Monday 10/25/99
Lichens and Bryophytes (Lichens & Mosses)

NOTE LICHENS AND MOSSES ARE COMPLETELY UNRELATED TAXONOMICALLY BUT ARE SIMPLY COVERED IN THE SAME LAB.



Lichens, an overview:
Lichens are a symbiotic association of a fungus (Ascomycota) and a green algae (Chlorophyta) or a fungus and a blue-green algae (Cyanophyta).  The fungal component is the mycobiont while the algal component is the phycobiont.  It appears the fungus parasitizes the algae by extracting carbohydrates from green algae and nitrogen from blue-green algae.  The photosynthetic organism does not derive a nutritional benefit from the fungus but may use to fungus to anchor to a substrate and for some protection from herbivory.  The lichen is divided into three layers; the cortex which the dense, outer, fungal layer, the medulla which is an a loosely packed, inner, fungal layers, and an algal layer which may be sandwiched in between these two layers are scattered in the medulla.  The shape of the symbiont is primarily determined by the organization of the hyphae while the pigmentation (shade of green or yellow) is primarily from the algae.

THERE ARE FOUR LIFE FORMS OF LICHENS

Crustose Lichens - growth from that is appressed, "crusty" in appearance, and in direct contact with substrate (ex. flattened, crusty lichen on a tree branch).  Has organization of the above but with an lower cortex and scattered algal cells.
Foliose Lichens - similar to crustose lichens in cross-section but leafy in appearance.
Fig. 2 Crustose / Foliose lichen cross-section.
From left to right, upper cortex, scattered algal cells, medulla, and finally lower cortex.

 

Fruticose Lichens -highly branched structure with a thick outer cortex, medulla and distinct algal layers.
Fig. 1 Fruticose Lichen Cross Section showing cortex, medulla, algal cells.

Gelatinous Lichens - different from the above three lichens in that is is made mostly of blue-green algae (Cyanophyta) with only a little fungus present.  The gelatinous appearance is from the mucilage commonly found on the exterior of blue-green algae cells.


Bryophytes, an overview:
Bryophytes can be defined as green plants that lack well developed vascular tissues (such as a xylem and phloem).  They have motile sperm, chlorophylls a and b, and generally are very slow growing.  Because they don't have well developed vascular tissues, they cannot conduct water and nutrients the way a vascular plant would.  This forces their growth form to be low to the ground so they can absorb water and minerals through capillary action.  They are not considered to have true leaves or roots even though they may have structures that appear leafy.  Even though they may be referred to as "leaves" they are much simpler than leaves seen in plants thus far.  They often use rhizoids as an anchor to the substrate.  These rhizoids do not conduct water or nutrients but instead allow the plant to stay attached to its surface while absorbing water and minerals through capillary action.  They also have sporophytes which are spore producing structures.  The sporophyte results from fusion of the egg formed in the archegonium) and the flagellated, motile sperm formed in an antheridium.

The bryophyte seen in lab this week are the MOSSES.  Early in the semester we did see another group of Bryophytes the LIVERWORTS.  There is a third group called the HORNWORTS which will not be covered this semester.

MOSSES:  Handout provided in class goes into a detailed description of the reproductive cycle. Listed below is a similar description with some pictures listed on web page

Mosses are able to reproduce similarly to higher, flowering plants. Plants in the gametophyte stage are able to re-produce sexually, in a similar way to  flowering plants. However, the reproductive parts are small and not showy like flowers.  There are two stages, the gametophytic and the sporophytic.  First meiospores germinate into a protonema.  The protonema develops into the leafy gametophyte that we commonly see.   From there male, female or both structures are produced either at the growing tip or laterally to the plant branches (see Acrocarpous and Pleurocarpus below).   The male organs are known as  antheridia.  The antheridium encloses the sperm which are released when the antheridial wall ruptures.  The sperm use a film of water to swim toward the egg.  The female organs are known as (archegonia).  The archegonia is surrounded by neck and ventral canal cells.  These break to allow the sperm passage to the egg.  After fertilization occurs, a new plant develops, which is attached to the parent plant. This is the sporophyte.

The sporophyte is made of a capsule, which is on the end of a thin stalk (seta) (click here for capsule and seta picture).   The capsule has a covering known as an operculum.  There may also be tiny teeth present underneath the operculum called the peristome teeth.  The mature capsule contains large numbers of microscopic spores called meiospores. These are  eventually released and dispersed  (usually by wind) and each is then capable of germinating to eventually develop into a new gametophyte plant.
 

THERE ARE THREE LIFE FORMS OF MOSSES
Sphagnum Mosses - spirally arranged "leaves" and very branched and clustered.  Sphagnum mosses form their sporophytes like the acrocarpous mosses and have been observed to forcibly fire their spores up to 2 meters.

Acrocarpous Mosses - distinguished by the sporophyte being very erect, (i.e. extending up and away from the main plant body).   Since sporophytes are so often present this moss can usually be distinguished by erect structures.  This structure forms because the apical (growing tip) is where to sporophyte forms so it is out on the end of a branch or leaf.

Pleurocarpus Mosses - distinguished by the sporophyte being low to the ground.  May be harder to identify in the absence of sporophytes but mosses generally hugs the ground more so than acrocarpous mosses.  The sporophyte does not form on the growing tip but instead on a side (lateral) branch.


Link to Internet Resources for Bryologists and Lichenologists
 
 




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