“Of all the Houseleeks
neatest far
The jolly
Cobweb
Houseleeks are.”
Walter Ingwersen 1943
Sempervivums
are extremely hardy, mat-forming succulents of the family
Crassulaceae, commonly
known as ‘Hen
and Chicks’ or ‘Houseleeks’. Native to the mountains of
Europe,
the Middle East, and Central Asia, they grow close to the
ground, with
fleshy leaves grouped around one another in a very tightly
packed
rosette. The ‘hen’ is the main plant, and the ‘chicks’ are
the
offspring, which start as tiny buds on the main plant, and soon
sprout
their own roots, taking up residence close to the ‘hen’.
Hens and chicks have a very
long
history of cultivation. It is said that the Frankish King
Charlemagne (742-814 AD) suggested to his subjects that they
grow the
plant on their thatched roofs in order to guard against fires
caused by
thunderbolts, storms and sorcery. Since succulents do
retain
water in their leaves, they may help in the first two cases, but
as to
sorcery – perhaps not!
The genus name Sempervivum is Latin for
‘always
alive’ (evergreen). Arachnoideum,
the species name, relates to ‘spiders’. In this species
the tips
of the leaflets are connected by thin hairs that closely
resemble
cobwebs. Sempervivum
arachnoideum has many cultivars, of which ‘Forest
Frost’ is
one. Some arachnoideum
hybrids
have very dense cobwebbing, while others have little.
‘Forest
Frost’ seems to be positioned somewhere in the middle of the
pack in
terms of this characteristic.
When I first found this
unusual
cultivar at my local greenhouse, I had no expectation that it
would
bloom during the spring or summer period. I was
correct!
However, later in the summer, the same greenhouse had a couple
of
blooming examples, which I obtained in order to show its entire
life-cycle. This article has been produced by using images
taken
of the three ‘Forest Frost’ plants.
It should be kept in mind that
Sempervivums are monocarpic, meaning that a
particular hen flowers only once, and then it dies.
Fortunately
by the time that this happens (usually several years), it has
already
produced many chicks to take its place.
The first image in the
article, and
the series shown below, show several ‘hens’ (4 - 6 cm diameter),
and
their smaller chicks (1 – 2 cm diameter).
Two higher magnification
images
show the dense tufts of cobwebbing at the leaflets’ tips, and
the
strands that connect each tip to its nearest neighbours.
Although Sempervivums can reproduce
by seed
when the plant finally flowers, a much more common reproductive
technique is vegetative
reproduction.
This occurs when the hen ‘offsets’. Here, the base of the
hen
produces what look like thin, root-like stems called stolons with tiny chicks at
their
ends. Each offset develops roots of its own and becomes
independent of the parent when the stolon withers and
dies. Some Sempervivums
produce offsets on
the ends of long stolons, which produces a less tightly packed
family
group. Vegetative reproduction produces offspring with the
same
characteristics as the parent plant. The four images below
show
the tight packing of a family group. Notice the light
coloured
stolon visible in the middle left of the first image.
Another view can be seen in
the
images that follow.
The leaflets at the centre of
a
chick rosette are green in colour, while those further out have
a deep
purplish-brown coloration.
Cobweb density is very
variable in
different hens and chicks.
When viewed at a very close
range,
it is evident that the threads comprising the cobweb originate
from the
tip of a leaflet. While the chick is still very tiny, the
threads
become tangled and caught by nearby leaflets, and as the
offshoot
grows, a cobweb is produced.
For some unknown reason, the
occasional chick has many fewer threads. Notice that the
lack of
threads allows us to see the outer surfaces of the leaflets much
more
clearly. Hair-like projections cover all surfaces.
Many of the chicks are not
solidly
rooted to the soil in the container, and they jiggle when
touched. This one was precariously balanced between a
couple of
siblings.
When removed from the
container, it
is evident that there is no cobwebbing underneath.
A longitudinal cross-section
of a
chick reveals that the leaflets grow from a colourless,
cone-shape core.
The four images that follow
show
what appears to be the formation of a new chick, but the shape
isn’t
right. Instead of a ball, a stalk appears to be growing
away from
the base of the rosette.
A couple of weeks of patient
observation resulted in the view below. It appears as
though
several of the container’s hens have almost reached the end of
their
life-cycle, and are preparing to bloom. The light coloured
spherical structures on the tops of stalks are immature buds.
Before the buds appear, the
top of
the lengthening stalk has the appearance shown below. At a
later
stage, the stalk’s leaflets will take on the normal purplish
colour of
the species.
