PHLOEM

Phloem   Food conducting tissue of vascular plants   Complex tissue   Substitute term in German – leptom

 soft-walled conducting part of the phloem   sieve elements   companion cells   parenchyma

  Parallel term for the xylem is hadrom  for the conducting elements

  tracheary elements  parenchyma

Classification

 Primary phloem- from procambium

 External phloem or abaxial phloem – external to the xylem

 Internal phloem or adaxial phloem – internal to the xylem

 initiated later than the external

 Secondary phloem – from the vascular cambium

Vascular bundle in Zea mays

Cross section of Cucurbita maxima stem

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Portion of the cross section of Tilia stem

Elements of the phloem

  Sieve elements

  Companion cells

  Phloem parenchyma

 Axial parenchyma  Ray parenchyma

  Phloem fibers

Elements of the phloem

1. Sieve elements

  A. Sieve cells   less specialized sieve elements

  B. Sieve-tube member/ sieve-tube elements   more specialized sieve elements   longitudinal series of members form the sieve tube

Elements of the phloem

Sieve cell –an element with:   a. relatively unspecialized sieve areas   b. long and slender   c. taper at their ends or have steeply inclined end

walls

Sieve tube members - sieve elements with:   a. sieve plates mainly on end walls   b. the walls of laterally adjacent sieve tubes bear

sieve areas of lower degree of specialization

Elements of the phloem

Sieve cell and sieve tube members   Differ in the degree of differentiation of their sieve

areas   Differ in the distribution of these areas on the walls

 Sieve areas and sieve plates  A sieve area appears like a depression in a wall

with a number of dots- the transection of the pore content each surrounded by a ring of callose

Elements of the phloem

 Sieve areas and sieve plates

 Sieve plates – wall areas bearing the highly specialized sieve areas; commonly on the end walls

 1. simple sieve plate – consists of a single sieve area

 2. compound sieve plate – with many sieve areas arranged in reticulate, scalariform and any other manner

Simple perforation plate (magnified view)

Elements of the phloem

Development of sieve plate

  1. The future pore site is at first occupied by a single plasmodesma.

  2. Sheets of ER and platelets of callose become localized on opposing surfaces of each pore site with the ectoplast interposed between ER and the callose

  3. The sheets and platelets increase in diameter until they become as wide as the future pores.

Elements of the phloem

Development of sieve plate

  4. Eventually, the 2 opposing platelets at each pore site fuse because of the disappearance of the original separating wall

  5. A hole appears in the middle of the fused platelets and enlarges centrifugally.  

Phylogenetic specialization in sieve tube elements

  Lower vascular plants and gymnosperms  generally have sieve cells

  angiosperms  have sieve-tube members

Trends of specialization:   1. Progressive localization of highly specialized sieve

areas on the end walls

Phylogenetic specialization in sieve tube elements

Trends of specialization:  2. A gradual change in the orientation of these end

walls from very oblique to transverse

  3. A step-wise change from compound to simple sieve plates

  4. A progressive decrease in conspicuousness of the sieve areas on the side walls

  5. An increase in the percent of transverse area occupied by the sieve-area strands appears to have occurred

Phylogenetic specialization in sieve tube elements

Differentiation of sieve tube member

  Enucleate at functional maturity   Nucleolus is extruded   Tonoplast disappears in mature sieve tube elements   ER may break up into vesicles   Mitochondria become devoid of internal membranes   Dictyosomes disappear completely   Center of the cell is filled with the mixture of vacuolar

sap and disorganized cytoplasmic matter, chiefly slime

Elements of the phloem

2. Companion cells

  Arise from the same meristematic cell as the associated sieve-tube member

  Retain its nucleus at maturity

  frequently lacking in the earliest part of the primary phloem (protophloem)

  Sieve cells of gymnosperms and lower vascular plants without companion cells; they have albuminous cells

 3. Fibers

Elements of the phloem

4. Parenchyma cells

  10 phloem parenchyma – from the procambium  Storage of starch, fat and other organic food

materials

  20 phloem parenchyma  axial phloem parenchyma –from fusiform initials

 fusiform parenchyma cells – long parenchyma cell

 parenchyma strand – a series of short cells

Elements of the phloem

  20 phloem parenchyma  ray parenchyma – from ray initials

 procumbent ray cells – elongated in the radial direction

 Upright or erect ray- vertically elongated

  in active phloem – the parenchyma are unlignified   In inactive phloem- may remain unchanged or may

become sclerified   phellogen may develop from phloem parenchyma

and ray parenchyma  

Primary phloem

Protophloem   conducting tissue of the

actively growing parts of the plant

  functions for a brief period only

  components that remain after obliteration of the sieve elements differentiate into fibers

Primary phloem

Metaphloem

  matures after the growth in length of the surrounding tissue is completed; retained longer than the protophloem

  companion cells and phloem parenchyma are typically present in dicots

  in monocots sieve tube elements and companion cells form strands containing no phloem parenchyma

Primary phloem

Metaphloem

  fibers if present in dicots originate in the protophloem not metaphloem; old metaphloem may become strongly sclerified

  in monocots, sclerenchyma encloses the bundle as sheaths and may be present in the metaphloem

Secondary phloem

  Have 5 components

  Axial system  Sieve tube elements  Companion cells  Axial parenchyma  Phloem fibers

  Radial system  Phloem ray

Tilia stem, portion of a cross section