Mountain Laurel is sometimes referred to as “the most beautiful tree” because of its spectacular display of flowers in the spring.  Growing to a maximum height of about 6 metres, but normally closer to 2.5 metres, it is a many-stemmed thicket forming shrub, or small tree.  Many names have been bestowed on Mountain Laurel, including Sheep Laurel, Lambkill, Spoonwood, Calico-bush, and just plain Laurel.  Several of its names refer to the fact that all parts of the plant are poisonous.

The hybrid studied in this article, Kalmia latifolia ‘Elf Mountain’ is a dwarf evergreen shrub which grows to about a metre in height.  (My plant was truly dwarf, about 30 centimetres in height!)  Its leaves are oval, and glossy dark green.  The buds and flowers start faint pink, but the mature flowers are almost pure white.  The image at the beginning of the article shows a typical view of the shrub during the blooming period.

It is the large number, and unusual shape, of both buds and flowers that set the Mountain Laurel apart from other plants.  Perhaps its most unusual feature is the fact that its bright red anthers fit into pits in the surface of the flower’s corolla, and spring out to release their pollen when disturbed by a visiting insect.




Even the buds are striking in this plant! A whorl of reddish-green sepals surrounds the immature corolla, and forms the flower’s calyx.  The unopened corolla appears deeply grooved with pink-tipped projections near its base.  Within the bud, the flower’s anthers are neatly arranged to fit in the interior of these projections.


In the two views that follow, notice that a bud’s stalk, calyx and corolla are covered with tiny hairs.  Although it is not evident from the photographs, the hairs are glandular, and produce really copious quantities of an extremely sticky, viscous liquid that rapidly coats ones fingers!  It would definitely not be a good idea to have a taste!


Additional images of unopened buds can be seen below.




Much closer views of a bud allow individual glandular hairs to be seen.



Even clearer views of these hairs can be seen in the images of a flower’s stalk and whorl of sepals.  Notice in the last images that thin strands of the sticky liquid can be seen stretching between hairs, like cobwebs.



The point of connection of each flower’s stalk to the stem is particularly sculptural.  Note the colour contrast between the light green stem, and the red stalks with white hairs.  Also notice the pale green pointed leaflets (more properly called stipules) that grow from the point of connection of flower stalk to stem.



It is impossible to touch any part of Kalmia latifolia ‘Elf Mountain’ without getting the sticky fluid on one’s fingers.  I have never worked with a plant that was so unpleasant to handle, and that includes spiny thistles!


Under the microscope, the glandular hairs look vaguely alien.  Often the tips of several hairs are glued together by their own fluid.



One result of this phenomenon is that insects that happen to alight on any of the plant’s surfaces are instantly trapped by the sticky glue.  The small insect shown in the sequence of images that follows tried valiantly, and ultimately unsuccessfully, for over an hour to escape its adhesion to a bud stalk.





The three images that follow show buds in the process of opening.  Hints of the flower’s reproductive structures can be glimpsed through the opening in its corolla.



In this partly open flower, its round, bright red stigma is visible.


Fully blooming flowers have a corolla consisting of a single fused petal with a 5-lobed margin.  About half way up the side of the corolla, there are 10 niches or pits, accompanied by corresponding bumps on the outside.  When the flower blooms, each pit is filled with a red anther and the anther’s supporting filament is curved down towards the base of the corolla.  Probably as a visual cue to insects, there is a 10-pointed, star-shaped red band near the corolla’s base, with each point on the star pointing towards an anther.  Growing from the flower’s centre is a single pistil with a white style supporting a red stigma.





If a section of corolla is examined under the microscope, its cellular structure becomes visible.  The photomicrograph on the right shows a fold in the corolla.


Much higher magnification, as in the image on the right below, reveals the ridged surface details of individual cells.


Bright red cells from the star-shaped pattern at the centre of the corolla can be seen below.


The images that follow show the neat packing of anthers in depressions on the interior surface of the corolla, and the bent filaments connected to them.



The slightest touch, caused perhaps by the leg of an insect, causes the anther and its filament to spring away from its pit.


If you study the flower in the image below, you may be able to detect a hint of yellow pollen on each red anther.


When a flower first opens, an anther’s surface is unbroken, and has a red colour.


As time passes, the red colouration begins to turn reddish-brown.


Eventually two pores open in each anther, revealing the pollen grains within.  (Only one of the pores is visible in the images.)  The left image also shows particularly well, the yellowish stigma pad on top of its style support.


Photomicrographs showing anthers with their pollen revealing pores open can be seen in the series below.  The pollen grains appear indeterminate in shape.



Below are two images showing the short hair-like protuberances on the surface of a filament.


Here are several images that show Kalmia latifolia ‘Elf Mountain’’s pistils.



A still closer view of the receptive surface of the stigma, with its five ‘bumps’, can be seen below.  Note also that the two pores on the surface of the bi-lobed anther, seen below the pistil, are clearly visible.


Notice in the image below, that all but three of the anthers have sprung away from their pits.  The geometry is such that anthers often strike the style as they ‘spring’.  This helps to dislodge pollen from their pores.


Additional macro-images showing the surface of a stigma follow.


The two photomicrographs below show the cellular structure of the top edge of the flower’s style (red),  and the stigma dome (yellow).


Even the style has glandular hairs producing the sticky fluid mentioned earlier.


The plant’s glossy green leaves start out a much paler shade, and over time, develop the darker tint seen at the base of the shrub.



I wonder what the tiny white specks might be on the surface of a new leaf?  Yes, you’re right; they are glandular hairs!  As I mentioned before – every surface is coated with the annoying sticky fluid!



All green parts of Kalmia latifolia contain the poisons andromedotoxin and arbutin.  In case you might wish to include them in a salad, it might be a good idea to keep in mind what a medical treatise has to say about the symptoms of Mountain Laurel poisoning!

“Poisoning produces anorexia, repeated swallowing, profuse salivation, depression, lack of coordination, vomiting, frequent defecation, watering of the eyes, irregular or difficulty breathing, weakness, cardiac distress, convulsions, coma, and eventually death.”


Photographic Equipment

The low magnification, (to 1:1), macro-photographs were taken using a 13 megapixel Canon 5D full frame DSLR, using a Canon EF 180 mm 1:3.5 L Macro lens.

An 10 megapixel Canon 40D DSLR, equipped with a specialized high magnification (1x to 5x) Canon macro lens, the MP-E 65 mm 1:2.8, was used to take the remainder of the images.

The photomicrographs were taken using a Leitz SM-Pol microscope (using a dark ground condenser), and the Coolpix 4500.


A Flower Garden of Macroscopic Delights

A complete graphical index of all of my flower articles can be found here.


The Colourful World of Chemical Crystals

A complete graphical index of all of my crystal articles can be found here.

 All comments to the author Brian Johnston are welcomed.