Phloem - II

Primary Phloem

• Protophloem• Metaphloem

Metaphloem Sunflower and Corn

Secondary Phloem in Dicots

– Sieve tube members– Companion cells– Fibers– Sclereids– Regular parenchyma

• Axial parenchyma• Ray parenchyma• Dilatation tissue – axial or ray

Secondary phloem in pumpkin stem

1 = STM; 2 = Sieve plate; 3 = Companion cell; 4 = Vascular cambium 5 = Vessel elements

1458 pound champion for 2003

Secondary phloem is short lived

• As new secondary phloem is produced, older STM and CC are crushed and may be obliterated

• This is caused by the expansion of the stem diameter during secondary growth

Secondary phloem in Tilia

Secondary phloem in Tilia

Secondary phloem in Vitis (grape)

Secondary phloem in pine stem

Tanin cells

Secondary phloem in pine radial section

Dilatation Tissue

• As stem (or root) increases in diameter, circumferential growth of the phloem must occur to keep the tissue from tearing

• Two types of dilatation tissue– Proliferative tissue: when axial parenchyma

begin to divide and expand– Expansion tissue: when ray parenchyma

begin to divide and expand producing a dilated ray

Secondary growth

Example of Dilatation Tissue

Dilated rays in Tilia show extreme amounts of dilatation tissue