Transpiration

Original Post:
>>
Valerie Lowery@ZEON
05/27/99 02:31 PM

I haven't had much of a problem with hollyhocks, despite our environment.
What the cause must be for the early death is a mystery.  I must disagree
about the importance of humidity in gardening, however.  Here in
Louisville, KY, humidity goes along with our heat in summer (hot, hazy,
humid).  My grey felt-leaved plants (lamb's ear, snow-in-summer, etc) rot
away in mid-summer, sometimes recovering when cooler, less humid temps come
back around in the fall.  Hybrid tea roses are infamous for blackspot here
despite religious showerings of chemicals.  Our local gardening gurus all
talk about the humidity factor, allowing that certain plants are doomed
here because of this stifling atmosphere.  Most plants can take in the
moisture through transpiration.  This is the ideal environment for plants
with a tropical nature, such as orchids and ferns.  In fact, this is the
principle behind foliar feedings.  However, certain plants cannot take the
moisture content that inevitably will bring along fungal growth and some
bacterial attacks.  Look at the susceptibility of monarda and phlox as
examples of what can happen in a humid environment.  Air circulation helps
dry off the leaves, but cannot always effectively carry off all moisture.
>>

Good analysis, Val.  However, please allow me to gently make a minor 
correction.

Plants do not "take in moisture through transpiration."  By definition, 
transpiration is just the opposite -- it is the LOSS of moisture from plant 
surfaces (leaves, stems) due to evaporation.  

The "principle behind foliar feeding" is that plants can take in a limited 
amount of nutrients from their leaf surfaces through absorption.  However the 
primary source of nutrients for plants is through their roots.

You do make some excellent points, Val.  I just wanted to make this point 
clear to newby botanists on the list who might have been confused by your use 
of the term 'transpiration'.  Do keep up the good work!  --Janis



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