[ sci.bio.botany restored, in case anyone more knowledgable can
comment. ]

In article ,
"Franz Heymann" writes:
|
| Yes. I was referring to the known effect by which glass 'magnifies'
| the strength of sunlight, as it affects plants. I believe that it
| a reradiation effect.
|
| What does that mean?
| The intensity of the UV per unit wavelength increment is almost
| negligible compared to that in the yellow-green region of the spectrum
| of sunlight reaching the surface of the earth. If I understand
| correctly what you mean by reradiation, namely absorption of UV and
| reradiating at a longer wavelength, then the absorbed UV will be so
| littlle that it will not resulet in a measurable increase in the
| intensity of the botanically active frequencies.

Why did you think that I meant ultraviolet? I didn't. But, on that
topic, ultraviolet is as effective at damaging plant cells as it is
at damaging animal ones, and plants that grow in high ultraviolet
locations have developed protection mechanisms.

No, what I mean is surface heating. This is the effect by which the
surface of an object can become much hotter than either the body of
the object or the air temperature. One point is that glass reflects
long (far) wavelength infrared well, though it transmits short (near),
and that causes the greenhouse effect, but you can get it even with
materials that transmit uniformly.

What can happen is that an object under glass can receive the direct
radiation, and a proportion of the reflected radiation from ALL of
the objects under the glass (i.e. a focussing effect). This does
not have to be a precise focus to double or even triple the total
radiation it is receiving, and explains why the exact location is
an important factor.

Now, it might appear that this would raise the temperature of the
leaf as a whole, but it is not necessarily so. Transpiration will
keep the leaf cool, just as sweating does for humans, but that will
not stop the surface cells between the pores from getting very hot.
It is quite possible that a significant amount of human sunburn
(under dry conditions, when sweat evaporates rapidly) is due to this,
rather than purely to ultraviolet.

Now, I have no PROOF of the above, but it is the only explanation
that I can think of that matches the properties of the effect that
I know about, and of course the biology and physics.


Regards,
Nick Maclaren.