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Chapter: Introduction to Botany: Tissues and organs; How the Plant is built

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Morphology of the Root

There are two types of root systems. The first is a fibrous root system which has multiple big roots that branch and form a dense mass which does not have a visible primary root (“grass-like”).

The Root

 

 

Root is a latest evolutionary innovation in the vegetative plant anatomy. Many “primitive” plants (all mosses and even some ferns like Psilotum) do not have roots; some flowering water plants like the> rootless duckweed (Wolffia) or the coontail (Ceratophyllum) have also reduced their roots. However, large homoio-hydric plants need the constant supply of water and minerals, and this evolu-tionary challenge was responded with appearance of the root system.

 

Root inan axial organ of plant with geotropic growth. One of root functions isto supply anchorage of the plant body in soil or on various surfaces. Other functions include water and mineral absorption and transport, food storage, and communication with other plants.

 

Morphology of the Root

 

There are two types of root systems. The first is a fibrous root system which has multiple big roots that branch and form a dense mass which does not have a visible primary root (“grass-like”). The other is the tap root system which has one main root that has branching into lateral roots (“carrot-like”).

 

Along with having different systems, there are different types of roots: primaryroot originated from the root of the seedling, secondary (lateral) roots origi-nate from the primary roots, and adventitious roots originate on stems (some-times also on leaves), the example are prop roots of screw pine (Pandanus).

 

Roots employ many different modifications which help to protect, interact and storage. For example, roots of parasitic plants are modified into haustoria which sink themselves into the vascular tissue of a host plant and live off of the host plant’s water and nutrients.

 

Roots of mangroves (plants growing in ocean coastal swamps) are frequently modified into supportive aerial roots (“legs”). Since these swamp plants need oxygen to allow cell respiration in underground parts, there are pneumatophores, specialized roots which grow upward (!) and passively catch the air via multi-ple pores. Plants which grow on sand (psammophytes, see above) have another problem: their substrate constantly disappears. To avoid this, plants developed contractile roots which may shorten and pull plant body deeper into the sand.

 

Root nodules present on the roots of nitrogen-fixing plants, they contain dia-zotrophic bacteria capable to deoxidize athmospheric nitrogen into ammonia:N2 ! NH3. Root nodules contain also hemoglobin-like proteins which facilitate nitrogen fixation by keeping oxygen concentration low. Nitrogen-fixing plants are especially frequent amongfaboid rosids: legumes (Leguminosae family) and many other genera (like alder, Alnus, or Shepherdia, buffaloberry) have root nod-ules with bacteria. Some other plants (mosquito fern, Azolla and dinosaur plant, Gunnera) employ cyanobacteria for the same purpose.

 

Mycorrhiza is a root modification started when fungus penetrates root andmakes it more efficient in mineral and water absorption: it will exchange these for organic compounds. In addition to mycorrhizal fungi, endophytic fungi inhabit other plant organs and tissues.

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