Assertions re plant "cellularity"

Una Smith una at phy.duke.edu
Wed Jul 8 10:28:18 EST 1992


I asked:

> : > : [...]
> : > : What are the fundamental characters common to all things we
> : > : categorize as plants, whether unicellular or multicellular?

And Tony Travis replied:

> : > A plant is an organism that has the ability to use sunlight to feed  
on
> : > C02 from the air using water and minerals from the soil.

How boring! I thought.  That doesn't tell me much about how the
trick is accomplished.  Later, Tony added:

> Read Una's question carefully - she did not say what are higher plants.

Thanks, Tony.  My question was, I'll admit, a bit disingenuous, but
I assumed that most readers in bionet.plants would not be mislead
into thinking it a naive question.

> It has been suggested that chloroplasts originate from ancient
> endosymbionts and retain some degree of genetic autonomy.  My 
> definition was intended to include *any* organism capable of 'feeding'
> itself by fixing CO2 photosynthetically.
> 
> I accept that saprophytic/parasitic plants do this indirectly but are
> they _incapable_ of photosynthesis, or do they not just invest more
> resources in reproduction than they are capable of acquiring
> independently and supplement their energy balance from elsewhere ??

I don't think this is known, but I know people who are intent on
finding out.

I have no problem with saprophytes;  they're thought to be derived
from normal plants, and have lost functions after figuring out how
to get what they need from other, functional plants.  These are
certainly interesting, though:  we don't really know how these
species differ from their photosynthetic relatives.

> My definition might also be:
> 
> A plant is an organism that has the ability to decrease its own entropy
> by storing the energy from sunlight as carbohydrate.

[...]

Ok, so plants are autotrophs.  My question remains:  what are the
necessary and sufficient fundamental characters of autotrophs 
which use sunlight as an energy source?  (Ocean-bottom thermal vent
organisms are also autotrophs, no?)

> Ultimately, the ability of all organisms to organise themselves against
> the thermodynamically favourable slide into chaos depends on the
> capture of energy by photosynthetic organisms.  This, in turn, depends
> on the entropy of the rest of the universe increasing.  In particular,
> the entropy of the Sun increases as a consequence of the nuclear
> reactions that yield (amongs other things) the energy of sunlight.

But wait a minute, Tony;  I've been told that life did not start
out with autotrophic organisms.  Plants came much later, long after
prokaryotes had acquired mitochondria and all sorts of other stuff
(but I don't know where chloroplasts fit into the evolutionary tree,
relative to the eukaryotic condition).  So life seems to have 
found ways of counteracting the evil forces of entropy before it
figured out how to make photon-catching devices (and thus plants).
 
> The decrease in entropy of cyber-space also depends on plants ;-)

Meaning those of us who strive for higher signal/noise ratios
in these newsgroup debates have plants to thank for our presence?

	Una



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