BRYOPHYTES Feb 24
As already mentioned, land plants called embryophytes.
-this is because gametophytes of all land plants retain the zygote through its initial cell divisions that form the embryonic sporophyte.
-thus the formation of the embryo on the parental gametophyte is an attribute shared by all land plants that we can use to hypothesize that they form an evolutionary group.
-as we mentioned earlier, we can hypothesize that the genus Coleochaete may be the green alga that is most closely related to the embryophytes.
-As we have discussed, the tendency of Coleochaete to retain its zygote on the haploid parental body is unique and is an attribute that helps to show how the life cycle of embryophytes might have evolved.
Fossils that are thought to be green algae are first found in rocks 900 million years old.
-the first fossils of the Charophyceae green algae (Coleochaete is a member) appear about 480 million years later
-some 420 million years ago during the Geologic Period known as the Silurian.
-During Upper Silurian, begin to find fossils called Parka
-these are thought to be Coleochaete-like organisms.
-Parka may have been a shore-line alga that was subjected to periodic drying
-it is during Silurian about the same time that we find Parka fossils that the first unequivocal fossils of land plants are found.
-These fossils are identified as land plant because they possess:
-1. cuticles-waxy layers on the outer surface of the epidermis that serves to prevent desiccation by providing a barrier to water movement.
-2. stomates-air passages are present in the cuticles of the plants which show that they were living above water level and needed to aerate the internal tissues of the body.
-3. vascular tissues-the internal tissues of these plant bodies clearly possesses strands of cells with thickened cell walls like those of xylem conducting elements
-which leads to idea that they were for conducting water.
-4. fossils clearly represent sporophytes because they have attached sporangia.
Draw
The earliest fossils of land plants are in the genus Cooksonia.
-fossils of the sporophytes have bodies that are leafless axes.
-axes branch dichotomously
-sporangia borne terminally on branches.
-internally the sporophyte has a cylinder of vascular tissue with a cluster of water conducting cells in the center of the axis.
This same set of attributes fits most of the early land plant fossils through the Lower Devonian, the geologic period that follows the Silurian. (415mya)
With the end of the Lower Devonian, fossils of land plants show a great evolutionary radiation of body forms.
-Presumably, this is related to the variety of new habitats and strategies for existence on land to which these plants were adapting.
We have no extant plants that demonstrate these simple body forms that we believe represent the extension of Cooksonia or any other early land plant to the present.
-The one extant land plant with a similar form to these early land plants is Psilotum.
-Psilotum enigmatic, but generally believed to be more closely related to later plants than those that first colonized land.
The simplest extant land plants are those in the groups: hepatophytes (liverworts), anthocerophytes (hornworts) and bryophytes (mosses).
The earliest fossils for these groups are for liverworts from the Lower and Middle Devonian.
Begin discussing these three extant groups by focusing on liverworts.
Liverworts have no vascular tissue in either the gametophyte or the sporophyte.
-Lack of vascular tissue should immediately indicate to you that the bodies of both the gametophyte and sporophyte are simple and not very thick so that there is no need for water to be distributed to tissues that are some distance from the surface.
Liverworts usually divided into 2 broad groups: thalloid & leafy liverworts.
-these groups named on the basis of the appearance of the gametophyte body.
Thalloid liverworts have a gametophyte that is a thallus.
Draw
-Thallus is largely undifferentiated, thin body with upper and lower sides.
-the thallus bears rhizoids (for anchorage) and thin scales on its lower surface.
-The upper surface of thallus has pores that lead to subsurface air chambers
-these presumably facilitate gas exchange for photosynthesis and respiration.
Multicellular gametangia are formed on the gametophyte of liverworts and all other land plants.
-as among some fungi and algae groups the male gametangia are called antheridia
-land plant antheridium basically sterile jacket of cells surrounding spermatogenous cells.
-female gametangia of land plants are called archegonia.
-Archegonium is elongate, vase-like structure.
-To accomplish fertilization, sperm must swim from the antheridium to the archegonium;
then enter the neck of archegonium;
canal cells of neck dissolve with entry of sperm;
sperm swims to egg for syngamy.
Common thalloid liverwort Marchantia forms specialized erect branches that bear gametangia.
-These fertile branches have enlarged head region where gametangia are borne.
-antheridiophore is branch bearing male gametangia.
-antheridiophore with umbrella-like antheridial head.
-Beneath upper surface of antheridial head, antheridia found in cavities.
