1. PHLOEM

2. 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

3. Classification
 Primary phloem- from procambium
 Externalphloem 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

4. Vascular bundle in Zea mays

5. a ma
Cross section of Cucurbita maxima stem

6. Portion of the cross section of Tilia stem

7. Elements of the phloem
 Sieve elements
 Companion cells
 Phloem parenchyma
 Axialparenchyma
 Ray parenchyma
 Phloem fibers

9. 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

10. 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

12. 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

13. 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

15. Simple perforation plate (magnified view)

17. 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.

18. 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.

24. 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

25. 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

26. Phylogenetic specialization in sieve tube elements

27. 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

30. 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

31. 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

32. 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

33. 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

34. 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

35. 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

36. Secondary phloem
 Have 5 components
 Axial system
 Sieve tube elements
 Companion cells
 Axial parenchyma
 Phloem fibers
 Radial system
 Phloem ray

37. Tilia stem, portion of a cross section