Page 72 - PG 101-Course notes مذكرة النظري 24-25 with spec
P. 72
Medicinal plants (PG 101) Level 1 Clinical Pharmacy-PharmD
b- Phloem
• Phloem is a complex tissue, composed of several different kinds of cells.
• The main phloem elements are the sieve elements which
are the principal food conducting tissue of the vascular
plants.
• They are composed of:
o Sieve tubes
o Companion cells
o Phloem parenchyma
o Phloem fiber
• They may be one or several companion cells associated with
the sieve tube.
1. Sieve cells (gymnosperm and lower vascular plants):
- Individual cells each developed from a single procambial cell.
- Long, slender at maturity without perforated end walls and nucleus.
2. Sieve tubes (angiosperm):
- Originate from a series of longitudinal procambial cells.
- The procambial cells become longer and broader and their end walls form one or
more sieve areas.
- At full maturity, the sieve tube lacks nucleus but contain cytoplasm.
- The protoplasm of adjoining cells are connected by cytoplasmic strands penetrate
through sieve pores
- Sieve element cell divided into two cells:
o Large cells: known as sieve tube.
o Small cells: companion cells.
Types of Sieve areas “sieve plates”:
- Simple: with one sieve area.
- Compound: several sieve areas.
- When sieve tubes cease to function, bands of callus formed from a
polysaccharide substance called callose deposit.
48
