Give Me Strength

This cross section of the stem of a soybean seedling shows the early stages in a developmental process that will produce a stem capable of supporting the mature plant. I stained the section with two fluorescent dyes - calcofluor, which binds to cellulose cell walls and fluorescences blue in ultraviolet light and auramine O, which binds to lignin and fluorescences yellow. It's the lignin laid down in cell walls that gives the stem the strength it will need to support the leaves and flower.

Working from the bottom left-hand corner towards top right, the core of the stem is filled with blue, thin-walled pith cells, which are simply packing tissue. Some of these have become slightly lignified and are fluorescing yellow and some, that are arranged in vertical rows of between two and five cells have distinctly thicker walls - these are xylem vessels, which are dead cells that form tubes that conduct water up the plant from the roots.

Above those lies a broad band of blue-fluorescing thin-walled cells that are very small and arranged like piles of bricks. This is the cambium - the plant's stem cells that divide continuously to produce new xylem on the inside and new phloem elements on their outer surface. The small, bright blue-fluorescing cells on the outside of the cambium are the phloem sieve tubes and associated companion cells, which conduct sugars produced by photosynthesis in the leaves to other parts of the plant.

The sinuous layer of yellow-fluorescing cells above the phloem are becoming lignified and these will contribute major structural rigidity to the stem as it grows, forming a continuous cylinder inside the stem. Outside of these lies the stem cortex, with blue cells becoming smaller in the layers just below the epidermis - and then the outer epidermis of the stem is covered in the yellow-fluorescing cuticle, which restricts water loss and defends that plant against pathogens.

At the stage when this section was taken the stem was about 3mm. in diameter and about 10 centimetres tall.

Swallowed by the Sun Animacule

I've long been fascinated by heliozoans aka 'sun animacules' and have posted on them before, but thought I would again because I've captured some pictures of one without squashing it under a coverslip - and also photographed it digesting its last meal. Briefly, heliozoans are single-celled amoebae which, instead of flowing around and engulfing their food with cytoplasmic arms called pseudopodia, radiate spines of cytoplasm supported by an internal scaffold of microtubules. They roll through the water like a one-tenth-of-a-millimetre-in-diameter sea mine, bringing dead to anything small that contacts those spines. This is a through-focus sequence of this microscopic protist menace, so in the image above we are looking down on the upper surface, onto what looks like a sphere of cytoplasm radiating needles in every dimension... 

.... and here the focus has moved down a bit, onto the surface of that cytoplasm, which is broadly divided up into a pattern of hexagonal blisters....

.... and now we are looking at the contents of its central food vacuole - and this heliozoan has somehow managed to ingest a testate rhizopod - a form of amoeba that lives in a shell shaped like a hot air balloon. Incidentally, all the pictures on this post will be clearer if you double-click on them to enlarge them in a separate window. Below is the same sequence, but this time at x400 magnification instead of x100......

..... so here are the surface hexagons, with the base of those lethal radiating spines....

..... and here is the blistered pattern - which looks like cells but this is a single-celled protist - so they must be formed with the aid of microtubules, I guess ......

.... and here's its food vacuole, with ingested testate rhizopod. It will digest the cytoplasm of its prey and spit out the empty shell.

It's easy to loose a sense of scale when peering down the microscope at small organisms - a sense of their place in the universe. So the picture above shows the place where this 100 micron predator came from - from the moss around the edge of this small pool on bleak moorland in Weardale, County Durham . The pool is about a metre long, so if you lined up 10,000 heliozoans spine-tip-to-spine-tip they'd stretch from one end to the other ....

..... and that tiny pool is somewhere in the middle distance in this landscape ......... and beyond that the solar system and beyond that the universe, home to an exquisite tiny life form that has been rolling through our planet's water, ingesting whatever it touches, for well over a billion years. Time travel is possible if you have a microscope....

Newly-hatched Snails

A couple of weeks ago I posted some pictures of garden snail eggs that had been found by Chantelle Kerr, one of our research students, that showed the calcium carbonate crystals in the egg shells. Well now she has hatched them out and here they are ....

Each is about the size of the head of a dressmaker's pin, less that 2mm. in diameter, and the shell is translucent so you can see the snail's organs inside. I think the yellowish blob at the top of the shell might be the digestive gland. This one is taking a cautious peek out from under the shell, at the scary world outside ...

 ... checking that the coast is clear.........

........ and making a dash for safety.

It seems that they ate the egg shells when they emerged, so all those calcium carbonate crystals have now been recycled into that translucent shell.

If it survives to adulthood, this is what it'll look like. There go the lettuces....

Micro-rock-pooling: an Intertidal Insect

There aren’t many insect species that live in the intertidal zone. The two you’re most likely to encounter are the springtail called Anurida maritima that scoots across the surface of rock pools on the upper shore and seaweed flies Coelopa frigida that breed in vast numbers in the rotting heaps of seaweed that pile up on the strandline. Even fewer insects complete their life cycle actually submerged in salt water but one is this little chironomid midge called Clunio marinus, that I found amongst the fronds of Cladophora seaweed in rockpools on the beach at Whitburn near Sunderland yesterday.

This specimen was about 5mm. long and living in a tube constructed from fine sand grains amongst the fronds of the seaweed.

It fed by hauling itself through the branches of the seaweed using two short leg-like appendages just behind the head.

The midge larva seemed to be nibbling away at much smaller organisms that encrusting the fronds, using a pair of pointed jaws. As far as I could tell it didn’t eat any of the seaweed – just the organisms that encrusted it.


Chironomid midges are extremely common in fresh water, breeding in vast numbers on ponds and even in small bodies of water like waterbutts in gardens. This species, though, as evolved to survive the rigours of life in salt water rockpools on the middle and upper seashore, where temperature and salinity levels can be extremely variable.

You can find a web site devoted to chironomid midge biology here