Phloem

Aug 09, 2022, 16:45 IST

complex permanent tissues

The permanent tissues whose cells are heterogeneous i.e., cells having dissimilar origin and structure but performing common functions, is called complex tissue.A complex tissue is a collection of different types of cells which forms a structural unit and perform a specific function. Xylem and phloem, present in all vascular plants, are complex tissues.

About Phloem

Like xylem, phloem (Nageli, 1858 term leptom for phloem by Haberlandt, 1914) is also a specialized complex tissue mainly responsible for the translocation of food materials. The basic components of phloem are:Sieve Elements

The translocating elements of phloem are collectively known as sieve elements. There are two kinds of sieve elements: (i) sieve cells and (ii) sieve tube elements or sieve tubes. Sieve cells, which occur in pteridophytes and gymnosperms, are single cells with specialized sieve areas usually in their lateral walls.Sieve tubes, which occur in angiosperms, are longitudinal file of cells (sieve tube components) with more specialized sieve area (sieve plate) usually in or near end wall.The most characteristic feature of sieve tubes is the presence of thickened cross walls between two adjacent sieve tube members, which are known as sieve plates. A sieve plate has a large number of perforations.There are two types of sieve plates, simple and compound. Simple sieve plates are transversely placed and consists of sieve area while the compound sieve plates are somewhat obliquely inclined with several sieve areas. Long sieve tube elements usually have a compound sieve plate, whereas the short elements have simple sieve plates.Degeneration of nucleus starts during differentiation of sieve elements. In mature sieve elements, there is a thin layer of parietal cytoplasm and a large central vacuole. A slimy substance of proteinaceous nature is also present. The slime originates in the cytoplasm in the form of discrete bodies, known as slime bodies, that occur either singly or in multiples and are variously shaped. In young sieve elements, these bodies consists of aggregates of tubules, designated as P-proteins.During the maturation of sieve elements, these tubules are dispersed in the cytoplasm to form groups of striated fibrils, designated as P2 proteins. The walls of sieve elements vary in thickness but they are usually thicker than those of the neighbouring parenchyma cells. Sieve elements usually have primary walls which consist mainly of cellulose.However, sometimes special thickenings, known as nacreous thickenings also develop. These thickenings have glistening properties and sometimes they almost occlude the cell lumen. Non functional sieve tubes may become filled with tylosoids (proliferations from contiguous parenchyma cells) in some plants.

Companion Cells

With the sieve tube elements of angiosperms, highly specialized parenchyma cells are associated which are called companion cells.Normally, only one companion cell is associated with each sieve tube cell in the primary phloem of herbaceous plants. But in the secondary phloem of woody plants many short companion cells are associated with each sieve tube member.They are living cells having dense granular cytoplasm, a prominent elongated nucleus and numerous small vacuoles.Companion cells, along with phloem parenchyma, play an important role in the maintenance of a pressure gradient in the sieve tube. They form a link between sieve tube members and other cells and control the passage of materials.

Phloem Parenchyma

They are typical parenchyma cells found associated with sieve tube cells. The parenchyma cells of the primary phloem are longitudinally oriented like sieve elements in the vascular tissue.

In the secondary phloem, they occur as two systems, the vertical and horizontal systems. The vertical system runs longitudinally along with the sieve elements and the horizontal system forms phloem rays that run horizontally across the organ.The phloem parenchyma cells are mainly concerned with storage of starch, fat and other organic food materials, accumulation of tannins, resins, crystals, mucilage, etc. and lateral transport of soluble food materials and water. Phloem parenchyma is absent in monocots.

Phloem Fibres or Bast Fibres

They are typically elongated cells with interlocked ends and lignified walls having simple pits.

They differ from xylem fibres in having small simple pits.

Besides providing support, phloem fibres also play a role in the transport of food materials.

Phloem fibres of plants like jute, flax and hemp are retted in water and extracted for making ropes and coarse textiles.

Phloem Transfer Cells

The phloem cells which are responsible for short distance transport of metabolites are called phloem transfer cells. These cells are characterized by wall ingrowths and a cytoplasm rich in organelles. They have a specialized type of non lignified secondary cell wall deposited on the inner side of the primary wall. Plasmodesmata usually occur between the two adjacent transfer cells.