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By the end of this section, you will be able to:
  • Describe the traits shared by green algae and land plants
  • Explain the reasons why Charales are considered the closest relative to land plants
  • Understand that current phylogenetic relationships are reshaped by comparative analysis of DNA sequences

Streptophytes

Until recently, all photosynthetic eukaryotes were considered members of the kingdom Plantae. The brown, red, and gold algae, however, have been reassigned to the Protista kingdom. This is because apart from their ability to capture light energy and fix CO 2 , they lack many structural and biochemical traits that distinguish plants from protists. The position of green algae is more ambiguous. Green algae contain the same carotenoids and chlorophyll a and b as land plants, whereas other algae have different accessory pigments and types of chlorophyll molecules in addition to chlorophyll a . Both green algae and land plants also store carbohydrates as starch. Cells in green algae divide along cell plates called phragmoplasts, and their cell walls are layered in the same manner as the cell walls of embryophytes. Consequently, land plants and closely related green algae are now part of a new monophyletic group called Streptophyta .

The remaining green algae, which belong to a group called Chlorophyta, include more than 7000 different species that live in fresh or brackish water, in seawater, or in snow patches. A few green algae even survive on soil, provided it is covered by a thin film of moisture in which they can live. Periodic dry spells provide a selective advantage to algae that can survive water stress. Some green algae may already be familiar, in particular Spirogyra and desmids. Their cells contain chloroplasts that display a dizzying variety of shapes, and their cell walls contain cellulose, as do land plants. Some green algae are single cells, such as Chlorella and Chlamydomonas , which adds to the ambiguity of green algae classification, because plants are multicellular. Other algae, like Ulva (commonly called sea lettuce), form colonies ( [link] ).

Light micrograph A shows rectangular Spirogyra cells linked in a chain. Light micrograph B shows a oval green desmid cell. Electron micrograph C shows egg-shaped Chlamydomonas cells attached to thin stalks. Photo D shows a colony of Ulva that resembles leaf lettuce.
Chlorophyta include (a) Spirogyra , (b) desmids, (c) Chlamydomonas , and (d) Ulva. Desmids and Chlamydomonas are single-celled organisms, Spirogyra forms chains of cells, and Ulva forms colonies resembling leaves (credit b: modification of work by Derek Keats; credit c: modification of work by Dartmouth Electron Microscope Facility, Dartmouth College; credit d: modification of work by Holger Krisp; scale-bar data from Matt Russell)

Reproduction of green algae

Green algae reproduce both asexually, by fragmentation or dispersal of spores, or sexually, by producing gametes that fuse during fertilization. In a single-celled organism such as Chlamydomonas , there is no mitosis after fertilization. In the multicellular Ulva , a sporophyte grows by mitosis after fertilization. Both Chlamydomonas and Ulva produce flagellated gametes.

Charales

Green algae in the order Charales, and the coleochaetes (microscopic green algae that enclose their spores in sporopollenin), are considered the closest living relatives of embryophytes. The Charales can be traced back 420 million years. They live in a range of fresh water habitats and vary in size from a few millimeters to a meter in length. The representative species is Chara ( [link] ), often called muskgrass or skunkweed because of its unpleasant smell. Large cells form the thallus: the main stem of the alga. Branches arising from the nodes are made of smaller cells. Male and female reproductive structures are found on the nodes, and the sperm have flagella. Unlike land plants, Charales do not undergo alternation of generations in their lifecycle. Charales exhibit a number of traits that are significant in their adaptation to land life. They produce the compounds lignin and sporopollenin, and form plasmodesmata that connect the cytoplasm of adjacent cells. The egg, and later, the zygote, form in a protected chamber on the parent plant.

 Chara is a water plant with a long, flexible green stem. Whorls of slender stalks periodically radiate from the stem.
The representative alga, Chara, is a noxious weed in Florida, where it clogs waterways. (credit: South Florida Information Access, U.S. Geological Survey)

New information from recent, extensive DNA sequence analysis of green algae indicates that the Zygnematales are more closely related to the embryophytes than the Charales. The Zygnematales include the familiar genus Spirogyra. As techniques in DNA analysis improve and new information on comparative genomics arises, the phylogenetic connections between species will change. Clearly, plant biologists have not yet solved the mystery of the origin of land plants.

Section summary

Green algae share more traits with land plants than other algae, according to structure and DNA analysis. Charales form sporopollenin and precursors of lignin, phragmoplasts, and have flagellated sperm. They do not exhibit alternation of generations.

Questions & Answers

Is the "growth and maintenance phase" in a cell's life cycle when cell division is about to occur
Somto Reply
what is the common name of Basidiomycetes
Ogechukwu Reply
الاجزاء النباتية لابد من تعقيمها قبل زراعتها في القوارير
yes
tariq
whats this?
tariq
do you speak arabic?!
what are bio elements
Shahzad Reply
which are present In Body And such elements Have Great role in our Body there are 16 bio elements that maintains human Body but on The basis of amount There are 6 bio elements present in Concen. of 99% and More Valuable And Highly Concen. element is Oxygent with 65 %
Haider
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Lamina Reply
how je pollution brouhgt about
Lamina
non is pollution brouhgt about
Lamina
describe the anatomy of cell division
Ivanovic Reply
Complex traits such as height result from 
Ruben Reply
what is the difference between chloroplasts and mitochondria
Nkalubo Reply
chloroplast in plants and bacterial cell ; mitochondria in animal cells
aung
Diagram of a living cell
Eliza Reply
what is cell
Sule
A cell is the smallest basic unit of life.
John
what's biology
Ogochukwu Reply
this is da study of living and non-living thing in an eco-system
Nutty
it is the study of living and non living organism in the ecology
Akufia
I agree with you dat biology is d study of living nd nonliving features
Winner
why do plants store carbohydrates in form of starch and not glucose?
Nutty Reply
Describe the structure of starch?
Nutty
wat is diffusion
Winner
water is life!.. Discuss?
Nutty Reply
why do plants store carbohydrates in form if starch not glucose!
Nutty
study of living thing
Dennis Reply
what is beyond a liveing cell
Raymond
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Gabriel Reply
d study of living nd non living thing
Winner
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Evelyn Reply
The surgical removal of d spermduct
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Source:  OpenStax, Biology. OpenStax CNX. Feb 29, 2016 Download for free at http://cnx.org/content/col11448/1.10
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