First, prop roots are modified for firm anchorage and support to absorb sunlight for photosynthesis efficiently. Prop roots are adventitious roots that emerge vertically downwards to prevent plant collaspe by gravity or wet slippery soils, resembling a "pitchfork" (Batzer & Sharitz, 2006). This is absent in a typical root, although it has a similar function. An example is Rhizopora mangle, a species of red mangrove. Its roots have secondary thickening and differentiation of vascular tissues for strong resistance and protection along with high composition of storage parenchyma for starch storage. Aerial and unsubmerged prop roots were found to have thinner xylem to prevent cavitation (Greig & Mauseth, 1991).
Another form of root plasticity is buttress roots which are swollen flanks which penetrate into soil for firm anchorage and stability. They have secondary thickening of vascular tissues and high 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 root like a pencil (Tomlinson, 2004). This facilitates oxygen uptake to underground roots in saline and flooded environments. They have high compositions of aerenchyma and parenchyma within enlarged cortex which 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. These roots have numerous lenticels on root surfaces acting as pores to absorb oxygen which is a unique feature absent in typical roots. Also, continuously dividing phelloderm and cork cambial cells causes secondary thickening and elongation at apical meristem (Dawes, 1998). One interesting point is that pneumatophores of mangroves have no root hairs which differs from a typical root thus endodermis layer absorbs nutrients effectively (Tomlinson, 2004).
Aerial roots are adventitious and protrude down as they emerge from epiphyte stems. This is seen in most orchids and an example is chistra parishii. Their roots have a unique velamen tissue which is multiple layers of epidermis with thick-walled dead cells for water uptake and transport to root cortex under environments with low nutrients and water (Mishra, 2009). In addition, it also assists the epiphyte attachment to host (Dickison, 2000). They possess higher compositions of storage parenchyma and extensive xylem vessels. Parenchyma cells also contain chloroplasts for photosynthesis.
Haustorical roots are found in parasitic epiphytes to attach 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 penetrate tissues of host to absorb nutrients and water with a 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 xylem tracheids of host. 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 of starch grains, oil droplets, resins, tannins 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 cells developed from secondary xylem and phloem in cortex for food storage (Mishra, 2009). It also has more cambium cells which forms enlarged xylem tissues to facilitate water and mineral uptake (Biology Online, 2005).
Water storage roots specialize in storing water for plants in arid and xerophytic environments. An example is Adrenium obesum (desert rose). Extensive root systems are shallow for optimal water absorption (Dickison, 2000). It has high composition of parenchyma cells 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).
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Contractile roots are broad shortened roots which pull perennial herbaceous plants deep underground to hide food resources from predators (Reyneke & Van Der Schijff, 1974). Examples include corms and bulbs. It consists primarily of contractile parenchyma which contracts causing cell walls of endodermis, exodermis, endodermis, phloem, periderm and pith to be pressured longitudinally (Mishra, 2009). This causes increases in width and decreases in 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, vascular and meristematic tissues, with exceptions to serve primary root functions such as mineral and water absorption. Nonetheless, the statement is valid.