AWA: Academic Writing at Auckland

The evolution of the vascular system in plants is one that has been studied in order to show how plants have adapted both to grow into different forms of one same species, and also to explain the varying rates of photosynthetic ability that we see today between gymnosperm and angiosperm species. This altogether helps us understand the success and vast spread of angiosperms that we see today.

An important structure in the evolution of the vascular system is the bifacial vascular cambium. The vascular cambium, located in the xylem, has cells that constantly divide, creating more xylem and phloem cells, allowing for secondary growth (Larsen, 2012). These cells are hugely important for photosynthesis, as the xylem contains tracheids- in gymnosperms and angiosperms- and vessels in angiosperms which are responsible for the conducting of water from roots through the stem and trunk and to the leaves, allowing for transpiration and photosynthesis (Rowe and Speck, 2005). The bifacial vascular cambium is thought to have arisen in lignophytes/ woody plants  in the Devonian period (c. 360 million years ago (Myr) and is was a major revolution for the development of different growth forms in such plants. It made secondary growth possible by a transformation in vascular tissue, from closed bundles to open vascular bundles. The cambium, which produces secondary phloem (inner bark) externally and secondary xylem (wood) towards the centre (Spicer et al.,2010; Schweingruber et al.,2007) allowed for widening of the plant structure, as well as strong, woody stems that meant trees could grow taller, promoting superior access to light, and therefore more photosynthetic activity. The bifacial vascular cambium is therefore a structure that allowed plants to be more adaptable to different environments, as opposed to the previous unifacial meristems that were employed in extinct lycopsids and sphenopsids (Larsen, 2012). The development of the vascular cambium allowed for the seeding plants, i.e. angiosperms to change their size and growth form to suit whatever environmental demands they were under (Brodribb et al.,2010).

The next development of the vascular system in lignophytes is the development of tracheids in gymnosperms and angiosperms, and furthermore the development of the vessel in angiosperms. These two evolved to replace hydroids (a more simple vascular cell type employed in bryophytes) in response to a greater need for water, and decreasing CO2 levels in the Carboniferous and Permian periods (Sperry, 2003).

Tracheids-in gymnosperms- play a role both in the transport of water, but also in providing strength and structure for the homoxylous wood. Because of this support function, they are shorter, but also narrower than vessels. Sperry et al., (2003), proposes that being shortened is not a big disadvantage, as end wall resistance is less severe, and therefore water conductivity is still efficient. However, in terms of being narrow, although tracheids allow for more cavity resistance in water stressed environments, it is ultimately disadvantageous for the vascular system, as it creates greater hydraulic resistance, thus allowing less conductivity than a wider conduit would allow. This decreases in water conductance further leads to less photosynthetic efficiency (Sperry et al., 2003).

Angiosperms on the other hand, employ both tracheids and vessels, therefore are at an immediate advantage. Vessels are not restricted in their size, and are therefore much longer and wider (have a bigger lumen diameter) than tracheids. This is partly to-do with the fact that angiosperms have separate lignified fibers for the support of their homoxylous wood. This decreases the demand on the vessel to provide strength to plant structure, and thus vessels are able to markedly increase both their length and circumference. Although both these factors are advantageous for water conductivity, the maximum length of vessels is limited to prevent end wall resistance which slows water conductance. Wider vessels are also more prone to cavitation and implosion which is devastating for the system. These factors explain the fact that, if angiosperms are in a environmentally stable environment with adequate water supply, they will have very efficient hydraulic flow, with up to 60 percent higher hydraulic conductance per leaf area than typical gymnospermous plants, such as conifers (Lusk et al., 2003). However, in water stressed environments, gymnosperms with their narrow tracheids, are less prone to cavitation. They will therefore be more successful in water stressed, freezing, and generally very dry environments- which is typically observed, with gymnosperms being successful in extremities  (Sperry et al., 2003; 2006).

Despite the success of gymnosperms in extreme environments, angiosperms have a capacity for growth which is unparalleled by any other terrestrial plants. Such impressive rates of photosynthesis make up the foundation of most land ecosystems. Apart from vessels providing immense hydraulic conductivity, what gives angiosperms a competitive edge is still debated. Brodribb et al., 2010, suggest that a rapid expansion (3-4 fold) in the leaf vein network allowed for angiosperms to achieve even more efficient water supply to leaves, and therefore further increase photosynthetic capacity. This increase in veination occurred 140-100 mya, and meant that angiosperms leaves transformed from simple, relatively inefficient structures, into photosynthetic powerhouses, allowing them to spread and diversify, dominating their competitors (Brodribb et al., 2010).

Another major advantage for angiosperms is that they can grow incredibly quickly in the juvenile stage, thus giving them early access to sunlight, and further outcompeting their slower growing gymnospermous counterparts (conifer juveniles) (Bond, 1989). This rapid growth leads to suppression of conifer seedlings at sites where the two live simultaneously, a major advantage for the angiosperms in such circumstances. It also allows angiosperms to dominate in early-successional stage environments (Bond, 1989). Some even argue that it is this juvenile growth success is accountable for the energy output is between gymnosperms and angiosperms, and that if leaf longevity is accounted for, conifers and angiosperms have similar net productivity in adult stages (Bond, 1989).

