Close-up View of the Hydrangea
Hydrangea macrophylla 'Blaumeise'
by Brian Johnston (Canada)
Hydrangeas are a common sight, both
as low-growing shrubs, and as potted plants. They are very
popular, probably because of their large and impressive flower-heads,
which tend to be white, shades of blue, or shades of pink. The
two commonest varieties are the ‘pompom’
or ‘mop-head’, and the ‘lace-cap’. The subject of this
article is of the lace-cap type, and is sometimes called the “Blue sky”, “Blue tit” or “Teller blue” hydrangea. This
latter name comes from the German for plate,
and refers to the startling gentian blue shade of the flowers.
It is interesting to note that soil chemistry determines the colour of
(non-white) hydrangeas!. If the soil is highly acidic, the plant
can absorb aluminum from the soil to produce blue flowers. In
neutral soil, the aluminum cannot be absorbed, and the flowers are
pink. By controlling the pH, a range of flower colourations can
be produced by growers.
Hydrangea was chosen for the
genus name because the plant’s fruit has the shape of an antique water
vessel. Hydrangea is
made up from the Greek words hydor
meaning water, and aggeion meaning receptacle. The name is a good
one for another reason. Hydrangeas consume huge quantities of
water. In order to keep my potted plant ‘moderately moist’, I was
forced to water it at least three times a day in the 20% relative
humidity in which it was kept! The species name macrophylla refers to the plant’s
The main characteristics of the lace-cap hydrangea can be seen in the
images that follow. Around the fringe of the slightly dome-shaped
flower-head are positioned about a dozen, infertile, large (two to three
centimetres in diameter), blue flowers. The central dome consists
of many tiny (two to four millimetres in height), unspectacular fertile flowers. Notice that
the larger infertile flowers start out with a large whitish-beige
centre, which later fills in with blue. In the third and fourth
images, the central fertile flower buds also transform from beige to
blue as they develop. The images show the rather random maturing
of these central buds.
Notice in the image below, that the underside of an outer flower’s
petal is lighter in colour than its top side. Also note the
exaggerated veining that exists only on the petal’s underside.
Immature infertile flowers with their large beige centres, and
partially folded petals, can be seen in the following images.
Hydrangeas have very tough, woody stems at their base, but these
transition to green and fleshy near the origin of the flower
stalks. Notice the red transition area, and unusual blue stalks
beneath the flowers in the image on the right!
Mature, completely blue flowers, with contrasting white veining can be
seen below. There is a four-lobed structure at the centre of
these infertile flowers.
The point of attachment of stalk to flower can be seen below.
Now let’s turn our attention to the fertile flowers that make up the
lighter coloured mound at the centre of the flower-head.
Although to the naked eye, these tiny flowers appear rather mundane,
when they are viewed close-up, they are spectacular in their own
right! While in bud form, each is pumpkin-shaped. As each
bud develops, it grows in size, and transitions from beige, through
white, to a final deep blue colour. This colour is deepest at the
pumpkin creases, and lighter in between.
Eventually, the buds begin to bloom - but not in the way we are
accustomed to! A tiny opening appears at the top of the bud which
grows in size over a period of about a week. Within the opening
many yellow anthers (male
pollen producing organs), and the yellow pistil are visible. (At this
magnification it is difficult to distinguish between the two.)
The higher magnification image that follows shows the two-lobed anthers
surrounding the slightly recessed three-lobed stigma (female pollen accepting
If the top edge of the opening in a flower is examined under the
microscope, it can be seen to have spherical, translucent protuberances.
One day later, a fertile flower has completely opened, revealing the
enclosed reproductive structures.
Here is another example.
The cellular structure of the broad face of an anther can be seen in
the photomicrographs that follow. The anther’s supporting filament is visible in the left
The narrower face of an anther has pale green columnar cells that can
be seen more clearly in the image at right.
The columnar cells shown above, end in irregular amoeba-shaped
structures on the anther’s surface. These latter structures can
be seen on the surface of a mature anther (first image), and an
immature one (last two images).
A photomicrograph showing the three-lobed stigma, and upper portions of
the supporting style, can be seen at left below. The image on the
right shows a higher magnification view of the cellular structure of
the junction of stigma and style.
Now, we’re going to return to the infertile flowers, and have a look at
their structure. Each flower consists of four overlapping petals
with a four lobed ‘button’ at its centre.
Close-up views of this lobed structure can be seen below. As far
as I can determine, it has no function other than as an ornament.
If the upper surface of a petal is examined microscopically, its
structure can be seen to be made up of numerous roughly spherical
cells. The low magnification image on the left shows, in
addition, the lighter coloured veins.
The back of a petal is more interesting. Notice the fine hairs
that emanate from the surface of the vein.
Higher magnification reveals the pock-marked surface of a hair.
Note that the image on the right is ‘true-colour’. The image on
the left has been ‘auto-leveled’ in Photoshop
to enhance contrast. In certain situations auto-levels does a
Here is another example.
The large, deep green leaves of this hydrangea are heart-shaped, and
very three-dimensionally indented. Each leaf has a saw-toothed
At low magnification, the veining is clearly visible.
The long parallel cells that constitute the structure of a vein can be
In some areas on the underside of a leaf, the stoma and guard-cells
that control gas entry into the leaf can be seen clearly (right image).
Hydrangea ‘Blaumeise’ was produced by the Federal Research Institute
for Horticulture in Wadenswil, Switzerland. It is certainly a
striking example of cultivar production. The depth of the
infertile blue flowers’ colour, and their longevity, make this a
spectacular garden shrub and potted plant!
The macro-photographs were taken with an eight megapixel Canon 20D DSLR
equipped with a Canon EF 100 mm f 2.8 Macro lens which focuses to
1:1. A Canon 250D achromatic close-up lens was used to obtain
higher magnifications in several images.
Microscopy UK or their contributors.
Published in the June
2007 edition of Micscape.
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