodiophytes are diverse in appearance and include
approximately 1000 herbaceous (=non-woody) species distributed in three main groups: the club mosses
(Lycopodiales), spike mosses (Selaginellales), and quillworts (Isoetales; not covered in this lab).
!!! Note that although these plants are called “mosses” they are not true bryophytes.
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2.A. Lycopodium
   Observe the sporophytes of Lycopodium (club moss) available in lab. Lycopodium sporophytes are
dichotomously branched and characterized by the presence of small leaves arranged helically on the stems,
as well as adventitious roots emerging from the stems.
   Now examine a longitudinal section through a Lycopodium strobilus (502 L2). Lycopodium is
homosporous and the spores are formed in sporangia borne on leaves called sporophylls. In the
Lycopodiophyta each sporophyll bears one sporangium on its upper (adaxial) surface. The sporophylls are
closely grouped together to form a strobilus. Try to find strobili on the Lycopodium sporophytes in the lab.
How many types of spores do you see in the sporangia? __________________________________________
Are the spores of Lycopodium meiospores? ____________________________________________________
   Draw the strobilus of Lycopodium as seen in this cross section.

2.B. Selaginella
   Observe the sporophytes of Selaginella (spike moss) available in lab. These sporophytes are similar to
those of Lycopodium, but they have a characteristic 4-ranked arrangement of the microphylls.
   Examine a longitudinal section of the Selaginella strobilus (504 L2). The major differences between this
genus and Lycopodium center on the sporangia. Selaginella is heterosporous (= unequal spores &
sporangia), producing two types of sporangia:
         megasporangia on leaves called megasporophylls, and
         microsporangia on leaves called microsporophylls
These sporangium-bearing leaves (one sporangium per leaf) are aggregated in strobili, just like in Lycopodium. The
spores contained in the microsporangia are called microspores, and the ones contained in the megasporangia are
megaspores. Locate the microsporangia with microspores and the megasporangia with megaspores in the section
you are examining (compare with the image in your lecture materials). The differences between the two types of
spores should make it obvious why they are given different names and why this plant is called heterosporous.
Are both microspores and megaspores meiospores? ______________________________________________
   Draw and label the longitudinal section of the Selaginella strobilus. Make sure you show both types of
sporangia and spores.
   Study the heterosporous life cycle of Selaginella in the lecture materials. Be sure you understand the
differences between mega- and the micro- structures and which of these give rise to eggs and sperm. Note


that the gametophyte phase, although multicellular, is highly reduced and develops entirely within the micro-
and megaspore walls. This type of life cycle is also seen in the precursors of seed plants.

2.C. Fossil Arborescent Lycopodiophyta
   Examine the fossil specimens illustrating the stem surface, branches, rooting structures, and strobili of
extinct lycopodiophytes. These were heterosporous plants, like Selaginella. Unlike living lycopodiophytes,
some of the extinct lineages were woody and produced tree-like growth. You may need to take a trip to the
neighboring lab to see some of this material (and more fossil material used later in this lab exercise) as the
number of fossil specimens that we have is limited.




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3. Phylum Pteridophyta
Ferns are by far the most diverse and evolutionarily successful group of seed-free vascular plants. Over
11,000 species are described, making this group second only to the flowering plants in diversity. One of the
major reasons for the success of this group may be a single morphological innovation: their well developed
leaves, also known as fronds and sometimes called megaphylls. Ferns are mostly homosporous, though
some are heterosporous (the ones we will be studying today are all homosporous), and they are represent
several divergent evolutionary lineages
   Observe the examples of fern sporophytes provided in the laboratory. Note the large leaves borne on
short stems. Examine the lower surfaces of leaves for the presence of sporangia grouped in sori (singular =
sorus). Note that young leaves are tightly coiled forming fiddleheads, a condition known as circinate
vernation.
   Examine a fern sorus under the dissecting microscope. Note the individual sporangia, and find the
unevenly thickened layer of cells known as the annulus, around each sporangium. As the sporangium heats
up and dries, you might witness the explosive release of spores.
   Examine a prepared slide of a frond transverse section in Polystichum (552 L3). Note the arrangement of
the sporangia in sori and the indusium, an umbrella-like covering on each sorus (compare with the image in
your lecture materials). Locate the annulus on the sporangia and find spores inside the sporangia.
What is the function of the indusium? ________________________________________________________
   Study the sporophytes of the other fern representatives on display in the laboratory. In some, the fronds
are divided into photosynthetic (vegetative) segments and fertile segments (which look like sporangium-
bearing stems although they are actually part of the same frond). Others have strikingly large pinnately
compound fronds as compared with the stems that look like lumps of tissue with no elongation.
   Ferns are another group of plants that, along with the arborescent lycopodiophytes, generated the
Carboniferous coals between 330 and 300 million yeas ago. And, just like the lycopodiophytes, some ferns
were arborescent too at the time. Examine the compressions of leaf segments, the slice of a fossil tree fern
trunk, and the sections through the numerous adventitious roots of these fossil plants. Since ferns are non-
woody, tree ferns (in both fossil and living species) use a “root mantle” of adventitious roots that forms
buttresses around the stem, at the base of the trunk, supporting it. Note the excellent preservation that reveals
individual cells in the xylem of these fossils.
   Draw the sorus and label the sporangia and indusium.
   Examine the clay pots in the lab for the presence of ferns gametophytes. Note their resemblance to the
thalloid gametophytes of the liverworts. Both archegonia and antheridia may be found on the lower surface
of a single gametophyte thallus.
   Examine prepared slides of fern gametophytes (550 V1, 550 T2, 550 O2) and locate the rhizoids,
archegonia and antheridia (compare with the images in your lecture materials). Note that the part of the
archegonium that contains the egg is sunken in the thallus of the gametophyte.
What is the function of rhizoids? ____________________________________________________________
   Draw a fern gametophyte and label the antheridia and archegonia.

