What is bark ? Give its importance . Why are certain barks used in medicines ?

Explanation

Bark is the outer protective covering of stems and roots of woody plants. It is mostly made up of cork that consists of several layers of compactly arranged dead reactangular cells. The cork cells are impermeable due to deposition of suberin. They contain air, tannins, resins and alkaloids.
Importance of bark.
(i) Protective against water loss due to imperable nature of cell walls.
(ii) Protective against invasion of microorganisms due to bark's impervious nature.
(iii) Insulation from extremes of temperature , mechanical injury and browsing by animals.
(iv) Bark provides cork which is used commercially in stoppers of bottles, insulation boards, shock absorbers, linolium and sports goods.
(v) Tannin rick bark is used in dyes.
(vi) Inner bark of cinnamon tree (Cinnamomum zeylanicum) is a common aromatic spice and is also to lace certain beverages and candies/chocolates.
Bark in medicines
(i) Quinine, an antimalaria drug is obtained from the bark of Cinchona tree.(Note. Quinine is a bitter toxic alkaloid which is formerly used as a tonix. Now this drug is not used in the treatment of malaria.)
(ii) Taxol , an expensive anti-cancer drug, is obtained from bark of yew tree (Box. 3.10). Medicinal barks contains alkaloids.
Taxol is a wonder drug for treating cancers other than blood cancer. It is used mainly against ovarian cancers. US cell biologist found in 1979 that taxol killed cancer cells by stabilizing the spindle fibres (i.e., by stoping cell division ). Taxol drug is obtained originally from bark of Pacific Yew tree (Taxus brevifolia) and discovered by Indian born biochemist M.C. Wani and M. Wall in 1964 . The coniferous tree. T. brevifolia is the only source of this anti-cancer drug and this plant is not found outside America. A kilogram of bark of yew tree yields only 50-150 milligrams of taxol. This most expensive (2 to 3 lac US dollar per kg) anti-cancer drug has also been artificially synthesized in the laboratory (Jain et al., 2012).