Thanks for your enlightening post on this "thorny" issue! (Warning:
Incorrigable Punster! Don't Incorrige). I feel that we often ignore
the flora side of the evolution issue on this reflector.
As a geologist, I have spent a lot of field time in the deserts of
the southwest U.S. and northern Mexico and am painfully aware of
issue of thorns, spines, stickers, prickers, barbs, briars, bristles,
nettles, burrs, and goatheads. Almost every plant in those deserts
are protected by some wonderfully designed mechanism for protection
from browsers! Two of my favorite(?) designs are catsclaw (acacia)
and 'jumping' cholla cactus.
The catsclaw thorns have a recurved shape almost exactly like the
claws of a cat. They work the same way too. When some creature,
or geologist, stumbles into catsclaw, the thorns snags the flesh and
hold on. As you try to pull away, the curvature of the thorns force
those thorns to drive their way farther into your flesh. The more
you struggle, the more you become entangled. For this reason, we
commonly refer to catsclaw as the 'wait-a-minute bush'.
Jumping cholla is a cactus that grows upward as loosely attached
segmented pods. Each pod is covered with one to two inch long spines.
(Because these spines are so long and closely spaced, from a distance
in bright sunlight the cactus look 'fuzzy', and some people call them
Teddy Bear Cholla). These pods are so easily detached that any animal
stirring around them can knock them off. Each spine is needle thin,
sharp, micro-barbed and covered with a smooth sheath. I was carefully
working my way around one of these cacti when a pod "jumped" and landed
on my elbow. When the pod landed, that force was enough to drive
the barbed spines through the sheath and into my skin. I flipped the
pod off using the flat blade of a pocket knife but all of the spines
remained embedded and had to be removed with plyers. The pod had
accomplished its "purpose" in that it was transported away from its
parent and "planted" in a new location. By the way, there are some
stories of animals and people that have become so entangled in
'jumping' cholla that their joints were effectively pinned together
holding them immoble until they died of dehydration and exposure.
You wrote:
>Looking in plant development books on how thorns develop I found that
>there are several sorts of thorns -- some from leaf buds, some from
>branch buds, some from epidermal structures.
I just thought of another - desert oak leaves are so stiff that each
point on every leaf is effectively a thorn thus discouraging many
browsers. I am constantly amazed at the designs and variety of
protection mechanisms in these plants. Just about every part seems to
have been used by some plant as a means of defense agains the animal
kingdom. Why so much variety?
>It looks as though the origin of thorns could be seen as degeneration
>or distortion of structures intended for more fruitful growth.
Again this brings up the question: How can we distinguish between
degeneration and generation? Did those wonderfully designed thorns
form as the result of the degeneration of genetically-rich plant
structures into thorns as John Morris would suggest? ...or were they
generated by mutation, genetic drift, etc. and preserved by natural
selection as a typical evolutionary scenario would suggest? I would
hesitate to call those effective thorns 'degenerative distortions'
...or, as John Morris does, defend them as God's creations for the
purpose of reminding me of my sins.
>How about the mode of origin of other aspects of "fallen" nature, such as
>parasites, protective coloration, poisons? Is anyone looking into these?
Good question! ...?
Steve Smith
[The opinions expressed here are my own
and should not be attributed to my employer]
::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::
:: ////// Steven M. Smith Office: (303)236-1192 ::
:: |----OO U.S. Geological Survey Message: (303)236-1800 ::
:: C > Box 25046, M.S. 973, DFC Fax: (303)236-3200 ::
:: \__~/ Denver, CO 80225 smsmith@helios.cr.usgs.gov ::
::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::