4. Fossil Ferns
    Examine the pteridophyte fossils. These plants, along with the arborescent lycopodiophytes, generated
the Carboniferous coals between 330 and 300 million years ago. You can see compressions of parts of
marattialean leaves, the slice of a trunk of a marattialean tree fern, and the ‘peel’ displaying sections through
the numerous adventitious roots of these fossil plants. Since ferns are non-woody, tree ferns (in both fossil
and living species) use a “root mantle” of adventitious roots that forms buttresses around the stem, at the base
of the trunk, supporting it. Note the excellent preservation that reveals individual cells in the xylem of these
fossils.
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5. Phylum Sphenophyta
This once diverse phylum is now represented by a single living genus of homosporous vascular plants,
Equisetum (horsetail, scouring rush), with15 living species. However, this lack of taxonomic diversity is
compensated by the ecological breadth of this single genus. Indeed, Equisetum is considered by some the
single most successful living plant genus. Not only does it have a very long history (possibly dating back to
the Carboniferous, 300 million years ago, which would make it the oldest surviving plant genus on Earth),
but species of Equisetum are found across all latitudes and altitudes around the globe. To top this, Equisetum
has tremendous capabilities to reproduce asexually from fragments of rhizomes, which make it virtually
irremovable by traditional methods once it has colonized a patch of land.
   Examine living sporophytes of Equisetum. Note the erect, ribbed, jointed stems (with nodes and
internodes), and the whorled arrangement of the stem branches and scale-like leaves. These characteristics
set this phylum apart from all other seed-free vascular plants.
   Examine a strobilus of Equisetum and note the absence of sporophylls. The spores are produced in
sporangia borne on modified branches of the stem called sporangiophores.
   Examine a transverse section of an Equisetum strobilus (501 L2) under the dissecting microscope. Find
the main axis of the strobilus (the stem that bears the sporangiophores), sporangiophores, and sporangia.
Note that each sporangium is filled with numerous spores.
What is the ploidy of Equisetum spores? ______________________________________________________
   Obtain an Equisetum sporangiophore from a fresh strobilus and break open a sporangium under the
dissecting microscope. The outer coat of the spores breaks in a helical pattern at maturity, producing strap-
shaped structures called elaters that stay attached to the spores (see picture in lecture materials). Note the
activity of the elaters – they writhe around as the air around them hydrates and dehydrates.
Where else have you seen elaters? ___________________________________________________________
How are the elaters of Equisetum different from the others? _______________________________________
  _____________________________________________________________________________________
  _____________________________________________________________________________________
Suggest a function for the elaters of Equisetum? ________________________________________________
  _____________________________________________________________________________________
  _____________________________________________________________________________________
    Draw an Equisetum sporangiophore with sporangia, as seen either in the fresh material or in the prepared
slide of the strobilus cross section.

6. Fossil Sphenophyta
   Examine the sphenophyte fossils. These include compressions of stems exhibiting the same longitudinal
ridge-and-groove structure and well marked nodes, as well as compressions of leaf whorls; note that in these
fossil sphenopsids the leaves were slightly larger than in living Equisetum, and unfused.




