Root

Radicle develops first, it break through the seed-coat and forms the first organ of the plant, the root. The embryonic root grows into root system which may include a main root branching repeatedly (taproot) and/or adventitious roots arising from the stem (fibrous root system). Roots anchor plant and absorb water and nutrients dissolved in it to transport them to other organs. Adventitious roots are underground or aerial organs that arise regularly from the base or the upper part of the stem but they might be induced irregularly in unexpected positions by external factors (eg. loss of radication, cuttings). They are of endogenous origin, from the endodermis or starch sheet of the stem, and pass the cortex to achieve its surface. Adventitious roots may supplement primary root system for absorption or specify for other functions.

MORPHOLOGY

Roots of plantsA. Taproot. B. Fibrous root. C. Adventitious root. D. Adventitious roots of cuts (cut on left, cut with roots after a few weeks on right).

The growth of primary root is generally orthotropic while lateral roots grow plagiotropically. Geotropism of adven-titious roots depends on their function.

In connection with extreme conditions, appearance and structure of roots might be changed and their basic functions strongly altered. They often become a thickened storage organ. Though almost all roots store some starch and other stored material, habit of roots modified for storage is typical with enlarged diameter because of the large amount of parenchymatic ground tissue composed of thin walled cells filled with storage material. These storage roots (swollen root, root tuber, taproot, etc.) may function for long time ensuring survival. Water content of these organs is essential at dry conditions (hot summer of continental climate; dry season of subtropical areas; semi-arid environment) to survive. Geophytes of the temperate zone have similar roots (eg.Ficaria verna) not because of the dry conditions but to grow quickly in springtime to avoid shade of other species of the vegetation. Apart of storage stem tubers and cereals, storage roots are elementary part of human diet in many countries.

Contractile roots help to achieve the proper positioning of underground bulbs and corms (eg.Lilium). Plants are able to regulate in this manner their depth according to the season.

Roots of some species can serve for vegetative reproduction (Robinia,Cerasus) especially in case of shoot loss or mechanical injury of root. From the adventitious buds initiated on the root complete plants regenerate.

Roots with haustorial function (suckers) enable parasites and hemi-parasites to use substances of the host plant.

Haustoria of the parasitic partner (Cuscuta,Orobanche) enter in the root or stem of host and reaching its phloem MORPHOLOGY

they take organic compounds from there. Hemi-parasite plants (Viscum,Loranthus) contain chloroplasts and they are able to photosynthesize, so these plants transport water from the xylem of host.

Beside roots growing in the soil a ray of modified roots are aerial roots specialized for different functions.

Aerial roots can improve the water and nutrient supply of a tall plant. This type of root arises from the upper nodal or internodial parts of the climbing stem. They reach the soil and are thin to prop the stem but rooting in the soil they transfer water for transpiration (Monstera).

Climbing roots arise from the stem and allow the plant with (at least at the beginning) feeble and thin stem to grow high to reach more light. These plants use supports or other plants in a non-invasive manner; they do not injure them directly (Hedera).

Tall plants should have additional support to guarantee the proper position of their body. Stilt roots arise from the lower nodes, get to the soil and anchor the shoot (Zea). Pop roots arise from the upper part of shoot, often from branches, and support the shoot standing against the ground usually rooting in it later (Pandanus,Ficus bengaliensis).

A special kind of support is provided by buttresses for the tropic giant trees (kapok tree). These roots grow in dif-ferent directions on the soil surface and form high, vertically flattened mechanical support to prevent to come down the high and heavy trunk.

Root thorns are strongly sclerenchymatized roots arising on the stem above the ground to save the plant against herbivores. The hard and sharp thorns may also save properties effectively if the plants are grown by people as a hedge. Thorns were used also for other purposes in different tools.

In swamps and marches soil is flooded and oxygen content is very low in it. Respiratory roots grow upward and has specialized root epidermis allowing gas exchange (Taxodium,Rhizophora). Large intercellular spaces and ducts form channels to ventilate the internal space. It permits to conduct oxygen to the cells of the underground parts.

Epiphytes (eg. epiphytic orchids) are usually completely separated from the ground and live in ”nests” fare above the soil surface. Their hygroscopic roots absorb water from the atmosphere (moisture, rain) with their specialized dermal tissue (velamen). Specialized and died cells (in fully differentiated stage) of velamen with reticulate cell wall thickening absorb the water and transmit it to the cells of cortex. From there it is transported to other organs.

Cortex is also important in photosynthesis of the epiphytic orchid plant. Its living cells contain chloroplasts and they are active in synthesis. They also store part of the synthetized organic material in starch.

Roots have got in touch with microorganisms living in the soil evolutionary very early. Fungi and bacteria formed different interrelations with their plant partners and some of these interactions were conserved as a stabile morpho-logical formation. (Close ties with algae, liverworts and mosses were assembled before the history of vascular plants: lichens, mycothallic forms, cyanobacteria and liverwort connections.) The main types of symbiotic relations between microorganisms and roots are the next:

Mycorrhiza: Tight metabolic and structural interrelation (thought to be mutualistic) between hyphae and roots, especially the tip part of the last one. Plant is able to absorb more efficiently the water and nutrients (eg. phosphorus) through the fungal partner from the soil while fungus gets organic compounds from the root. There are different types of mycorhhizal interaction but here only two of them, the ecto- and the endomycorrhiza will be mentioned.

In case of ectomycorhhiza, hyphae grow on the surface of the root (mantle) and between the cortical cells (Hartig-net) but do not enter into the cells. Elongation and ramification pattern of root is changed. Strategy of endomycor-rhizal fungus is quite different. Hyphae break through the wall of the cortex cells and grow into them pushing the plasma membrane inside. Important to see, that the entity of cell is not diminished by the fungus. There is an interface created by the partners located between the fungal wall and plasma membrane of plant but cellular integrity of plant cell is not destroyed.

Nitrogen is essential for plants and its availability is fundamental. The best solution is to be near to organisms which are able to fix the atmospheric nitrogen. Some prokaryotes form morphologically stable structures with roots for nitrogen fixation.

MORPHOLOGY

Rhizobiumbacteria induce cell divisions in the cortex of leguminosae roots and as a result of it, root nodules develop on them. Bacteria are engulfed into the plant cells and become bacteroids. In this form they coexist with the plant, they get sugars from the plant partner and transport fixed nitrogen into it.

Actinorrhiza is similar in its metabolism and structure but the partners areFrankia(Actinobacteria) and plants from other families (eg.Ulmus).

Cycads live together with photosynthetic cyanobacteria. The metabolic scenario is very similar as above but there is a big difference: cyanobacteria need light for their photosynthesis. Coralloid roots of cycads grow upward and cyanobacteria living in the cortex of the root can get light.