The important factor of varying growth rates is also explained by a difference in stomatal density and control in the two families (Lusk et al., 2003). Paleontologists have studied ancient stomatal densities in coniferous species, finding them to have changed relatively little over time. Thus while lower stomatal densities were probably advantageous in the relatively high CO2 levels of ancient times, they are now relatively inefficient in today's environmentally desaturated CO2 concentration. Angiosperms, however, having evolved later in the cretaceous, adapted to environments of lower CO2 availability, and thus have higher stomatal densities and superior controls of said stomata. This then maximised angiosperms transpiration and photosynthetic even further (Lusk et al., 2003).

Overall, it is still up for debate as to what, apart from their incredible photosynthetic efficiency, sets angiosperms apart from their competitors. It is likely to be a combination of reproductive, vascular, and morphological factors. However, some suggest that the anatomical diversity has had a major effect on angiosperm success- allowing them to develop into over 250,000 species with various forms, as opposed to gymnosperms, which currently sit at about 1000 species, with most growing in a similar structure (Brodribb et al, 2010; Isnard, 2010). Forms of angiosperms include trees, herbs, submerged aquatics, bulbs and epiphytes (Isnard, 2012). Perhaps because of the ease of survival due to angiosperm’s superior photosynthetic capacity, they are able to use their energy in developing different forms, and adapting to different environmental conditions as they see fit. For example, if an angiosperm is in a stressed environment, they can adapt their form to that of a simple herbaceous lifestyle, with a simple meristem and decreased cambial activity (Isnard, 2012).

Furthermore, contrary to previous explanations, botanists now believe that rather than developing over time from small herbaceous plants into large structures with secondary growth, angiosperms have in fact switched this order of evolution, only recently adapting to be able to live in herbaceous form, through many separate evolutions of the cambium (Isnard, 2012). Scientists have come to understand the long and intricate evolutionary history of both advancement and simplification in the cambial activity of angiosperms. For example, its advancement has allowed for vessels and support fibers, whereas decreasing activity has led to loss of the vascular cambium altogether in monocotyledons (Rowe and Speck, 2005). These changes in cambial activity have had massive effects on the diversity of life forms, plasticity, and ecological niches within different clades. It is suggested that angiosperms have been able to exploit various cambial activity levels over time to extend their own diversity. In contrast, the gymnosperms (except the Gnetales) have a vascular cambium activity level that remains relatively across species and through ecological time, thus limiting gymnosperm life forms (Rowe and Speck, 2004; 2005).

Overall, angiosperms and gymnosperms are both successful for different reasons and in different environmental circumstances. Gymnosperms are incredibly long lived, and although less successful in the juvenile stage, are capable of a large photosynthetic output in their adult form in environments of high stress and low water availability, due to their narrow tracheids. Angiosperms, while not as successful in such stressful environments, have an immense range of life forms that enable them to inhabit a large range of ecological conditions. In less stressful environments, they take advantage of long, wide vessels, and a large leaf vein network for maximum water conductivity, and numerous, well controlled stomata for increased transpiration. These factors allow them to reach immense photosynthetic output, ultimately leading to their vast expanse and success. Furthermore, different evolutions in the vascular cambium in angiosperms have allowed for varying levels of increased or decreased levels of cambial activity in different species, leading to the vast spread of growth forms they employ. This is perhaps how they were able to diversify to such immense levels, and be successful across the globe.

References

Bond, W.J. 1989. The tortoise and the hare: ecology of angiosperm dominance and gymnosperm persistence.Biological Journal of the Linnaean Society 36, 227249.

Brodribb, T. J., & Feild, T. S. (2010). Leaf hydraulic evolution led a surge in leaf photosynthetic capacity during early angiosperm diversification. Ecology letters, 13(2), 175-183.

Larson, P. R. 2012. The vascular cambium: development and structure. Springer Science & Business Media.

Isnard, S., Prosperi, J., Wanke, S., Wagner, S. T., Samain, M. S., Trueba, S., ... & Rowe, N. P. 2012. Growth form evolution in Piperales and its relevance for understanding angiosperm diversification: an integrative approach combining plant architecture, anatomy, and biomechanics. International Journal of Plant Sciences, 173(6), 610-639.

Sperry, J. S. 2003. Evolution of water transport and xylem structure. International Journal of Plant Sciences, 164(S3), S115-S127.

Sperry, J. S., Hacke, U. G., & Pittermann, J. 2006. Size and function in conifer tracheids and angiosperm vessels. American journal of botany, 93(10), 1490-1500.

Spicer, R., & Groover, A. 2010. Evolution of development of vascular cambia and secondary growth. New Phytologist, 186(3), 577-592.

Schweingruber, F. H., Börner, A., & Schulze, E. D. 2007. Atlas of woody plant stems: evolution, structure, and environmental modifications. Springer Science & Business Media.

Rowe, N., & Speck, T. 2005. Plant growth forms: an ecological and evolutionary perspective. New phytologist, 166(1), 61-72.

Rowe, N. P., & Speck, T. 2004. Hydraulics and mechanics of plants: novelty, innovation and evolution. The evolution of plant physiology, 297-325.