Phloem: Ontogeny and Phylogeny | Plants

Phloem consists of sieve cell, sieve tube, companion cell, phloem parenchyma and fibre.

1. Sieve Cell-Origin:

The mother cells of sieve cells vary in shape. They are slender, short cylindrical to elongate with both ends tapering. Numerous primary pit fields occur on lateral walls. During differentiation the mother cells elongate.

Vacuole appears in the cytoplasm that streams actively. The cell wall increases in thickness. Sieve areas develop in the position of primary pit-fields. Cytoplasmic strands appear in the sieve areas and they gradually become prominent and increase in size. At later stage callose develops surrounding the cytoplasmic strands.

Sieve cell predominates in pteridophyte and gymnosperm. It is reported among angiosperms in Anstrobaileya scandens and Sorbus aucuparia of Rosaceae. Sieve cells are considered as primitive. They are not arranged in axial files. Moreover the structure of end walls is similar to the lateral walls.

2. Sieve Tube:

Sieve tubes are arranged in axial files. The end wall of it is the sieve plate. Sieve plate is a specialized structure and differs from the sieve areas present on the lateral wall of a sieve tube and all walls of a sieve cell.

i. Origin and development of sieve tube:

Sieve tubes are present in primary and secondary phloem and accordingly they originate from procambium and cambium respectively (Fig. 10.7). The procambium and cambium are arranged in a longitudinal file. They contain living protoplast with prominent nuclei, endoplasmic reticulum, dictyosomes, tonoplast bound vacuoles, ribosomes, mitochondria, chloroplast etc.

The mother cells divide longitudinally to form two daughter cells. One of the daughter cells forms the companion cell and the other cell develops into sieve tube member. During development the companion cell may divide transversely to form more companion cells. The differentiating sieve tube member increases in length and its protoplast undergoes a profound change.

In mature sieve tube mitochondria, plastids, P-protein, plasmalemma and some endoplasmic reticulum persist. The other cellular components degenerate during differentiation. The nucleus disappears or may persist as collapsed body. Nucleus does not degenerate in Taxus, Neptunia oleracea etc. Ribosomes and dictyosomes, though abundant in differentiating sieve tube member, are absent from them at later stage.

At early stages of differentiation the endoplasmic reticulum (ER) is of rough type due to the presence of ribosomes. Later ER becomes smooth by loosing the attached ribosomes and aggregation in parallel stacks to form cisternae. The cisternae are arranged in parallel or perpendicular to long axis of the cell wall of the sieve tube member and occupy the parietal position.

As development progresses ER diminishes in amount. Some changes also occur in mitochondria where the inner membrane disorganizes and so lacks cristae. The vacuolar membrane ruptures and as a result there lies no delimitation between cytoplasm and vacuole. After the disappearance of tonoplast the P-protein occupies the parietal position of sieve tube member and sieve plate pores.

Sieve areas originate at the sieve plate, which is the common transverse wall of the sieve tube elements. The future sieve plate, at early stages, is smooth with no sign of primary pit fields. Later there appear the plasmodesmata, which mark the site of future sieve area. A single plasmodesma occupies the future pore site. Callose deposits encircling the plasmodesma.

Callose accumulates on both sides of the cell wall and assumes the shape of platelets. A pair of callose platelets, which is interrupted at the centre where plasmodesma exists, occupies the future pore site. In between the pair of platelets there occur the primary walls of the two adjacent cells and the middle lamella.

Callose deposition continues, platelets increase in thickness, plasmodesmatal canal enlarges and a cavity is developed at the middle lamella region. At a later stage the two cell walls between the paired platelets disappear and thus a pore is formed. The end result is the full differentiation of sieve plate and the protoplast of the contiguous sieve tube elements become continuous.

ii. Phytogeny of sieve tube:

Long sieve tubes are considered as primitive. The short one indicates advanced condition. The cambial initials, during evolutionary specialization, tend to become shortened and this caused the formation of short sieve tubes.

It is frequently noted that cambial initial undergoes transverse septation before the differentiation of sieve tube. As a result short sieve tubes are formed. So, Carlquist opined that the studies on the length of sieve tube must note whether it was directly derived from the cambial initial or formed after the septation of initial.

