Angiosperms are seed-producing plants that produce a fruit after fertilization with aid of stamens and carpels which enclose pollen grains and ovules respectively (Solomon et al., 2010). Through evolution, there is much diversity and modifications of angiosperm tissues such as root systems for their functions. A root is an essential plant organ which provides anchorage and stability and primarily absorbs mineral salts and water (Dickison, 2000). This essay explores if angiosperm roots are plastic, in which plasticity is ability to be variable and adaptable to plant's environment and functions by possessing modified structural differences from a typical root.
First, prop roots are modified for firm anchorage and support to absorb sunlight for photosynthesis. They adventitiously emerge vertically downwards to prevent plant collaspe by gravity or wet slippery soils, resembling a "pitchfork" (Batzer & Sharitz, 2006). This is not observed in typical roots, although it has similar functions. An example is Rhizopora mangle, a red mangrove species. Its roots have secondary thickening of vascular tissue for strong resistance and protection along with higher storage parenchyma composition for starch storage. Non-submerged prop roots have thinner xylems to prevent cavitation (Greig & Mauseth, 1991).
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Buttress roots are swollen planks which dig into soil for firm anchorage and stability. They have secondary thickening of vascular tissues and higher composition of sclerenchyma accompanied by a well-developed periderm for resistance and protection (Osborne, 2000). The windward and leeward laterals prevent tree collapse by gravity with application of tension or compression strut (Crook et al., 1997). An example is Salmalia malabarica (Soni & Soni, 2010). Growth of competitors is inhibited as these roots take up space so tree has higher availability of sunlight and water.
Pneumatophores are upward extensions from secondary roots which follow negative geotropism like a pencil (Tomlinson, 2004). They are a subtype of aerial roots and facilitates oxygen uptake to underground roots in saline and flooded environments. They have higher compositions of aerenchyma and parenchyma in enlarged cortex to provide internal air spaces for gas exchange and food storage respectively. (Mitsch et al., 2009). These roots are found in black and white mangroves. An example is Avicennia germinans. There are numerous lenticels on root surfaces acting like pores for oxygen absorption. This is a unique feature absent in typical roots. Also, secondary differentiation and apical meristem lengthening is caused by continuously dividing cork cambium and phelloderm (Dawes, 1998).
Aerial roots are adventitious and protrude down as they emerge from epiphytic stems. This is seen in orchids and an example is chistra parishii. Their roots have a unique velamen tissue which is made up of numerous epidermis layers of thick-walled non-living cells for water uptake and transport in environments with low nutrients and water (Mishra, 2009). It also assists epiphyte adhesion to host (Dickison, 2000). They possess higher compositions of storage parenchyma and extensive xylem vessels. Parenchyma also contains chloroplasts for photosynthesis.
Haustorical roots are found in parasitic epiphytes to adhere and stabilise them onto host's structure. It can have secondary functions of absorbing minerals and water and conducting photosynthesis. An example is Viscum album (mistletoe). Its roots pierce through host tissues to absorb nutrients and water with hydrostatic force (Pate and Calladine, 2000). There may be lack of phloem elements, root apical meristem, sclerenchyma, root cap and cortex to allow connections between thin-walled parenchyma and host xylem tracheids. These contribute to their chisel wedge shape. Thus, such epiphytes may have difficulties in growth even with rich soil nutrients (Mauseth, 2009).
Food storage roots functions to store starch grains, oil droplets, resins and amino acids. An example is Ipomoea batatas (sweet potato). Food storage is underground as stable humidity and temperature maintains functional storage cells and inhibits starch degradation (Mauseth, 2009). It also prevents herbivore consumption. Compared to typical roots, it has a higher composition of storage parenchyma developed from secondary vascular tissues for food storage (Mishra, 2009). It also has enlarged xylem tissues to facilitate water and mineral uptake (Biology Online, 2005).
Water storage roots specialize in storing water for plants in xerophytic and dry environments. An example is Adrenium obesum (desert rose). Shallow and extensive root systems maximize water absorption (Dickison, 2000). It has higher composition of parenchyma and larger xylem vessels containing tracheids (Nobel, 2002). Proline present in these roots can detect osmotic imbalance during droughts or excess nitrogen. In addition, Adrenium obesum possess latex which is highly toxic to prevent herbivore consumption (Ng et al., 2011).
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Contractile roots are broad shortened roots which keep perennial herbaceous plants underground so food resources remains hidden from herbivores (Reyneke & Van Der Schijff, 1974). Examples include corms and bulbs. It consists primarily of contractile parenchyma. Contraction of parenchyma causes cell walls of ground, vascular and dermal tissues (endodermis, exodermis, phloem, periderm and pith) to be pressured together (Mishra, 2009). This increases width and decreases length.
In conclusion, diverse root modifications and examples show angiosperm roots are indeed highly plastic based on tissue and cell compositions, structure and functions. Vast differences in tissue structures and compositions facilitate their respective functions efficiently. However, they still retain similarities in root structures such as presence of cortex, stele, root hairs and vascular tissues, with exceptions to serve primary root functions such as mineral and water absorption. Nonetheless, the statement is valid.