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                          Key to Some Common Ferns & Fern Allies


1a.     Leaves always small; sporangia usually hidden in compact fertile shoots called strobili [fern
allies]………………………………...…………………………………………………………………………2
1b.     Leaves usually large; sporangia usually visible and grouped in sori of many sporangia on lower
surface (or at margin) of sporophyll [ferns]……………………………..……………………………...……...6

       2a.      Stems erect, creeping on surface, or subterranean; nodes and internodes absent; leaves
       spirally arranged; strobili composed of overlapping sporophylls bearing a solitary sporangium on
       upper surface…………………………………………………………………………………………..4
       2b.      Erect stems usually photosynthetic and segmented into nodes and internodes ; stem surface
       with vertical ridges and grooves; branches and leaves in whorls; leaves fused into an black, brown,
       or yellow sheath around the base of the internode; strobili composed of sporangiophores bearing
       many (5-10) sporangia [Equisetum]……………………………………...…………………………...3

3a.      Stems totally unbranched, or branching sparsely below damaged top; strobilus on green
stem………………………….…………………………………………………………………….….E. hymale
3b.      Stem covered by numerous whorls of branches radiating from nodes of green stems; strobili
on tips of seasonal, yellowish, unbranched stems, but never on branched green stems….…….…..E. telmateia

       4a.    Foliage leaves usually less than 5 mm long; strobili 4-sided; two types of
       sporangia………………………………………………………….……………………...…Selaginella
       4b.    Foliage leaves more than 5 mm long; strobili round-cylindrical, one type of
       sporangium [Lycopodium]………………………………………………………………………….…5

5a.      Stem less than one foot long, seldom branched; solitary strobili borne on leafy
stems.…................................................………………………………….……………………….L. inundatum
5b.      Spreading plants of forest floors; stems often many feet long and profusely branched;
strobili borne on branched stems sparsely covered by reduced leaves.……………..….…………..L. clavatum

       6a.     Leaves of mature plants pinnately lobed or once pinnately compound ……………..……….7
       6b.     Leaves of mature plants 2 or more times pinnately compound……………...……...………10

7a.      Foliage leaves and sporophylls distinctly different; two long narrow sori on each leaflet
or lobe; one sorus on either side of midvein; lower segments of blade ear shaped……………….…Blechnum
7b.      Foliage leaves and sporophylls identical; many round sori on each leaflet or lobe;
indusium peltate or lacking……………………………...……………………………………………………..8

       8a.      Leaves radiating in all directions from a short, upright stem hidden by old leaf
       bases, roots, and scales; leaflets with sticker-like spines along the margin, and a
       "sword hilt" at the base; indusia peltate, but drying up at maturity……………………….Polystichum
       8b.      Erect leaves spaced along, but not hiding, a rhizome or creeping runner; leaflet
       margins entire or finely toothed; indusia never present [Polypodium]………………….....………….9

9a.    Leaves thick and leathery; rounded leaflet tips………………..………………....Polypodium scouleri
9b.    Leaves not unusually thick or tough; pointed leaflet tips…………….…...….Polypodium glycyrrhiza




                                                    5
        10a.     Obvious netted venation; elongated sori between the veins in two rows resembling
        chain links on each leaflet; persistent indusia open on side facing leaflet midvein………Woodwardia
        10b. Venation dichotomous, if at all evident; sori on veins or at end of veins along
        the margin………………………….………………………………………………………...……….11

11a.    Clump of closely-arranged leaves radiating in all directions from a short upright stem
hidden by old leaf bases; sori on veins on lower surface between margin and midvein…….………………..12
11b. Vertical leaves in a row along subterranean rhizomes, or creeping runners; sori at leaf
margin………………………………………………………..………………………………………………..13

        12a.     Lanceolate leaf blade wider in the middle than at base or apex; short, straw-colored
        petiole; 2 vascular bundles in petiole cross section; indusiate sori oblong to curved……..…Athyrium
        12b. Triangular leaf blade widest at base and tapering to a point at the apex; long, dark petiole;
        5 vascular bundles in petiole cross section; sorus and indusium reniform (kidney-shaped)..Dryopteris

13a.    Rhizome far below the soil surface; large, tri-pinnately compound leaves often more
than two feet long; triangular leaf blade; green, fleshy leaf stalk; sori at leaf margin…..................…Pteridium
13b. Creeping runner or shallow rhizome, visible near soil surface; leaves usually less than
18 inches long; blade with a forked organization; translucent leaflets in a fan-like array; dark,
wiry leaf stalk………………………………………………………………………...……………….Adiantum




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