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  • we continue our look at

  • topic 2 the ecosystem

  • this segment is concerned solely with

  • topic two point three point one

  • which is to construct simple keys and use

  • published keys

  • for the identification

  • of organisms

  • so lets find an organism that needs identifying

  • suppose you are out doing a

  • study of a

  • polluted pond or lake

  • and found you this

  • floating

  • as a result of maybe runoff

  • from a farm

  • and your job was to identify

  • the species

  • that you're looking at

  • so that you can go ahead and write

  • a report of what's going on

  • your first step

  • is likely to be

  • take a sample of it to the lab and look at it

  • with a microscope

  • when you look in there

  • your likely to find

  • this

  • this should tell you that you are looking at algae

  • provided that you've got some

  • basic knowledge of

  • what you're doing

  • and algae have

  • chloroplasts in their cells

  • lots of the green stuff

  • used in photosynthesis

  • and this

  • particular algae that we have

  • is made up of long

  • unbranched filaments

  • with this

  • basic information in mind

  • our next step

  • in identification

  • is this

  • key..and

  • see if you could figure out how it works just by looking at it

  • this of course is a very simple

  • version of

  • a dichotomous key and

  • the content of this

  • is adapted from this URL here

  • at the bottom

  • so if you have had a look

  • maybe you've figured out how it works

  • but this is how it works if you have any trouble

  • the key the dichotomous key is made up of

  • a series of statements

  • and each set of statements

  • is aimed at dividing

  • the big group

  • into

  • smaller groups

  • so that you can eventually zero in on the identity of the unknown species that

  • you're trying to

  • identify

  • for instance what we found in the pond is

  • microscopic

  • it's not macroscopic we needed a microscope to see details of it

  • so the first statement

  • or pair statements

  • which is why we get the name

  • dichotomous branching into two

  • the first pair of statements

  • says microscopic or macroscopic and

  • our response should be that it's a microscopic

  • so that

  • means that we are directed

  • to go to

  • number two

  • which then asks

  • does it contain chloroplasts or not and it does contain chloroplasts

  • so we go to three

  • and in three

  • are the cells solitary which means just by themselves

  • or are they in

  • colonies

  • or are they filamentous

  • well.. we might not be sure about what a colony is

  • but if it's grouped with solitary we've already identified that

  • the algae is filamentous so

  • that takes us to five

  • long thin

  • unbranched

  • filaments

  • unbrached filaments

  • so that takes us from five

  • over to seven

  • and at seven

  • we're asked are the chloroplasts in a

  • spiral like a spring or are they not in a spiral

  • and we don't have any evidence that they are in a spiral

  • so that leads us to eight

  • do we have a single chloroplasts or many small chloroplasts

  • and you can see here

  • that our specimen has many small chloroplasts

  • and that leads us

  • to identify our specimen

  • as

  • microspora

  • and this is how you use

  • a simple dichotomous key

  • having used it though you

  • are likely to be asking

  • why a key

  • why do you need to

  • go through this system when maybe you could just look through the pages of the

  • book or

  • go online and search for kinds of algae

  • and that could very well work

  • if you just had maybe fifty types of algae to pick from

  • that could certainly work

  • or maybe even a hundred

  • but if you had eight thousand

  • species

  • and more

  • of algae

  • it could be the bit time consuming

  • think about looking for somebody in a town with

  • eight thousand homes

  • it would take some time to carry out your search

  • but if you

  • had some important information about who we were looking for

  • and each time you asked a question

  • of somebody

  • it narrowed your search down

  • then it will be a whole lot easier

  • so this is what

  • a dichotomous key does

  • it takes

  • information that you know simple information that you can observe

  • and it very quickly allows you

  • to move

  • toward identifying the

  • name

  • of what you have in front of you

  • trying to

  • figure out

  • the keys

  • are usually made by biologists

  • for biologists

  • so in the real world of biology in trying to identify things

  • its definitely and asset if you have a

  • basic knowledge of some of the terms

  • like the terms

  • we've used here to understand the key like

  • microscopic and microscopic

  • to know what chloroplasts are

  • to know what

  • filamentous means and

  • what solitary and colonial mean

  • that would also be helpful

  • so a basic understanding

  • of the structure

  • of what you're looking at

  • certainly is helpful

  • because

  • ultimately to identify we have to work with things that are easy to observe

  • it is no point talking about things that are

  • really complicated and hard to see

  • we would like to..quickly identify what we are looking at

  • so let's stop here now and have

  • a look at your activity

  • your activity requires you to make a key for

  • eight organisms that..you are

  • hopefully very familiar with a beetle

  • scorpion

  • centipede and

  • dragonfly ladybug spider and butterfy

  • so go ahead and

  • look back at

  • our sample key

  • look at how it's made up

  • pairs of statements

  • and try to organize

  • our eight

  • specimens here

  • in a key of your own

  • and a hint

  • is to look for features like

  • body segments

  • wings and legs

  • things that are easy to observe and would be very useful

  • in separating

  • the members of this group

  • your answer doesn't have to

  • exactly match this to be right

  • so you could have a very good answer that

  • does not match this and if you have it

  • totally wrong and you think you need to another shot at this

  • go to your refrigerator

  • and take out

  • vegetables or some food items

  • and try making a key to identify the names

  • of what you have in the refrigerator

  • for maybe for the purpose of some

  • alien being that you have staying with you

  • who won't know the names of what's in the refrigerator

  • but they might be able to identify

  • some of the features

we continue our look at

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B1 中級

2.3.1-製作和使用簡單的識別鑰匙; (2.3.1-Making and using simple identification keys.)

  • 20 3
    Yrchinese 發佈於 2021 年 01 月 14 日
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