Euphrasia

Eyebrights - Euphrasia

Eyebrights are small hemiparasitic flowering plants. They rely on nutrients drawn from neighbouring plants, such as grasses, especially
during early growth but also to supplement the nutrition of the established plant. They obtain these nutrients via haustoria, outgrowths of
root epidermal cells which adhere to host roots and develop a full vascular continuity between host and parasite. The plants are
attractive with small, but very attractive flowers. Eyebrights are difficult to identify down to the species level. There are some 20 or more
British species, but between them they form at least 71 recognised hybrids! The form above keys out as Euphrasia nemorosa (Common
Eyebright, American Eyebright) which coincides with the habitat of a chalk meadow. Sometimes included in the figwort family the
Eyebrights are included in the broomrape family (Orobanchaceae) in Stace's key to British flora (3rd ed.2010).

Above: the leaf 'teeth' characteristically
terminate in a sharp spine, the lowest
pair more-or-less at right-angles (top).
Both leaf surfaces are of a similar hue
in E. nemorosa.

Above: trichomes at the base of the stamen filaments (basally
pointing). Similar trichomes occur on the corolla, especially in the
yellow 'honey spot' on the lower lip. Most of these trichomes consist of
two cells, a short basal cell and a longer, pointed apical cell. However,
some appeared to consist of a single cell or three cells (this needs to
be verified). Some had a few visible granules inside whilst others had
a roughened (pitted?) surface. In addition there were trichomes of a
clearly glandular function (colleters). The shaft of these short hairs
consisted of three cells, the terminal cell being swollen into a sphere
which was filled with vesicular cytoplasm in some cases, or with a
yellowish secretion in others. Perhaps the secretion serves as an
odorant to atrtract insects or as a tastant to guide them into the flower.

Above and left: The petals of plants are often covered in
conical epithelial cells. These have several functions. In
roses, the conical papillae or tubercles have microscopic
ridges, formed from arrays of microscopic rods, which are of
the correct spacing to diffract blue light. This creates a
structural blue colouration which gives the petals their
iridescent sheen. In structural colours the colour results
from purely physical interactions with the light rather than by
chemical pigmentation. It is also possible that these cells are
secretory.

These conical cells also alter the wettability of the petal
surface. They create a superhydrophobic surface which
reduces wettability and increases self-cleaning. This is used
in the leaves of the Sacred Lotus (Nelumbo nucifera). The
same principle also applies to many flowers, encouraging
water and sediment to flow freely off the surface, keeping it
clean and attractive to insects!

Above: the two stamen types. Left: a stamen from the upper corolla, with its violet filament and asymmetrical spikes one the tip of
each anther lobe. There is also a characteristic twist at the end of the filament connecting to the anther (though this might depend
on developmental stage if the anther position changes). Right: a stamen from the lower corolla, still bilobed but with equal-sized
spikes on each lobe tip and with no violet pigment in the filament (though the basal part of the filament is pigmented yellow. In both
cases, a single vascular strand (with a visible xylem vessel) can be seen in the middle of each filament. Each locule dehisces (splits
open) in a large elliptical zone on the basal surface (underneath the upper surface visible here) leaving white twisted strands of wall
tissue hanging as a hairy fringe around the opening. This suggests how twisting of the materials in the anther wall as it dries brings
about dehiscence by setting up stresses in the tissue causing it to split.

In the intact flower, the 4 stamens are arranged in two pairs, with their pollen releasing surfaces pressed together by the pressure of
the surrounding hood formed by the sides of the upper lip, so preventing the escape of pollen. The lower lip forms a landing
platform. A large insect, such as a bee, landing on the platform and forcing its head into the entrance, pushes its head against the
stigma, to deliver any pollen it is carrying, and then deforms the corolla, releasing the pressure so that the stamen pairs spring
apart and sprinkle the bee's head with pollen. This is presumably helped by the projecting spurs on the anthers which, if touched by
the insect, will rock the anther to dislodge the pollen.

