ing of the tissues, as happens
with most other types of plants.
In epiphytic orchids, the ve-
lamen covering th e roots instantly
absorbs whatever water it comes in
contact with. The water then enters the
root, crosses the outer epidermis and moves
toward the xylem (water-conducting tissue)
by moving in the spaces between the cells or
actually through them. Once the water reaches the
xylem, which is a series of open tubes within the
stele (that “string” that’s left when a root dies and
rots), it moves quickly upward, continuing through
xylem branches and into the tiny veins in the leaves,
and ultimately into the cells themselves through os-
mosis.
In order to keep the whole thing working, it is
important that water keeps moving in an unbroken
fl ow. That is helped by the fact that water molecules
are cohesive and bind together. Those bonds are
strong enough to keep the molecules stuck together
even as they travel up to great heights (think about
trees!). However, if the transpirational pull becomes
greater than the water supply absorbed by the roots,
cohesion is broken and an air bubble, or embo-
lism, forms. Unless the bonds can be reestablished
relatively quickly (sometimes water will enter the
xylem from surrounding cells and fi ll the gap and
force the air to dissolve),
the fl ow of water is perma-
nently interrupted. Water
can be diverted around the
embolism by moving later-
ally into other xylem tubes,
but if too many embolisms
are present the part of the
plant above them will die.
Fortunately, that is less
common in orchids, as the
rate of water transfer within
the plant is much slower than that of many annuals
and perennials, and the transpiration rate has been
reduced through some evolutionary metabolic modi-
fi cations
How Culture Affects Transpirational Pull
The obvious reason for a problem is a lack of
watering and infrequent watering. If you tend to
water infrequently, or insuffi ciently, the roots can-
not take up as much water as they can when they
are watered frequently. Even if the individual par-
ticles in the potting medium stay wet, that does not
mean they can easily transfer it to the roots. Such a
practice tends to “strain” the cohesivity of the water
already within the plant, slowing the whole growth
process. Likewise, growing in too dry of an environ-
ment accelerates the transpirational fl ow, which can
outstrip the roots’ ability to replace the water, lead-
ing to desiccation and wilting, usually seen fi rst in
fl owers.
Probably the least obvious cultural issue is ex-
cessive humidity. Yes, orchids have evolved, in
many cases, to live in high-humidity environments,
but if the relative humidity is consistently very high,
the rate of evaporation of water from the leaf sto-
mata is stifl ed, which slows the transpiration process
and ultimately slows the growth of your plants.
Summing this up, for optimal growth of your
plants, water should be applied frequently and
copiously to the airy root system, and the relative
humidity should be kept moderately high, but not
saturated. GG
Ray Barkalow has been growing orchids for over 45 years,
and owns First Rays, which offers horticultural products
to the hobby grower. He may be contacted at raybark@
fi rstrays.com and you can visit his website at FirstRays.com.
wWw.
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