5.2.2  The pipeline: leaf vein architecture

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Figure 5.3 Transverse section of a pine needle. Note the simplicity of its venation in comparison to that of a grass (Figures 5.4 and 5.5) and a dicotyledon (Figure 5.7). A single central vein has two strands of phloem (p) and xylem (x), embedded in transfusion tissue (t).These vascular tissues are separated from chlorophyllous mesophyll (red) by an endodermis (e) with Casparian strips and suberised lamellae, through which water must pass outwards from the xylem and sugar must pass inwards to the phloem. Stomata in the epidermis appear bluish. Rhodamine B stain, fluorescence optics. Scale bar = 100 µm (Photograph courtesy M. McCully)


Figure 5.4   The same leaf as in the frontispiece for Chapter 5, showing one large and three small longitudinal veins, with transverse veins connecting them. Water moves along the leaf only in large (supply) veins, and enters small (distribution) veins through transverse veins. In small veins it can move in either direction in response to local pressure gradients. Small veins could carry up to two-thirds ofthe evaporated water. Semi-dark-field optics, blue filter. Scale bar = 100 µm (Photograph courtesy M. McCully)

The simplest vein architecture is found in conifer needles where a single unbranched strand of xylem and phloem is surrounded by mesophyll (Figure 5.3). Vascular strands are enclosed by an endodermis that separates them from the mesophyll, and are embedded in a mixture of parenchyma cells and tracheids called transfusion tissue. Water from the xylem permeates radially outward through transfusion tissue, endodermis and mesophyll to evaporate below lines of stomata in the epidermis.

Leaves of angiosperms have much more complicated venation than conifer needles. Look at a grass leaf with your hand lens. Parallel veins run the length of the leaf, but they are not all the same size. A few large veins have several small veins lying between them (see Chapter 5 frontispiece). On closer inspection with a light microscope, all these parallel veins are connected at intervals by very small transverse veins (Figure 5.4). There are in fact two vein systems with different functions: large veins supply water rapidly to the whole length of a leaf blade while small veins and their transverse connections distribute water locally, drawing it from the large veins. Water in large veins flows only towards the tip, but in small veins it can flow either forwards or backwards along the leaf blade or transversely between adjacent parallel veins. The distinction of flow patterns in large and small veins arises as a result of different vessel sizes. Large veins have wide vessels (c. 30µm diameter), while small veins have narrow vessels (c. 10µm diameter). Applying Poiseuille’s Law (Section 5.4.5(a)) for a fixed pressure gradient, volume flow in a large vessel will be 304/104 = 810 times the flow in a small vessel. Put another way, pressure gradients along the leaf in large vessels will be very slight, but steep pressure gradients can develop locally in the mesophyll that will direct local flows in narrow vessels. Large veins supply water rapidly over the whole lamina while small veins distribute it locally and slowly.

You do not see the vessels with your hand lens, you see the sheaths that surround xylem and phloem, much as an endodermis surrounds vascular strands of a conifer needle. Grass leaf veins have one (maize) or two (wheat) sheaths of parenchyma cells enclosing the parallel veins and containing the xylem and phloem tissues, and all materials passing out of or into the transport tissues must go through these parenchymatous sheaths (Figure 5.5).


Figure 5.5 Transverse hand-section of a fresh wheat leaf showing a single large (supply) vein comprising three large vessels (v). The vein is surrounded by two sheaths of living cells, the inner mestome sheath (arrowheads), and outer parenchyina sheath (*). The mestome sheath of these veins is impermeable to water. There is no apoplasmic path for water through the mestome sheath of large veins, except through a connecting transverse vein. In small veins, by contrast, two or three mestome sheath cells next to the xylem permit flow of water and solutes through the cell wall apoplasm. Water enters the symplasm at the inner boundary of the parenchyma sheath (Canny 1990). Phloem (p).Toluidine blue stain, bright-field optics. Scale bar = 100 µm (see Colour Plate 17) (Photograph courtesy M. McCully and M. Canny)


Figure 5.6 Whole mount of a cleared leaf of Eucalyptus crenulata showing the more complicated arrangement of supply and distribution veins characteristic of a dicotyledonous leaf. Islands marked out by large veins with large vessels, in which water is moved rapidly all over the lamina, surround islets of small veins with small vessels in which water is slowly distributed locally to the mesophyll. The ratio of small to large veins in a dicotyleclonous leaf is much larger than in a grass leaf (see Figure 5.4). A factor of 10 is not uncommon, suggesting that c. 90% of evaporated water comes from the small veins. Partial phase-contrast optics. Scale bar = 1 mm (Photograph courtesy M. McCully and M. Canny)


Figure 5.7   Transverse section of a leaf of soybean showing two of the smallest veins surrounded by bundle sheath cells. Veins of this size are the end of the branching network shown in Figure 5.6, and supply most water that is evaporated. Before processing, this leaf had been transpiring in a solution of fluorescent dye for 40 min. The small vesels in each vien contain dye solution which has become concentrated by water loss to the symplasm and out through the bundle sheath. Dye has started to diffuse away from small vessels in the cell wall apoplasm of bundle sheath cells. Anhydrous freeze-substitution and sectioning, fluorescence optics. Scale bar = 50 µm (Photograph courtesy M. McCully and M. Canny)

A dicotyledonous leaf contains the same two vein systems as a grass leaf, but these are differently arranged. Large supply veins are prominent, comprising a midrib and two orders of branches off it, often standing out from the surface of the lamina. These contain wide vessels and carry water rapidly to the leaf margins. Between them lie distribution veins, another two branch orders of small veins dividing the mesophyll up into islets about 1–2mm across, and within these islets a fifth and final order of branches of the finest veins (Figure 5.6). The fourth- and fifth-order branches have only narrow vessels. As in grass leaves, these vascular tissues are enclosed by bundle sheath cells through which materials must pass when leaving the xylem or entering the phloem (Figures 5.7 and 5.22).

Any distribution network such as the branching vessels of decreasing size in leaves is found to obey Murray’s Law. This states that the cube of the radius of a parent vessel is equal to the sum of cubes of the radii of the daughter vessels (e.g. a 50µm vessel would branch into five 30µm vessels). Such a pattern of branching produces optimal flow in several senses: minimum energy cost of driving that flow, minimum energy cost of maintaining the pipeline, constant shear stress at the walls of pipes, and rapid flow in supply pipes with slow flow in distributing pipes to permit exchange through the pipe walls (LaBarbera 1990).