Many immature buds push their
way
through the packed leaflets in the top section of the stalk.
In some cases the bud location
is
limited to the very tip of the stalk. By this point the
leaflets’
green colour has transitioned to purplish-green.
Notice the great variation in
bud
size within a particular group. There is just a hint of
the
flowers’ final pink colour in a few of the larger buds.
A sequence of images taken
with
increasing magnification follows showing the buds at the top of
the
stalk shown at left.
Each of the buds is ringed by
many
green sepals which
have pink,
sharply pointed tips. These sepals surround pink flower petals. The whorl of
sepals is
referred to as the calyx.
In
the last few images, taken at the highest macrophotographic
magnification possible with my equipment, it is evident that the
hairs
on the sepal have bulbous red tips, which indicates that they
are
glandular in nature.
Eventually some of the buds
begin
to bloom. A closer view of the flowers reveals that they
are (actinomorphous),
shaped like a star,
with about 12 reddish-pink petals. The most
noticeable
characteristic of a flower is the ring of stamens arising from
the disk
at the base of the petals. Anthers are yellow in colour,
and
their supporting filaments are bright red.
Sempervivum
arachnoideum ‘Forest Frost’ buds are strikingly
beautiful both
in form, and colouration!
At
the
magnification shown below, it is apparent that both the
sepals, and
the flower’s petals are covered with fine hair-like
projections.
The whorl of pink petals is referred to as the flower’s corolla.
The petals of the flower grow
from
the edge of a yellow dome-shaped disk, and the male stamens grow
from a
ring where the petals meet the disk. Emerging from the
disk
itself is a group of bright red female pistils.
If the surface of a green
sepal is
examined under the microscope, the subtle green colouration is
reminiscent of a pastel painting. Note that a water mount
was
used, and this accounts for the occasional bubble in the field
of view.
Near the edges of a sepal,
some of
the glandular hairs seen earlier are visible. Notice that
the red
colouration in the glandular, bulbous tips is localized in two
distinct
areas!
The use of a different
lighting
technique emphasizes the bulbous tips of the hairs, but does so
at a
lower resolution.
More images of flowers at
different
stages of development follow. Next, we will look at one of
a
flower’s pink petals with the aid of the microscope.
Photomicrographs of the
surface and
edge of a petal can be seen below. Its cells are
irregular, both
in shape, and pink colouration. Hairs growing along the
petal’s
edge are visible in the second image.
A petal vein can be seen in
the
image on the left below. The image on the right reveals
the
presence of glandular hairs on the petal’s surface.
A higher magnification shows
the
longitudinal striations on the surface of individual cells (left
image), and the bulbous tips of glandular hairs (right image).
In the sequence of three
images
that follows, some of the anthers are covered by a thin purple
membrane
which hides the developing pollen grains beneath. In more
mature
anthers this protective purple membrane has disintegrated.
Notice the unusual surface
texture
of the anther at the centre of the left image. The image
at right
shows a membrane that has split longitudinally to reveal the
yellow of
underlying pollen grains.
When the membrane has
completely
disintegrated and fallen away, the remaining anther appears
considerably smaller. Each anther has two pollen releasing
pads,
with a darker central section that is connected to the
filament.
This forms a sort of anther ‘sandwich’.
The slightest contact is able
to
dislodge pollen from the anther, as can be seen in an area where
one
has touched the surface of a petal.
Below are photomicrographs
showing
the surface of a mature anther. The pollen grains can be
seen to
be ellipsoidal in shape.
Images showing pollen grains
adhering to the top of a filament (left), and the surface of a
petal
(right), can be seen below.
Ten bright red pistils, each
connected at its base to a pale green swollen ovary are visible
at the
centre of the flower shown in the two images that follow.
Under the microscope the
cellular
structure of the stigma and its supporting style can be seen
below. At the very tip of the pistil, the unusually short
lobes
that make up the surface of the stigma can be seen. In my
experience, these lobes are shorter than those in most flowers.
In the first image below, the
base
of one of a flower’s styles (red) is shown at the point of
connection
to its associated ovary (green). Note, in all of the
images, the
large swollen heads of the glandular hairs.
Hens and chicks are extremely
popular as rock garden plants. This popularity has
resulted in an
amazing number of ‘common’ names being given to the them.
A few
of them are: Houseleek, Jupiter’s Eye, Jupiter’s Beard, Thor’s
Beard,
Bullock’s Eye, Sengreen, Ayron, Ayegreen, Aaron’s Rod, Hens and
Chicks,
Liveforever, and Thunder Plant!!
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.
A 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.