For the theme session of WOOD, I will take a closer look at the microstructure of trees and write about the structure, function and optimization of vessels and tracheids.
It is well known that trees suck up the water by their roots und transfer it up to the leaves. When trees are freshly cut, it is interesting to see how much liquid they actually contain and how long it takes to dry the chopped wood. What is even more interesting is, how they transport all this water to the heights above 100 m. One might think that there must be some cells inside the tree, constantly pulsing and squeezing the liquid and therefore pushing it up by some kind of peristaltic movement. This is not the case. What makes this possible, are physical laws of water molecules and a very optimized microstructure of wood.
Vessels and tracheids
By observing a cross-section of a tree trunk, small individual holes can even be observed with the naked eye and a lot of even smaller holes can be seen under magnification (Fig. 1). These holes are so called vessels and tracheids – serving as pipelines within the trunk, transporting sap (= water with nutrients) within the tree (Fig. 2).
Tracheids are the main conducting elements of gymnosperms (e.g. conifers) and pteridophytes (e.g. fern). These are single cells with a narrow diameter, imperforated with numerous pores (pits). Their walls are thickened with lignin (Fig. 1). Being unicellular, they are restricted to lengths less than 6 mm. On the other side, vessels are the main conductive elements of angiosperms (flowering plants). They are formed by vessel members, which are joined into long continuous tubes with a comparatively wide diameter, have more pores than tracheids but thinner cell walls. They can reach lengths ranging from 1.17 cm to 27 cm .
Figure 1: Magnified view of wood vessels and tracheids (first photo 110x magnified)
Functionality of tracheids and vessels
Since tracheids and vessels resemble capillaries, water rises in the narrow tubes due to capillary forces. These forces on their own are not strong enough for water uplift of more than several meters. Physical laws of cohesion (attraction between same molecules) and adhesion (attraction between dissimilar molecules) of water play the major role here. Due to cohesion, water molecules get cohered (united) to form a continuous unbroken water column and due to adhesion, water molecules get adhesed with xylem cells. Evaporated water molecules from the leaves cause a pressure drop and this gap is replaced by the water molecules next to them, which pulls up the molecules underneath. This is how water is translocated from the roots upwards . A pulling pressure is generated on the top of water column due to transpiration and this tension is transmitted downward to root. The upliftment of water in plants of any heights is possible as water column is continuous. Water lost by transpiration is continuously compensated by water absorbed through roots.
o Optimized shape for maximal efficiency
Different literature sources emphasize the importance of diameter and length of vessels/ tracheids for conducting capacity. With increased diameter, the lumen area increases and the longer the conduit, the fewer end-wall crossings must be made per unit length . During the process of evolution, nature has optimized such ratios to perfection for many different tree species, existing in different climatic environments. A statistical study with a computer model has shown, that the conduit area conductivity is greatest at 67% end-wall resistivity , which is practically the same value which was measured for tracheids . This suggests that tracheid diameters are optimized to maximize conducting efficiency for a fixed tracheid length (don’t forget that their length is limited due to unicellularity).
Unlike tracheids, vessels have no developmental limit on length, because it depends only on how many vessel elements can be lined up. Although vessles are an order of magnitude longer than tracheids of the same diameter, they are not necessarily more efficient because they lack the low end-wall resistance of tracheids with torus-margo membrane (= hydraulically advanced membrane with numerous pits working as valves to control water transfer) (see Fig. 2 for detail). Instead, vessels gain conducting efficiency over tracheids by achieving wider maximum diameters.
Figure 2: Structure of Xylem – vessels, tracheids, fibers, parenchyma 
Cavitation is by definition the formation of vapour cavities in a liquid. It occurs in the xylem of plants when the tension of water within the xylem becomes so great that sap vaporizes locally and dissolved air within the water expand to fill either the vessel elements of tracheids . Large plants repair cavitation in different ways, one of them is via osmosis through bordered pits. This causes the water to enter and then redissolve the air. Water moves through lateral wall pits below bubble and moves upward, then it again moves in the former vessel through pits above air bubble (Fig. 3). Thus continuity of water remains unbroken from the top to the bottom . Air bubbles are frequently found during freeze and thaw cycles or hot days at high evaporation rates. Such air bubbles may break the water column inside the vessels/ tracheids and block the upliftment of water.
Figure 3: Schematic representation of upward movement of water (shown by arrows) in xylem vessels (A) and tracheids (B) bypassing the embolized zones .
Xylem conduits, in addition to transporting water efficiently while avoiding cavitation, must also protect themselves against breakdown. Negative sap pressure puts the vessels/ tracheids’ wall under compression, drawing them inward. Unless reinforced with lignin, cellulose walls cannot resist this compression. The evolution of thick and lignified walls was a solution for transport under significant negative pressure . In the wood of conifers, the tracheids must be strong enough to hold the water column and Support the tree at the same time. In angiosperm wood, fibers take over much of the plant support task, meaning that they tightly surround vessels which can consequently grow larger and only need to withstand the negative pressure.
To summarize, the design of wood tracheids and vessels is optimized for different types of plants, living in different climatic environments. Their multifunctionality (serving as conducting elements, mechanical support, etc.) is also their main limiting factor dominated by the physical laws.
I guess it is not easy to understand all the hydraulic principles of the water transport inside the complex system of trees, if one is not familiar with this topic. For better understanding I suggest to take a look at this video, which summarizes very good, why trees are such an amazing inventions of nature.
-  Sperry J.S., et al., 2006, Size and function in conifer tracheids and angiosperm vessels, Am. J. Bot. vol. 93 no. 10 1490-1500
-  Sinha R.K., 2004, Modern plant physiology, Alpha Science International Ltd, Google books 12.03.2017
-  Pittermann J., Sperry J. S., Hacke U. G. et al., 2006, Inter-tracheid pitting and the hydraulic efficiency of conifer wood: the role of tracheid allometry and cavitation protection, American Journal of Botany 93:1105-1113
-  https://en.wikipedia.org/wiki/Cavitation#Vascular_plants
-  Article shared by Koratkar S. – Cavitation and Embolism in vascular plants, 13.03.2017
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