Geometry of Integration and Differentiation in Plant Stem
This section through the stem of a geranium reveals the close-packed bundles of differentiated vessels and specialised cells. The geometrical arrangement and close-packed integration produces a complex structure, strong but flexiblae, and capable of differential movement. All the cells have a structural role in addition to other functions, and structural capacity emerges from their interaction. The large pale tubes in the centre are xylem vessels that transport water and nutrients up from the root. These vessels are reinforced by spiral helices of lignin; the geometry of the spiral helix allows elongation for growth without loss of strength. The five bundles of pale green vessels are phloem cells, part of the vascular system for the distribution of carbohydrates and hormones, and the smaller purple cells on the perimeter are parenchyma cells, which are thin-walled and flexible and can increase or decrease in size by taking up or losing water. These changes cause deformations, which is how the plant achieves movements such as bending towards the light or around an obstacle. The arrangement of cells of different sizes and orientations constrains the movement in the direction that is needed. The stem is enclosed by the epidermis, a double layer of cells, some of which are specialised, with hairs or oil glands.
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