First, prop roots are modified for firm anchorage and support to absorb sunlight for photosynthesis. They are adventitious and 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. Aerial prop roots have thinner xylem to prevent cavitation (Greig & Mauseth, 1991).
Buttress roots are swollen flanks which penetrate 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). These roots also prevent growth of competitors by taking up space so tree has higher availability of sunlight and water.
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Pneumatophores are vertical aerial roots which follow negative geotropism, thus extending upwards from secondary roots like a pencil (Tomlinson, 2004). This 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 as pores for oxygen absorption. This is a unique feature absent in typical roots. Also, secondary thickening and elongation at apical meristem is caused by continuous dividing phelloderm and cork cambium (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 multiple epidermis layers with thick-walled dead cells for water uptake and transport in environments with low nutrients and water (Mishra, 2009). It also assists epiphyte attachment 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 bark or branch. 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). Phloem elements, sclerenchyma, root cap, root apical meristem and cortex may be absent to allow thin-walled parenchyma to contact host xylem tracheids. These contribute to their chisel-like wedge shape. Thus, such epiphytes may have difficulties in growth even with rich nutrients in soil (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 humidity and temperature is stable thus enabling functional storage cells and inhibiting 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 in cortex 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 arid environments. An example is Adrenium obesum (desert rose). Extensive root systems are shallow for optimal water absorption (Dickison, 2000). It has higher composition of parenchyma at cortex and larger xylem vessels containing tracheids for efficient water absorption and uptake (Nobel, 2002). Proline present in these roots can detect osmotic imbalance during droughts or excess nitrogen. In addition, Adrenium obesum has latex which is highly toxic to prevent herbivore consumption (Ng et al., 2011).
Contractile roots are broad shortened roots which keep perennial herbaceous plants underground to hide food resources 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 causes increases in width and decreases in length.
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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.