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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 large leaves.

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 organ).

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 image.

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 remarkable job.

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 edge.

At low magnification, the veining is clearly visible.

The long parallel cells that constitute the structure of a vein can be seen below.

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!

Photographic Equipment

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.

Further Information

Hydrangeas! http://www.hydrangeashydrangeas.com/

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© Microscopy UK or their contributors.

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