The sieve tube that has a long end wall with numerous sieve areas is considered as primitive. Small pores in the sieve plate are regarded as primitive by Zahur (1959). Hemenway (1913) regards that the lateral sieve areas, during evolutionary specialization, become smaller and less conspicuous.

The terminal sieve areas become more specialized and form the sieve plate. The sieve plate may be compound or simple and the latter is an advanced feature. The position of a sieve plate also changed from oblique to almost transverse during phylogenetic advance.

So the primitive sieve tube has compound sieve plate with oblique position, whereas the advanced one has simple sieve plate with horizontal orientation. A significant statistical correlation was found between the compound sieve plate and plate with oblique orientation. A similar correlation was also found between simple plate and those that are horizontal in position.

Zahur noted that there exists a correlation between the length of a sieve tube and length of a sieve plate. Zahur grouped the sieve tubes into three categories based on the length of it, position of sieve plate and number of sieve areas present per sieve plate. These types were compared to other features in a statistical fashion to indicate phylogenetic advance.

The three categories are:

(1) Sieve tube long, sieve plate is oblique in position and the number of sieve areas is more than ten per plate,

(2) Sieve tube medium in length, sieve plate is oblique in position and the number of sieve areas per plate ranges between two to ten, and

(3) Sieve tube is short, sieve plate is almost transverse to transverse in position and the number of sieve area is single per plate.

Zahur concludes that in course of evolution:

(1) The length of sieve tube is decreased and this is due to either reduction in length of cambial initial or septation of the initial;

(2) The length of end walls decreased with the reduction in number of sieve areas per sieve plate;

(3) In the secondary phloem of dicots, the size of sieve areas increased, and

(4) In the functional sieve tube the presence or absence of nucleoli has no phylogenetic significance and this feature may be regarded as taxonomic criteria.

The sequence of specialization in the metaphloem of monocots seems to have occurred in the following way: roots to stems and terminating in rhizomes, corms and inflorescence axes. The least advanced sieve tubes are noted in roots and the most advanced forms are found in rhizomes, corms and inflorescence axes, whereas the intermediate forms are present in the aerial stems.

3. Companion Cell:

The companion cell and sieve tube originate from a common parent cell. It is present in the primary and secondary phloem and accordingly the parent cell originates either from procambium or cambium. The parent cell divides by unequal longitudinal division and the smaller cell develops into companion cell.

Companion cells remain associated with sieve tubes. Zahur, in the secondary phloem of dicots, distinguished three types of companion cell to indicate phylogenetic specialization. The basis of distinction is length and septation of companion cell.

The followings are the three types:

(1) Companion cell is much shorter than the accompanying sieve tube.

(2) Companion cell is almost as long as the accompanying sieve tube.

(3) Companion cell is as long as the accompanying sieve tube, but the companion cell is septate. Zahur considers that the above three types form a natural group. Zahur regards that the first type, where the companion cell is much shorter than the accompanying sieve tube, is primitive.

The other two types are advanced. The decrease in the number of companion cell indicates evolutionary advance. In course of evolution the length of sieve tube is shortened. This shortening may be correlated with the increasing contact between the companion cell and sieve tube (Carlquist).

4. Phloem Parenchyma and Fibre:

Procambium gives rise to phloem parenchyma of primary phloem. In secondary phloem the axial phloem parenchyma and phloem rays are developed from fusiform initial and ray initial of cambium respectively. Some parenchyma cells originate from a common mother cell of sieve elements. Fibres occur in both primary and secondary phloem and accordingly their origin differs.

The primary phloem fibre originates from procambium where as the secondary phloem fibre originates from cambium. The fusiform initial of cambium gives rise to fibre, which composes the axial system of the organ in which it occurs. In many dicotyledonous stems the parenchyma of protophloem often differentiates into fibre in later stages of development, i.e. when the protophloem elements become functionless.

The phloem parenchyma and fibre of secondary phloem bear no phylogenetic trend in phloem evolution. The distribution and morphology of them may be of comparative value (Zahur).

Sieve elements are the most labile cells of a plant. The fossils do not provide any useful details of phloem structure though other tissues show excellent preservation. An idea of fossil phloem can be obtained from Palmoxylon and Rhynia. Therefore the phylogeny of phloem is inferred from extant plants.