Wednesday, March 18, 2015
Dune Builder
The photograph at the top of this post shows a cross section of the leaf of marram grass Ammophila arenaria, the grass that’s primarily responsible for trapping wind-blown sand and building the dune systems around our coast that are such important wildlife habitats (bottom photo). Marram grass survives in the arid environment of a sand dune by rolling up its leaves during long periods of drought, so that all the leaves’ breathing pores or stomata (see http://beyondthehumaneye.blogspot.com/search/label/stomata) are inside the rolled leaf, minimising water loss. This cross section of a partially rolled leaf has been stained with fluorescent dyes to light up different cell types within the leaf, with the outside surface of the leaf at the bottom of the picture (smooth, curved surface) and the inner convoluted surface at the top. The outer surface of the leaf at the bottom is composed of a layer of thick walled cells, covered with a thick cuticle to resist wind-blown sand abrasion and this layer also acts like a spring, giving the leaf a natural tendency to roll up under drought conditions. The stomata are hidden on the inner surface of the leaf amongst those stubbly hairs near the bottom of those convolutions – which in the whole leaf are actually ridges and furrows that run along the whole length of the leaf. The clusters of thin-walled blue cells at the base (i.e. in the ‘valleys’) of the furrows of the convolutions are responsible for unrolling the leaf – when it rains and the plant takes up water these thin walled cells inflate like balloons, forcing the leaf to unroll. Other features that you can see in this leaf cross section are the snaking rows of reddish cells which are actually the cells containing most of the chlorophyll, that carry out photosynthesis – in order to make the dyes fluoresce I had to irradiate this leaf section with blue-violet light, which paradoxically makes green chlorophyll fluoresce red. The other distinctive features are the scattered structures that look like ‘smiley faces’ with a pair of large ‘eyes’ with a blue open ‘mouth’ – these are the leaf veins that conduct water and sugars along the leaf – they’re the plant’s internal plumbing system.
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