Friday, September 14, 2012

Lab 4: Alternation of generations

Green algae:

“Green algae” are a deeply paraphyletic group of aquatic photosynthetic organisms.  They are divided into Chlorophytes and Streptophytes, the latter of which includes all of land plants.

Chlorphyll b, stach, stellate flagellar structure, and certain gene transfers are some of the features shared between the Chlorohytes and Streptophytes (which collectively comprise the green plants, or the Viridophytes).  The split between the Chlorophytes and the Streptophytes occurred ~1billion years ago.

The Chlorophytes include three major clades: Chlorophyceae, Trebouxiophyceae, and Ulvophyceae. We have examples of each of these three groups in lab.   

Examine the prepared slides of Volvox and Platydorina (Chlorophyceae) which represent colonial forms of green algae. 


Examine the preserved specimens of Acetabularia (Ulvophyceae).


Trebouxia (Trebouxiophyceae) is a frequent component of lichen, a composite organism involving a symbiotic relationship between a fungus and a photosynthetic partner, and is yet another member of the Chlorophyte group.


The Streptophytes include the land plants (or Embryophytes), as well as numerous lineages of “green algae.”  Within the Streptophytes, Zygnematales, Coleochaetales, and Charales are consistently recognized as the algal lineages most closely related to land plants, although the exact relationships of these groups remain unclear.  Glycolate oxidase system is a synapormophy (other than molecular characters) uniting the Streptophyte clade as a whole.

Examine the prepared slide of Spirogyra (Zygnematales).

Examine the prepared slide of Coleochaete (Coleochaetales).

Examine the living material of Chara (Charales). Charales is frequently recognized as sister to the land plants. The following are some of the features supporting this relationship: an encasement of egg, sperm morphology, and numerous plastids per cell. 



Embryophytes are the term given to all land plants.  The following are the 5 synapomorphies for land plants: multicellular sporophytes, embryo, gametangia, sporangium, cuticle.


“Bryophytes” include all of the earliest diverging, yet extant lineages of land plants.  They are non-vascular and therefore do not contain specialized anatomy of phloem and xylem.  They are a paraphyletic group that includes liverworts, mosses, and hornworts.  However, the relationships of these bryophytes are still uncertain. 

Stomata are specialized epidermal cells involved in gas exchange.  Stomata evolved at the most recent common ancestor of all land plants, excluding liverworts.

Note the alternation of generations for the moss.  The gametophyte stage is free living; that is, it is totally independent of it's sporophyte parent.  The dominant stage is the gametophyte.  Bryophytes are homosporous.

Compare a hornwort, liverwort and moss.


Lycophytes and Ferns:

Lycophytes are called “club mosses,” yet are not true mosses.  Lycophytes have microphylls (relatively simple leaves).  The gametophyte stage is also free living, like in bryophytes; that is, it is totally independent of it's sporophyte parent.  The dominant stage is the sporophyte.  However, some lycophytes are homosporous and some are heterosporous.

Lycopodium is homosporous.

Selaginella is heterosporous.

The following are examples of Lycophytes:

Ferns are also called Monilophytes and are morphologically very diverse.  This variation will be the focus of next week’s lab.  Note the alternation of generations for the fern (Ceratopteris).  The gametophyte stage is yet again free living, like in bryophytes.  Ferns are homosporous.  The dominant stage is the sporophyte.

Many ferns have a characteristic heart-shaped gametophyte, as in the bisexual prothallus below (another term for fern gametophytes).  The archegonia is on the abaxial side under the notch. Ceratopteris also has male gametophytes.

(hermaphrodite gametophyte, abaxial surface showing archegonia just above rhizoids)

After adding ~1mL of water to a petri dish with Ceratopteris gameotphytes, you saw the antheridia releasing sperm that are visible.  The antheridia were large, bulging cells especially apparent on the male gametophytes.

The following are examples of ferns:
Ceratopteris (sporophyte)

Seed plants (Gymnosperms and Angiosperms):

All seed plants are heterosporous and have seeds.  Heterospory has evolved several times in the course of land plants.  The gametophyte stage is NOT free living; that is, it is totally DEPENDENT on it's sporophyte parent.  The dominant stage is the sporophyte.

Note the alternation of generations for the seed plants (a gymnosperm and an angiosperm). 

The following are examples of gymnosperms:

The following are examples of flowering plants:

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