Anatomy of angiosperms: Lenticels and rhytidome

Anatomy of angiosperms: Lenticels and rhytidome

• 1.
  LENTICELS
• 2.
  •It is aloosely packed mass of cells in the bark of a woody plant,visible on the surface of a stem as a raised powdery spot through which gaseous exchange occurs. •Lenticels are first formed immediately beneath the stomata or group of stomata and therefore the number of lenticels depends upon the number of stomata or group of stomata.
• 3.
  Development or formationof lenticels •The parenchyma cells found near about the sub- stomatal cavity loose their chlorophyll and divide irregularly in different planes giving rise to a mass of colourless,rounded, thin-walled loose cells called complementary cells. •Such cells are also produced by phellogen towards outerside instead of cork cells. •Complementary cells increase in number due to which pressure is exerted against the epidermis and it ruptures.
• 4.
  •The thin-walled loosecomplementary cells may alternate with more dense and compact cells called the closing cells.Closing cells together form a layer called closing layer. •In spring season,the lenticels are completely filled up eith complementary cells but at the end of spring season,the lenticels becomes closed by the formation of closing layer. •Complementary cells are thin-walled,rounded and loose with sufficiently developed intercellular spaces among them.Their cell walls are not suberised.
• 6.
  Types of lenticels Thereare three structural types of lenticels are recognised in woody angiosperms. •The first type •In this type,the filling tissue is composed of suberized cells.This tissue,though,having intercellular spaces,may be more or less compact and may show annual growth layerswith thinner-walled looser tissue appearing earlier and thicker-walled more compact tissue later. •In species of Malus,Persea,Populus.
• 7.
  •The second type •Itconsist of a mass of a mass of more or less loosely structured non-suberized filling tissue. •At the end of season the filing tissue is succeeded by a more compact layer of suberized cells. •In species of Quercus,Tilia. •The third type •The filling tissue is layered because loose non-suberized tissue regularly alternates with compact suberized tissue.The compact tissue forms the closing layer.
• 8.
  •The closing layersare broken by the new growth.
• 9.
  Functions of lenticels •Thelenticels perform the function of gaseous exchange during nights or when the stomata are closed. •A little transpiration may occur through lenticels(0.01%).This is called lenticular transpiration.
• 10.
  RHYTIDOME
• 11.
  •In most ofplants,as soon as the first phellogen ceases to function,second phellogen develops below the first one.In this way,additional layers of periderm are formed in the deeper regions of the stem. •The phellogen layers may exceed even upto phloem. •As the phellogen arises in deeper regions and cuts cork cells(phellem) towards outside,all the living cells outside the phellogen do not get water supply and nutrients and become dead. •These dead cells outside the phellogen constitute the rhytidome.
• 13.
  •The manner inwhich periderm originate possesses a characteristic effect upon the appearance of the rhytidome. •When the sequent periderms develop as overlapping scale-like layers then the outer bark is termed as scale bark. •If the phellogen arise around the whole circumference of the stem,a ring bark is formed.