In this article we will discuss about:- 1. Origin of Xylem Parenchyma 2. Phylogeny of Ray Parenchyma 3. Phylogeny of Axial Parenchyma.
Origin of Xylem Parenchyma:
Xylem parenchymas cells are present both in primary and secondary xylem; accordingly their origin also differs. In primary xylem they originate from procambium. In secondary xylem ray parenchyma cells originate from the ray initials of cambium. The fusiform initial of cambium gives rise to axial parenchyma along with tracheary element and fibres.
Fusiform initials of cambium normally divide vertically in the longitudinal plane. Transverse divisions do occasionally occur during the formation of additional initials, but these are much less frequent than vertical division. The vertical divisions are mainly periclinal.
In these cells anticlinal division also occur to keep pace with the growth of stem in girth. During longitudinal division cell wall first originates between the two newly formed nuclei and gradually extends towards the ends of cell. The cell wall formation may not be completed for some time after mitosis.
Phylogeny of Ray Parenchyma:
In radial longitudinal section it is observed that the ray parenchyma is composed of square (isodiametric), erect (upright or vertically elongate) and procumbent (radially elongate) cells. It is considered that the square cells are morphologically equivalent to erect cells. The ray cells may be homocellular and heterocellular.
The homocellular rays are composed either of square cells or of erect cells, or of procumbent cells, or of erect and square cells. The heterocellular rays consist of both square and procumbent cells or of erect and procumbent cells. Rays may be uniseriate when they are composed of one cell in width, biseriate (ray is two cells in width) or multiseriate where the rays consist of a portion more than two cells wide.
The latter may have uniseriate wing similar to uniseriate rays. Multiseriate ray, as seen in tangential longitudinal section (TLS), is tapered towards both upper and lower margins. Tapered ends are described as wings, which is commonly uniseriate. The outline of rays appears to be fusiform in TLS. Primitive and advanced wood exhibit the following types of rays.
Carlquist (1961) showed some trends of evolution of ray types in dicots based on the works of Kribs (1935) with modifications and recognized the following type of rays:
Heterogeneous I:
It consists of uniseriate ray and multiseriate ray with uniseriate wings. The cells of wings are more or less similar to the cells of uniseriate rays. Both rays are of marked vertical length, i.e. very high. The cells are square, erect and procumbent.
Heterogeneous II(A):
This type is composed of multiseriate rays with short height and short wings. The cells are square, erect and procumbent. This type also consists of uniseriate ray with square and erect cells.
Heterogeneous I(B):
The rays are uniseriate and the cells are square, erect and procumbent.
Heterogeneous III:
The rays are uniseriate and the cells are square, erect and procumbent.
Homogeneous I:
The rays are uniseriate and multiseriate with very short wings. The multiseriate rays are mostly fusiform. The cells of multiseriate portion together may be oval, round or radially elongated in outline. The cells of the uniseriate tips are identical with the cells of multiseriate portions of rays. The cells are procumbent.
Homogeneous II:
The rays are multiseriate with extremely short wings and the cells are procumbent. The shape of multiseriate rays is fusiform. The cells may be round or radially elongated.
Homogeneous III:
This type consists of uniseriate rays with procumbent cells. Carlquist showed the trends of evolution of rays in dicots which is presented in Fig. 10.1 in diagrammatic figures.
Woods having heterogeneous I type of ray is considered as primitive. The advanced form of rays is either uniseriate or multiseriate with procumbent cells (Homogeneous I, II and III). It is regarded that the rays with erect cells increased in length and continued elongation resulted the conversion of ray initial into fusiform initial. The ultimate result is raylessness.
Phylogeny of Axial Parenchyma:
The fusiform initial of cambium gives rise to axial (vertical) parenchyma. The distributions of axial parenchyma are studied in transverse sections. Axial parenchyma may lie independent or associated with vessels. Accordingly timbers may be apotracheal (parenchyma are not associated with vessel) and paratracheal (parenchyma are distinctly associated with vessels).
The common apotracheal forms are:
(i) Diffuse (axial parenchyma occurs as isolated strand),
(ii) Diffuse-in-aggregates (axial parenchyma occurs as aggregates),
(iii) Banded (axial parenchyma appears as bands; the bands may be narrow or wide),
(iv) Marginal [parenchyma occurs either at the beginning of growth ring (initial) or at the end of growth ring (terminal)].
The common paratracheal forms are:
(i) Scanty (the parenchyma cells do not form a continuous sheath surrounding a vessel),
(ii) Vasicentric (parenchyma cells encircle the vessels),
(iii) Abaxial (vasicentric parenchyma occurs more in width on abaxial side of vessel),
(iv) Adaxial (the parenchymatous sheath is more in width on adaxial side of vessel),
(v) Aliform (vasicentric parenchyma extends laterally in the form of wings), and
(vi) Confluent (vasicentric parenchyma extends and coalesces with other forming a continuous band) (Fig. 10.2).
The phylogenetic sequences among the distributional type of axial parenchyma are discussed below (Fig. 10.3).
1. The primitive wood may exhibit no parenchyma; ex. vesselless wood of Winteraceae.
2. The primitive wood exhibits diffuse parenchyma. The cells of fusiform cambium differentiate into parenchyma.
3. The advanced form of wood exhibits diffuse-in-aggregate, where the tendency towards grouping of axial parenchyma is noticed.
4. The advanced form of wood exhibits apotracheal-banded parenchyma. The wide band of apotracheal parenchyma is considered to be more advanced than narrow band of parenchyma. It is assumed that diffuse-in- aggregate gives rise to narrow banded apotracheal type that ultimately forms wide band apotracheal type.
5. Marginal parenchyma (collective term of terminal and initial parenchyma) is formed as a result of changes in climatic conditions. So it is assumed that marginal parenchyma arose independently.
6. Scanty parenchyma seems to be less specialized than vasicentric.
7. The banded types gave rise to vasicentric, aliform and confluent types.
Xylem fibre:
i. Origin of xylem fibre:
In primary xylem fibres originate from procambium whereas they are developed from fusiform initial of cambium in case of secondary xylem.
ii. Phylogeny of xylem fibre:
Fibres and tracheids are phylogenetically related and it is suggested that the former evolved from the latter. During evolution the length of fibre decreased, the bordered pits reduced in size and the thickness of cell wall increased. In secondary xylem of dicots, libriform fibre evolved in the sequence of tracheid, fibre-tracheid, and libriform fibre.
Bordered pits are prominent in tracheids. The pit border is diminished in fibre-tracheid where the bordered pits have less developed border. Disappearance of border on pit occurs in libriform fibre. Here the pits are simple or nearly so. The cell wall is thick in fibre-tracheid and thicker in libriform fibre. The diameter of fibre becomes narrower.