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Let us learn about the Stem of Pinus. After reading this article you will learn about: 1. Anatomy of Young Stem of Pinus 2. Secondary Growth in Thickness of Stem of Pinus.
Anatomy of Young Stem of Pinus:
It resembles the anatomy of dicotyledonous stem in many respects. The general arrangement of the various tissues from the circumference to the centre is the same. However, it differs, from the dicot stem in having a large number of resin ducts filled with resin. These ducts are found to be distributed almost throughout the stem.
The epidermis has an irregular outline. Endodermis and pericycle are like those of the dicotyledonous stem, but the pericycle contains no sclerenchyma. The vascular bundles are not wedge-shaped, as in the dicotyledons. Phloem consists of annular and spiral tracheids which are irregularly disposed towards the centre.
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Meta-xylem consists of exclusively tracheids with bordered pits. The tracheids are arranged in radial rows as seen in the transverse section of the stem. The pits of the pine wood are large and mostly restricted to the radial walls. There are no true vessels.
The details of the anatomy of young stem of pine are as follows:
Epidermis:
It consists of a single layer of cells with a very thick cuticle.
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Sclerenchyma:
Sometimes a few patches of sclerenchyma occur here and there below the epidermis.
Cortex:
Many layers of more or less rounded parenchyma cells, with conspicuous resin ducts lying embedded in the cortex.
Endodermis:
A single layer lying internal to the cortex; the innermost layer of the cortex is treated as endodermis.
Pericycle:
It consists of parenchyma cells, there is no sclerenchyma in it.
Medullary rays:
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They run from the pith outwards between the vascular bundles.
Pith:
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There is a well defined pith, consisting of a mass of parenchyma cells. A few resin-ducts are also present in the pith.
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Vascular bundles:
These are collateral and open, and arranged in a ring, as in dicot stem.
Each bundle consists of phloem, cambium and xylem.
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Phloem:
The phloem consists of sieve tubes and phloem parenchyma, but no companion cells. It lies on the outer side of the bundle. Cambium—A few layers of thin walled, rectangular cells in between xylem and phloem.
Xylem:
It consists exclusively of tracheids; there are no true vessels.
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Resin ducts are also present here. Protoxylem lies towards the centre and consists of a few annular and spiral tracheids which are not disposed in any regular order. Metaxylem lies towards the cambium and consists of tracheids with bordered pits which develop on the radial walls. These tracheids are four sided and are arranged in definite rows.
Secondary Growth in Thickness of Stem of Pinus:
The secondary growth in pine stem takes place in exactly the same way as in a dicotyledonous stem.
However, the points of differentiation are as follows:
The pine stem is characterized by the presence of conspicuous resin-ducts which are distributed throughout the stem. The secondary wood consists exclusively of tracheids with numerous bordered pits on their radial walls.
As in the dicotyledonous stem, there are distinct annual rings, consisting of the autumn wood and spring wood. The autumn wood consists of narrow and thick-walled tracheids, and the spring wood of wider and thinner-walled tracheids. The secondary medullary rays are usually one layer of cells in thickness and a few in heights.
The phloem portion of the medullary ray consists of middle layers of starch—containing cells, called starch cells, and upper and lower layers of protein containing cells, called albuminous cells. The xylem portion of the medullary ray consists of similar starch cells in the middle, and empty cells with bordered pits, called tracheidal cells, in the upper and lower layers. Vessels are absent.
