Breathing Space

This is a thin section of the lower stem of water milfoil Myriophyllum sp. , stained with the fluorochrome calcofluor which binds to the cellulose of the cell walls and is fluorescing brightly in ultraviolet light. Marsh plants tend to be rooted in anaerobic mud and so have air channels (aerenchyma) that conduct oxygen down to the roots. 


Working from the outside inwards in this section, there is a well defined single outer layer of very small cells forming the epidermis, then inside that lies the stem cortex with 17 air channels arranged around the central stele, which contains the phloem (brightest flourescence) and the xylem.

The stem and leaves of water milfoil. The small white structures in the leaf axils are the stigmas of the female flowers.

Sticky Jack

This is a cross section of the stem of the plant commonly known as goosegrass or sticky Jack and more scientifically as Galium aparine. Sticky Jack is a very common weed that scrambles up through other vegetation using its covering of hooked hairs on the stem and leaves and which sticks to clothing with these when kids throw handfuls of the stuff at each other.

This image was produced using fluorescence microscopy, staining the cells with compounds that bind to the cell walls and fluoresce. The blue cells have walls made of cellulose and their blue fluorescence is due to the calcofluor that they've been stained with, which fluoresces blue in ultraviolet light. Calcofluor has been used as a 'blue whitener' in washing powders - it binds to the cellulose in cotton fabrics and fluoresces faintly blue in the UV component of sunlight. The yellow staining is due to another fluorescent dye (fluorochrome) called auramine O, which binds to cutin in the outer cuticle of the plant, and to dead, lignified cell walls that give the stem its strength - and it fluoresces yellow. The cuticle in this cross section is the thin yellow line covering the outer surface of the section. The yellow circle in the centre is composed of dead, lignified cells - not particularly well developed in goosegrass because it scrambles over surrounding vegetation rather then investing resources in producing a stout lignified stem of its own.

Taking the Pith

Even as a dyed-in-the-wool botanist I have to admit that rushes (Juncus species) are pretty dull plants – just spiky bunches of cylindrical leaves growing in mud around pond margins (bottom picture) or dominating large areas of poorly drained upland pastures. But, as the old saying goes, ‘beauty is more than skin deep’ and to find the really attractive feature of this plant you have to delve below the leaf surface. Peeling back the outer layer of green photosynthetic tissue reveals a cylinder of spongy pith, as light as thistledown. Pith peeled from rushes and dipped in tallow was once used to form the wicks of rush-lights, a smoky-flamed form of interior lighting that was eventually replaced by gas mantles and then electric light. Take a look at these pith cells under the microscope, in a cross-section of the cylindrical leaf (see top picture) and their real beauty emerges. The individual cells, shaped like starfish, form three-dimensional interior scaffolding for the cylindrical leaf. Each ‘starfish-cell’ is joined to its neighbours by the tips of its arms, forming a three-dimensional lattice (see second picture from top). The top two photomicrographs were taken using fluorescence microscopy, where the section of the leaf was treated with a compound (in this case auramine) which fluoresces when illuminated with blue light, giving an very attractive green glow that shows the three dimensional structure of the plant tissue particularly well. Double-click the top image for a better view. To find out more about rush lights, visit http://pilgrim.ceredigion.gov.uk/index.cfm?articleid=1239