Above: trichomes, including glandular colleters, on the corolla. These are most densely grouped on the yellow spot of the lower lip.
Long, pointed and apparently non-glandular trichomes also occur on the yellow basal portions of the lower stamen filaments. (It is
difficult to rule out a glandular function, but the shorter glandular hairs clearly have a very obvious glandular function).

Right: a variety of epidermal cells types were visible on the
corolla, depending which region was examined. Here we can
see some with wavy contours. Can you think what the
advantage of this is? The cells are held together by a sort of
'biological glue' called pectin and wavy edges increases the
surface area of contact between neighbouring cells and allows
them to interlock. This gives the joins strength and a similar
technique is used in wood-working. Perhaps here the
epidermis was subject to more mechanical stress, but was
tucked out of the main display area and so had no need of
conical 'self-cleaning' cells. A further study could map the
types of epidermal cell in different regions of the corolla.

The floral structure of Eyebrights is curious. They have four stamens which are attached at the base of their filaments about midway
along the inside of the corolla tube. The corolla tube consists of the fused petals (presumably 4 or 5 petals originally) which are fused
about half-way along their length with the end segments free. The flower is zygomorphic, and the corolla has a distinct upper lip (of two
lobes) and an enlarged lower lip (with three principle lobes, each notched).

Oddly there are two forms of stamen. Attached to the upper lobe is one pair of stamens attached almost at the base of the free part of
the upper lip. The filaments have violet pigment along most of their length and end in bilocular (with two compartments) anthers which
thus appear double-headed. Each compartment ends in a terminal spine, but one of the spines (the basal-most) is clearly much longer.
The inside of the corolla has a small yellow spot around the base of each filament. The base of the filaments has a few straight trichomes
(hairs). The second pair of stamens are attached to the lower lip at the base of their filaments, just below the point of fusion of the corolla
lobes. They lack the violet pigment but the bases are pigmented yellow, as is the inner corolla at this point, where the large visible yellow
spot is clearly visible on the lower lip. These yellow spots possibly lure insects deep inside the flower by mimicking pollen. The lower
stamens also have more trichomes on the basal region of their filaments. These long straight trichomes possibly serve a sensory
function, perhaps registering insect visitors to trigger the next developmental stage. Alternatively, they may block the path of small
insects attempting to steal nectar, that is if this plant produces any nectar.

Another key characteristic to aid species identification is the
straightness of the stems and branches, whether they are
straight, curved or wavy. Many had the straight or slightly
curved upright posture expected of Euphrasia nemorosa, but
some had slightly undulating branches.

Each of the five corolla lobes has three prominent violet veins.
Each vein contains a vascular strand (with a visible xylem
vessel). A view of the epidermis covering one such vein is
shown on the left. The purple pigment is clearly filling the
cytoplasm of the epidermal cells in a homogeneous,
non-granular manner. Presumable these act as guides to lure
the insect pollinators into the flower.

Above and above right, the yellow coloration of the 'honey spot'
on the lower lip is due to the presence of yellow granules in the
epidermal cells of the corolla in this region. The granules are
clear under the microscope and just visible in the
photomicrographs (click photos to enlarge). They occured
largely in flattened epidermal cells, but also in some of the
conical cells. The trichomes and colleters are most dense in
this region.

Things to consider when identifying Eyebrights are:
habitat, locale, number of nodes on the stem basal to the
first flower (the cotyledons are sometimes visible on the
most basal node), number of nodes before the first
branching, the relative sizes of stem leaves and bracts,
the shape and hairiness of the leaves and the length and
colour of the corolla. The Eyebrights pictured above came
from the top of a grassy chalk bank, and some at the
bottom. These had small flowers (corolla less than 7 mm
long) and almost hairless leaves. Some plants had
reddish leaves, especially towards the base. The stems
were reddish in all cases and had a fairly dense covering
of white hairs. The leaves were almost entirely hairless.

