Yogy's biology

06 Plant Water Relation - part 13 - Structure of stomatal apparatus Structure of stomatal apparatus :  Typical stomatal apparatus consists of two guard cells -  Stoma  Accessory cells. 1. Stomata  : Are minute, elliptical pores bounded by two kidney/ dumbbell shaped  guard cells.  Is a type of epidermal tissue which may be called as  modified, epidermal parenchyma cell.   Are  kidney shaped in dicotyledons  Dumbbell-shaped in grasses. In  Cyperus , both kidney- and dumb bellshaped guard cells are present.  Guard cells Are living, nucleated cells with unevenly thick walls.  Inner wall  : thick and inelastic; Lateral wall  is thin and elastic.  Contain  few chloroplasts  which are capable of poor photosynthesis.  Guard cells have ability to change their size and form due to which  stoma   opens (widens) or closes (narrows).  Stoma Is an  elliptical pore formed due to specific arrangement of guard cells.  It is through the stoma, excess water is lost in the form of vapour.  2. Accessory c

06 Plant Water Relation - part 12 - Transpiration Transpiration :  Plants absorb water constantly and continously. Hardly  5% of the total water absorbed by roots that is utilised for  cell expansion and plant growth.  Remaining  95% water becomes surplus  which is then lost into the atmosphere, through its aerial parts.  Hardly  1% of surplus water is lost in the form of liquid  and  99% of surplus water, is lost in the form of vapour.  The loss of water in the form of liquid is called  guttation . It occurs through special structures called water  stomata or hydathodes.  The loss of water in the form of vapour is called  transpiration  that occurs through  leaves, stem, flowers and fruits.  Most of the transpiration occurs through the  leaves  (called  foliar transpiration ).  The actual water loss during transpiration occurs through three main sites -  cuticle, stomata and lenticels.  Accordingly, three types of transpiration are recognized viz -  Cuticular Stomatal and  Lenticular.

06 Plant Water Relation - part 11 - Transport of food Transport of food :  All the plant parts require  continous supply of food  for nutrition and developement.  In higher plants, there is a great differentiation and division of labour.  Chloroplasts  are confined to green cells of leaves where food is synthesized.  The non-green parts like  root and stem  must received food from leaves.  The part where food is synthesized is called  source  and while part where it is utilized, is called  sink.  Food has to travel from  source to sink.  This movement of food from one part to the other part, is called  translocation of food.  Path of translocation :  Food is to be translocated to  longer distance s in higher plants. Hence plants must have adequate channels for the transport of food.  Sieve tubes and vessels  are structurally ideally suited for longitudinal (vertical) translocation.  The ringing experiment structure and distribution of phloem chemical analysis of phloem sap and  use of

06 Plant Water Relation - part 10 - Transport of mineral ions Transport of mineral ions :  Soil serves as  main source for minerals.   Constitute most commonly occuring  solid, crystalline inorganic materials  obtained from earth's crust.  Play an important role in the day to day life of plant.  Are absorbed by plants in the  ionic (disolved) form,  mainly through roots and then transported. The analysis of  plant ash  demonstrates that minerals are absorbed by plants from soil and surroundings.  Absorption of minerals is  independent of that of water.  Absorbed mineral ions are pulled in  upward direction  along with  xylem sap  because of transpiration pull. This could be understood when the  ascending sap  is analysed.  Mineral ions are needed in the areas of the plant viz.  apical, lateral, young leaves, developing flowers, fruits, seeds and storage organs.  Hence, from the source (root), these are pulled and transported  ascendingly through the sap and gets unloaded by fine ve

06 Plant Water Relation - part 09 - Translocation of water Translocation of water:  The transport of water with  dissolved minerals from root to other aerial parts  like stem and leaves, against the gravity, is called  translocation or ascent of sap.  Translocation of water occurs through the lumen of conducting elements of  xylemtracheids and vessels,  in all vascular plants.  Ringing experiment has proved that  xylem is the path of ascent of sap .  Several mechanisms/ theories have been put forth to explain the mechanism of translocation of water.  The theories include -  Vital force theory Relay pump theory Physical force theory Root pressure theory,  etc.  We shall consider following three theories :  1. Root Pressure Theory (Vital Theory) :  According to this theory, the activity of  living cells of root  is responsible for translocation of water.  J. Pristley proposed this theory.  When a stem of potted plant is cut few inches above the soil by a sharp knife, xylem sap is seen fl

06 Plant Water Relation - part 08 - Mechanism of absorption of water Mechanism of absorption of water :  Mainly, there are two ways/ modes of absorption of water viz -  Passive absorption and  Active absorption . 1. Passive absorption :  It is the  main way of absorbing water through the roots  and not by the roots from soil into the plant.  The  driving force is transpiration pull  and it thus proceeds through  DPD gradient.  There is  no expenditure of energy (ATP)  as water moves in accordance to the concentration gradient. Hence, it is  passive absorption.  About  98%  of the total water absorbed in plants, occur  passively.  Passive absorption occurs during  day time  when transpiration is in progress.  It  stops at night  when transpiration stops.  Rapid transpiration  creates a  tension in the xylem vessel  due to  negative water potential. This tension is transmitted to xylem in the roots.  Consequently water is pulled upwards passively.  During passive absorption,  no ATP is u

06 Plant Water Relation - part 07 - Path of water across the root Path of water across the root (i.e. from epiblema upto xylem in the stelar region) :  Water is absorped by  root hair cell  through  imbibition diffusion osmosis , sequentially.  Consequently the cell becomes  turgid.   Its turgar pressure increases, but its DPD value decreases.  However, the immidiately adjacent cortical cell inner to it, has more DPD value , because its O. P. is more.  Therefore,  cortical cell will suck water from the turgid root hair cell.  It then becomes turgid.  The  flaccid root hair cell  now absorbs water from soil.  Water from the turgid cortical cell is sucked by inner cortical cell and the process goes on.  Thus, a  gradient of suction pressure (DPD)  is devloped from  cells of epiblema  to the cortex of the root.  Consequently water moves rapidly across the root through loosely arranged living cells of cortex, followed by passage  cells of endodermis and finally into the  cell of pericycle.