Supplement - Roots

Roots are the principal water-absorbing organs of a plant. They are present on essentially all vascular plants. In fact, a root, by definition, must have vascular tissues, i.e., water conduits in xylem and sugar conduits in phloem, arranged in a particular way ("exarch"). Much thinner, threadlike rhizoids (means "root-like") are present on the nonvascular plants, such as mosses and liverworts, and on gametophytes of vascular plants without seeds, such as ferns, horsetails, and club mosses. Rhizoids also absorb water but totally lack vascular tissues.

There are three primary functions of roots: (1) to anchor the plant to a substrate, (2) to absorb water and dissolved minerals, and (3) to store food reserves. Typically we see roots in soil, but there are specialized types of aerial roots (air roots) that enable climbing plants and epiphytes to become attached to rocks, bark, and other nonsoil substrates. In addition, parasitic plants may form specialized haustorial roots that form an attachment disc to the host during the first stage of colonization. To absorb water and dissolved minerals, a young sector of a root commonly possesses numerous single-celled projections called root hairs, which greatly increase the absorbing surface of the root and achieve much greater contact with soil particles. Water uptake into the young root is rapid because there is little resistance through the outer cell walls, and in general these walls contain virtually no water-repellent wax (cutin). Both young and old roots can be important repositories for carbohydrates, usually in the form of starch grains located in root cortex, but in addition older roots may store massive quantities of starch and even become specialized below-ground storage organs. Storage of carbohydrates in roots and other below-ground plant organs is an important plant strategy for surviving stress and dormancy, just as certain mammals store extra fuel as fat for winter.

The radicle is the initial root of a plant, the one that is generally present on the embryo within the seed. This forms the primary root of a young plant. In certain lineages, the embryo is so tiny and immature, such as in microseeds of orchids (Family Orchidaceae), that a radicle is not present.
There are several possible fates of the primary root.

     In gymnosperms and dicotyledons, the primary root commonly grows to become a thick central root, the taproot, which may or may not have thick lateral roots (branches). This structural organization is frequently termed a taproot system. 

    In monocotyledons, the radicle is very short-lived, and before it dies other adventitious roots have already originated from shoot or mesocotyl tissue to become the new root system, called a fibrous root system

Adventitious roots: adventitious roots arise out of stems. In some plants leaves can also be encouraged to form adventitious roots. Examples: adventitious roots of a palm; of a Canary Island date palm; specialized adventitious roots of an epiphytic orchid; of an aquatic plant that has unattached roots in moving water] Certain "root crops" that botanically are below-ground shoots, such as tubers, bulbs, rhizomes, and corms, form adventitious roots when planted in soil.

Vegetative reproduction (apomixis) of cacti and other succulent plants is also achieved largely by rooting either stems or leaves using methods to stimulate adventitious root formation.

Specialized Variations of Roots:
•    Nodal roots: adventitious roots that form characteristically in rings from stem tissues around a node.
•    Aerial roots: roots that are formed in and exposed to air, e.g., by epiphytes and hemiepiphytes; in some species, aerial roots grow downward from the tropical tree canopy toward the ground as extremely long, unbranched roots.
•    Prop or stilt roots: adventitious roots that develop on a trunk or lower branch that begin as aerial roots (another example; reaching for the water) but eventually grow into a substrate of some type; these roots in some cases seem to provide mechanical support, having either good compression or tensile properties to help support trees at their bases.
•    Buttress or tabular roots: vertically flattened roots that project out of the ground and lower trunk at the base of large trees. Models have suggested how these buttresses provide additional tensile forces to resist uprooting of large tropical trees.
•    Contractile roots: roots that become shortened in length (shrivel or shrink in length) and thereby draw the plant or plant part downward into the soil profile; many examples can be found among bulbous plants.
•    Pneumatophores: spongy, aerial roots of marsh or swamps, such as in mangal (mangroves), where roots are present in waterlogged soils and cannot obtain enough oxygen for maintaining healthy tissues. Here, pneumatophores are "breathing roots" that are emergent, and they have special air channels (lenticels) for gas exchange in the atmosphere (air enters at zones called "pneumathodes") and there is an internal pathway for getting O2 into the root and to supply submerged roots. The aerial loop of a mangrove root is sometimes called a "knee" or "peg root," but it is not clear that knees are necessarily breathing roots.
•    Caudex or lignotuber: a taproot that has fused with the stem may become woody. Lignotubers often occur in seasonally dry or fire-prone habitats, and the plants appear to use this strategy to recover from dormancy or fire.
•    Haustorial root: the root of particular parasitic plants that become cemented to the host axis via a sticky attachment disc before the root or sinker intrudes into the tissues of the host.
•    Strangling roots: the special name for roots of strangling figs (Ficus), which are primary hemiepiphytes that begin life as tropical epiphytes in trees and send down adventitious roots that become rooted in the soil. The roots surround the host trunk, eventually strangling the bark and killing the host tree.
•    Root tubers: swollen portions of a root that can have buds to produce new shoots; when broken off, these can grow into a new plant, so this is a form of cloning. In the older literature, these were sometimes referred to as fascicled roots.

*note some questions in the exam maybe found here