-archegoniophore is branch bearing female gametangia.
-archegonial head is like a lobed umbrella.
-archegonia inserted on bottom surface of archegonial head along the lobes.
-antheridiophores and archegoniophores borne on separate gametophytes.
-motile sperm must have water to swim from antheridia to archegonia.
-this is on important reason why liverworts (also hornworts & bryophytes) are limited to moist environments of at least periodically moist environments.
-One way sperm reach archegonia is through splashing rain drops.
-Rain drop hits top of antheridiophore and splashes sperm in water.
-If splashed drop happens to land on and surround an archegonium then sperm may accomplish fertilization.
-It has been demonstrated that sperm may be dispersed up to 150 cm by this method.
-Fertilization obviously accomplished because we see sporophytes developing in nature.
Sporophytes develop from the zygote.
-Zygote is inside of archegonium.
-Sporophyte effectively grows out of archegonium of gametophyte.
-Gametophytes of liverworts may have many sporophytes attached to them.
Sporophytes of liverworts, hornworts and mosses are all more simple than gametophytes of these respective groups.
-Sporophytes of all remain attached throughout existence.
-Among liverworts, sporophyte is dependent upon gametophyte for nutrition.
-Sporophytes show little differentiation in these groups.
-Consist of foot: hemisphere of tissue at base of sporophyte that remains anchored in gametophytic tissue.
Draw
-Short stalk or seta.
-Distal end of sporophyte is the sporangium (often called a capsule in these groups.)
Inside sporangium is sporogenous tissue: cells that will undergo meiosis.
-Meiosis leads to formation of spores.
- spores form in tetrads
- there are cellular structures on the spores that are called ELATORS These are helically arranged thickened cells on the spore wall that loosen and twist with changes in the relative humidity. This helps in spore dispersal.
Asexual reproduction among thalloid liverworts occurs via fragmentation of the body or formation of gemmae.
-Gemmae are formed in cup-shaped structures (gemmae cups)
-Gemmae cups form on the dorsal side of the thallus.
-Inside the gemmae cups are set of lens-shaped bodies called gemmae (singl. gemma)
-Gemmae float from the body when flooded.
-New haploid bodies form when gemmae begin to grow.
Earlier we noted that two major groups of liverworts may be identified on the basis of body form.
-Been discussing thalloid liverworts.
-Second category is "leafy liverworts"
Leafy liverworts have a distinctly dorsi ventral body but it consists of an elongate, cylindrical axis with "leaves."
-"Leaves" along the dorsal side of the axis are usually larger than those on the ventral side.
-"leaves" are flaps of tissue that are only one cell layer thick.
anthocerophyte are commonly called hornworts.
-this is a small group of only about 5 genera.
hornworts usually grow on moist soils
-sometimes grow on rock surfaces that are continuously splashed by stream water
-One is a tropical epiphyte: it grows on leaves of trees in tropics.
Hornworts have thalloid gametophyte body that is orbicular in outline.
-The thallus said to form a "rosette"
-e.g., whereas liverwort thallus branched and grew outward from a point of origin, a hornwort thallus basically grows upward with thalloid branches forming above part of older thallus.
-The thallus is attached to its substrate by rhizoids.
-the thallus often with internal cavities formed from breakdown of cells.
-Cavities become filled with mucilage in which cyanobacterium Nostoc lives.
-Both antheridia and archegonia form from cells just beneath the upper surface of thallus.
-As mentioned for liverworts, the zygote forms in archegonia after fertilization of the egg.
The sporophyte as it develops is a simple, elongate structure.
-it remains anchored by foot in gametophyte thallus.
-The sporophyte is photosynthetic and can manage formation of its own sugars for life processes.
-The sporangium composes almost the entire length of the sporophyte.
-Sporogenous cells inside of the sporangium undergo meiosis to form spore.
-Structures called pseudoelators, like elators of liverworts, also formed in sporangia.
-pseudoelators function like elators.
Sporophyte is source of name hornworts for these plants.
-gametophyte looks like it has a horn when sporophyte emerges.
-When the sporangium (horn)of the sporophyte dehisces to release spores:
-it dehisces (splits) along almost entire length and gives appearance of two horns.
Asexual reproduction among hornworts is by gemmae-like structures or by : "tubers"
-tubers are internally segregated clusters of cells that have ability to persist through stressful environmental conditions when rest of body may be worn away.