Strings Attached

Deciding on the prime time to pick a bean pod is s tricky business. Leave it too late and the pod will become tough and stringy - and the reason for that is because as it grows the pod begins to prepare to shed its seeds. Members of the pea family - the leguminosae - carry their seeds in pods that naturally become brittle when they dry and ripen, when tensions developed along the suture between the two pod halves and in the pod wall eventually become so great that the pod splits open violently, hurling out the seeds. Plant breeders have worked hard to breed this trait out of legume crops, but species like runner bean still produce long strands of woody, lignified cells in their pod walls as they ripen. In this fluorescence micrograph, showing a cross section of the upper suture of a developing pod, the bright yellow arcs of cells at the top are the 'strings' that you need to strip out of the pod before you eat and cook it if you've left it too long before harvest. The yellow cells just creeping into the picture at bottom left belong to the parchment layer that develops in the pod wall. Together, these thock-walled cells develop the tensions in the pod as it dries that will eventually split it open along its longitudinal sutures and release the seeds.

Twister

To appreciate the true beauty of mosses you really need to explore them with a hand lens or low power microscope. This is Tortula muralis, wall screw-moss and to find out how it acquired that colloquial name you need to take a close look at the spore capsules.

Wall screw moss grows in the mortar-filled cracks in walls, where it produces spore capsules that are carried aloft on stalks that are a couple of centimetres long at maturity. These are capsules in the very early stages of development, before their stalks lengthen, but if you take a really close look at a mature spore capsule....

 ...it looks like this. Notice how the capsule's stalk (seta) has twisted helically. If you take a close look at the capsule (double click on the image for an enlarged version) you can see that most of it is sheathed in a membranous covering - the calyptra. Gently pulling this off with a pair of forceps reveals....

 ... a lidded capsule underneath and if you pull the lid (operculum) away....

... it reveals a screw thread-like arrangement (the peristome) underneath, that gives the moss its common name. These threads twist up tightly in moist air but untwist in a dry atmosphere, allowing the minute spores to be shaken out when the seta trembles in the wind. In this image you can see that the operculum that has been removed has become temporarily stuck to the base of the capsule - normally it will just fall away.

Control Centre

This is a cell from a developing seed cotyledon of a broad bean Vicia faba plant. In the centre you can see the nucleus, floating like an icy-blue planet in a universe of cytoplasm - mission control, home the the DNA molecules that encode all the instructions for making the cell and, indeed, the whole plant. I stained the cell with a fluorescent dye that binds to the DNA in the nucleus and fluoresces blue when it's illuminated with ultraviolet light - the DNA (chromatin) is visible as the bright blue flecks on the nuclear membrane. The large, brightly fluorescing spot inside the nucleus is the nucleolus, where DNA is transcribed into ribosomal RNA subunits that are transported out of the nucleus and assembled into ribosomes in the cytoplasm. There the ribosomes form part of the machinery that translates the genetic code carried by messenger RNA, which is transcribed from the DNA in the nucleus, into proteins that are assembled from amino acid subunits. The nucleolus is highly visible in this cell because cotyledon cells in the seed manufacture and store proteins that are used when the seed germinates, to support the early growth of the seedling; this is a very busy nucleus and nucleolus because at this stage the cell is making a lot of protein. During early cotyledon development the cells are loosely packed together and you can see large triangular intercellular spaces between them. Later these will disappear, as the cells become packed full of proteins, lipids and starch, the cell walls thicken and the seed dries out during the seed ripening process. N.B. The cell walls are fluorescing blue in this image because of their own inherent biophysical properties, not because they contain DNA like that which is fluorescing blue in the nucleus.

Trees: the Inside Story

Almost as soon as plants colonised the land surface they began to compete for light, struggling to grow out of each other’s mutual shade. The ultimate solution, adopted by trees,  was to produce woody stems and grow tall, shading out competitors below. It's a very successful strategy - left to their own devices, many terrestrial ecosystems where water and warmth are adequate become forests. These (above) are cross sections of stems of two sycamore Acer pseudoplatanus seedlings, just a couple of weeks after germinating from a seed in spring, and already they have begun to produce woody thickening in some of their cells, visible here as the bright yellow fluorescent staining inside the stem (on the periphery of the large pith cells in its core). The very narrow yellow fluorescent line around the perimeter of the stem is the waxy cuticle secreted by the epidermal cells that protects the young stem – just a couple of millimetres in diameter at this stage - from water loss and invasion by pathogens. Double-click on the image for a clearer picture.

Fast-forward almost three years now and this seedling has grown into a sapling. In this cross section of a three year old lime (Tilia sp.) stem the big cells at the core are the pith. The three concentric rings of brown cells outside of that contain the xylem vessels that conduct water up and down the stem. They’re dead and their walls are strengthened with woody lignin, producing a strong, rigid support for the fast growing shoot and leaves. The width of those annual rings varies according the growing season – but I suspect that the outer, most recent ring is narrower because this shoot was harvested for microscopic sectioning sometime in mid-summer, before that year's annual growth was complete. Take a close look at the outer edge of the outer annual ring of xylem (double click the image to enlarge) and you may just be able to make out a distinct narrow zone of very small blue-stained cells, just a  few cells thick (at about 7 o'clock on the section). This is the cambium – the thin layer of living cells that divides to produce dead xylem cells on its inner face and living phloem cells, that conduct sugars from the leaves to the rest of the plant, on the outer side. Together the phloem and cambium are only a few cells thick and represent the most important living tissue inside the tree. Their protection is vital for the tree’s survival, so they are covered by a thick layer of bark tissue, also stained blue where the cells are alive but showing as grey-brown on the outer surface of the twig, where they are dying or dead. This is the tree’s waterproof,  self-repairing, insulating,  wound healing tissue, protecting the delicate living layer of cells inside. Growing tall by producing annual rings of growth is a long-term investment for a plant which only reaches full size after decade of growth, but the return on investment can then continue over centuries – and in some cases millennia - of annual flowering and seed production. As the stem adds annual rings, expanding in girth with every succeeding year, the outer dead bark layer splits into characteristic patterns, depending on the tree species.  The line of red cells in the bark tissues are fibres - dead cells that strengthen the young stem.