FLORIDA
STATE
HORTICULTURAL
197
SOCIETY
DO AVOCADO ROOTS DEVELOP
ROOT-HAIRS?
D. S. BURGIS and H. S. WOLFE
University of Florida, Gainesville
In the great majority
of vascular plants,
the absorbing area of the roots is tremendously
increased by the development of root-hars im
below the seeds, so that the upper portion of
the root system was in humid air, the lower
portion in water.
Seeds were first germinated in a mixture
mediately back of the area of elongation near
of sand and peat, and when the taproot was
the root tip.
about 5 inches long, each seedling was trans
ferred to a 2-liter jar containing the medium.
For many years it was believed
that the roots of citrus trees do not develop
root-hairs,
but
recent
investigations
shown this
belief to be erroneous,
have
although
root-hairs develop more abundantly under cer
tain conditions
than under others
and may
sometimes fail to develop at all.
No report could be found in the literature
on the presence or absence of root-hairs in the
avocado.
Writers who had occasion to refer
to avocado roots usually assumed that roothairs were present, a& in nearly all other trees,
but no one actually reported any study or ob
servation on the matter.
present
study
was
For this reason the
undertaken.
failed to find any root-hair
We
have
development in
the avocado, either under natural or under
artificial environmental conditions for root
growth.
Seedlings grown in water and in moist air were
supported by placing the seed on a piece of
board on top of the jar, the roots going down
through a hole slightly smaller than the seed
diameter. Seedlings grown in sand, peat and
a sand-peat mixture were transplanted to the
medium with the seed resting on the surface.
Glass museum jars were used for the fluid
media, and glazed crocks with a drainage hole
for the solid media.
A duplicate set of cultures was run in which
a nutrient solution was supplied instead of tap
water. This solution consisted of 6.47 g. su
perphosphate (20%) and 10.23 g. nitrate of
potash dissolved in 400 cc. of water. Of this
stock solution, 3 cc. was- added every two weeks
to the various cultures, except that only IY2
cc. was supplied the moist-air jar because it
contained only half the amount of water which
It is well known that the abundance of roothair development on plant roots decreases
was in the water jar.
markedly as the medium in which they are
prevent growth of algae.
Examination of the roots was made at bi
growing becomes higher in water content.
Many plants which develop root-hairs in soil
fail to develop them when grown in water.
On the other hand, saturated air seems to be
a very favorable environment for development
of root-hairs.
Good aeration seems to be a
requisite for good root-hair formation.
In the series of tests here reported, avocado
roots were allowed to develop in five different
environments: 1) water, 2) humid air, 3) sand,
4) peatmoss, and 5) sand-peat mixture (equal
parts).
The humid air condition was pro
vided by keeping the liquid level several inches
It was necessary to cover
all the glass jars with black paper in order to
weekly
intervals
for
five
months,
removing
small portions from the solid media and ex
amining roots in the fluid media through the
walls of the glass jars. No root-hairs were
found in any case, using a binocular micro
scope magnifying 8 diameters.
As a further
test, a root
tip was placed between glass
slides under the pressure of rubber bands and
allowed to grow in the peat medium thus for
three weeks. It has been reported that pres
sure of this sort is one factor in inducing
root-hair development, but none developed in
FLORIDA STATE HORTICULTURAL SOCIETY
198
this test, although the root tip continued to
grow vigorously.
Indeed, root growth was
good under all the test conditions here em
ployed.
Roots were also examined of trees- growing
in tubs in the greenhouse in a sandy loam soil,
and of trees of large size growing out-of-doors
in a Norfolk fine sand. Repeated examination
failed to discover any root-hairs on these root
tips.
The avocado forms in soil a much branched
system of secondary roots', each rootlet having
a white tip about 2 mm. long. A brown peri-
derm covers the portions of the rootlet older
than this white tip. It is evident that absorp
tion of water and nutrients is dependent on
the ramification of this root system so as to
provide a maximum amount of these absorb
ing tips, since no root-hair system is developed
to increase absorbing area in the usual way.
Figure 1 shows a portion of the root system
developed in peat where the porous nature of
the medium gives ideal conditions for roothair development.
Several tips are clearly
visible, magnified ab~out 3 times, but nothing
resembling root hairs.
THE RAMON TREE OF YUCATAN
(Brosimiim alicastrum)
DAVID FAIRCHILD, Coconut Grove
Mrs. Fairchild and I spent a few days with
the Sylvanus Morleys in Merida last Novem
ber.
Their life work on the Maya ruins of
years ago, shortly after his trip into Peten
where he found the pretty palm, Opsiandra
maya. The muleteer who was taking him in
Yucatan has made them well known among
to the wilds of that inhospitable country fed
the archaeologists of the world.
As we entered their "finca", "Chen Ku"
(Nest in the Well), I noticed that the place
his mules on the branches of the ramon tree
was filled with picturesque dark green trees,
—"No tiene ramon en su pais?" 'You don't
tall and irregular in shape like the poplars in
Holland for example. "What trees are these,
have ramon trees in your country?')
Sylvanus?"
stuck in my memory and I determined some
I
inquired.
"They
are
ramon
and when Cook told him he had never seen
the
tree
before,
Somehow
or
he
other
remarked incredulously,
this
innocent
remark
trees and the only plants on the finca out of
day to see this ramon tree which Cook's mule
which we make any money.
They form what
teer thought he should have in his country.
the Yucatecans call a "ramonal" and in the
dry season there come out from Merida the so
Brosimum alicastrum is its name and it has
called "ramoneros" who climb the ramon trees,
several relatives, one of which is the "Milk
Tree" of Venezuela, B. galactodendron, the
and with their machetes cut off most of the
milky juice or latex of which is sweet and is
branches and cart them to town where they
form one of the important sources of fodder
drunk by the natives like cow's milk.
I knew that there was a tree of this species
for the mules and horses and cattle.
in the Plant Introduction Garden at Chapman
We get
about 40 cents for the branches from a sin
gle ramon tree. Since the trees cost us noth
ing to keep and we have three hundred or so,
Field. I thought I recalled a plant of it which
grew from some seed brought in by my old
There came back to me, when he said this,
friend G. N. Collins in 1913 from Merida and
which withstood the great freeze of 1917 at
our old Buena Vista Garden, being killed back
a remark made by by friend 0. F. Cook many
but not killed out by a temperature of 25 F.
we make a little money from them."