Within Every Grass Leaf There Are Hidden Smiley Faces .....

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A vascular bundle in a transverse section of a grass leaf, stained with the fluorochromes Calcofluor M2R (blue fluorescence = cellulose) and auramine O (yellow fluorescence = lignified cell walls). The red fluorescence is chlorophyll autofluorescing red in the blue excitation beam of the microscope. 

The two big 'eyes' in this 'smiley face' (which is typical of a monocot vascular bundle) are metaxylem elements that transport water through the leaf. The bright blue fluorescence in the 'mouth' of the 'smiley face' is phloem, composed of larger sieve tubes and smaller rectangular (in cross section) companion cells, which together transport sugars, made by photosynthesis, out of the leaf. The bright yellow cells forming the neck of the 'smiley face' are lignified, providing a measure of rigidity in the leaf,  and the band of cells along the bottom of the section are epidermal cells covered by a cuticle.

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.

Plant Plumbing

Swiss cheese plant Monstera deliciosa is commonly grown as a decorative house plant but in its native Mexican rainforests it's a rampant climber, using its adventitious roots to cling to trees and climbing in much the same manner as ivy in temperate woodlands. Those holes in the Monstera leaf, whose resemblance to holes in Swiss cheese account for its name, let flecks of sunlight filter through to the layers of leaves below, all of which are transpiring water from their surface. If you cut a section through the stem, you can see the internal pipework that conducts water from the roots to the leaves.

In this transverse section of adventitious root, stained with fluorescent dyes that colour dead, woody cell walls yellow and living cellulose cell walls blue you can see the various cells that conduct liquids up and down the root. Embedded in that thick-walled strengthening tissue that gives the root (which in this case is used for clasping tree trunks and branches - this plant is a tropical climber)  rigidity and are fluorescing yellow, are large vessels that conduct water in a continuous tensile column from the roots to the leaf, pulled upwards by evaporation from the leaf surface. The smaller tubes, lined with a layer of blue-fluorescing cell walls, may be resin ducts. The outer cells on the left, part of the ring of small bundles of living cells that encircle the root, are the phloem cells that conduct sugars manufactured by photosynthesis in the leaf to other parts of the plant. Swiss cheese plants are such familiar items of interior decor that they hardly attract a second glance, but they have extraordinary hidden beauty, only visible under the microscope