However, they tended to give way to another form at the
bottom of the bank with larger flowers with very large lower
lips (probably Euphrasia pseudokerneri (Chalk
Eyebright)). This latter form had very hairy leaves, some
of the hairs visibly glandular (terminated in a small
sphere). Both forms have been recorded in similar places
(at the top and bottom of chalk slopes) and both are late
flowering (these were observed in September).The
presence of these two species on the same grass bank
suggests that they may be hybridising, as they readily do.
It is difficult to be sure to what extent the populations are
pure. The specimens at the top of the bank appeared to
be fairly pure nemorosa, except that some had more
horizontal stems which is thought to be more of a
pseudokerneri trait, so these may have been hybrids.

Click images to enlarge.

Above: the small corolla (6 mm from
base to end of upper lip) is partially
enclosed in the calyx of fused green
sepals.

Above: the carpel with long
white hairs at the top end and a
small brownish protuberance at
the base of unknown function
(possibly a nectary or
elaiosome?)

Above: a fruit capsule is
developing inside the calyx. In
E. nemorosa, the capsule
remains shorter than the calyx
(is this fruit mature?)

Above: The area had recently been mowed (for conservation
purposes) which hampers identification slightly, since you can not
count the number of nodes before the first flower on the main axis if
the top has been cut away! (It also detracts from the appearance of
the plant!) Within a week or so, however, the branches had taken
over and produced more flowers. If allowed to, these plants may
reach up to 20 cm tall.

Above: Sometimes the shoots form quite tangled-looking
masses. Personally, I find the growth habit of this plant and its
variations aesthetically pleasing! They look magnificent
through a field magnifier.

Above: Some individuals with reddish leaves can be seen
here.

Above: the anthers are spent in this flower, having released all their
pollen. All but three of the anthers have been moved up into the
upper lobe (on other flowers all anthers were in this position). This
suggests that, as in many other flowers, the male organs mature first
(protandry) followed by movements of the filaments and perhaps
lowering of the stigma as the female parts mature.

Conservation Status

Like so many wild flowers, all species of Euphrasia appear to be in decline. Although globally widespread (occurring in north America
as well as in Europe) its populations in Europe, and certainly in the British Isles, appear to be in decline. It thrives on ground that has
not been disturbed for agriculture, so shifting land use is likely a major factor. Many populations are likely to be hybrids.

Caveat: Before collecting any wild flower, in part or whole, check the conservation status (and legal status) of the plant in the local
area. Only collect what you need for scientific, recording or identification purposes. Only collect from populations large enough not
to be affected.

Article created:
22 Sept 2015
12 Dec 2015
24 July 2017

Zonation and Habitat Preferences

In the locale studied here, Euphrasia nemorosa at the top of the slope gave way to the larger flowered Euphrasia pseudokerneri (or
a E. pseudokerneri x E. nemorosa) at the 'bottom' of the slope, which was actually the middle as the slope is in two segments with
level paths at the top and mid-level. Is this what we would have expected? It depends: what we have to consider are something like
the Ellenberg indicator values (external link). These give the preferred ranges of light intensity, soil moisture, pH, nitrogen and salt
for different plant species. According to these scales, both E. nemorosa and E. pseudokerneri like to avoid shade, so a grass bank
is ideal, but E. pseudokerneri prefers the soil to be slightly drier (better drained) and more alkali (pH 8 rather than 6) than does E.
nemorosa. In our site, the top of the bank was a level and well-trodden path bordering woodland, and was favoured by the E.
nemorosa, whilst the mid-slope was likely to be less compacted and so perhaps better drained and favoured by E. pseudokerneri.
To be sure we could carry out a transect from the top to the bottom of the slope and record and identify the plants in a specified
area, such as by using quadrats (wire squares, typically 1 metre in width) to mark out sample areas in a random or systematic
sample (avoiding bias). However, if our initial observations hold, then it is not unreasonable in this site to suggest that the middle of
the slope was better drained and more alkali. The compacted soil at the top may have been further acidified by leaf litter and so may
have had a lower pH more suited to E. nemorosa. In this case, the pattern observed is quite possible. Equally, at other sites, the top
of the slope may be better drained and better suited to E. pseudokerneri.

Below: more E. nemorosa photographed from the same location in the
third week of July, 2017.