Secondary Growth in Aristolochia Stem (With Diagram)!
It has been stated that this stem which is a liane, differs from the normal ones in the process of secondary growth.
The most striking points of difference are the formation of only parenchymatous medullary rays by the interfascicular cambium, and consequent occurrence of secondary tissues in strands (Fig. 641).
In the primary condition the Aristolochia stem has the following structure. Epidermis is uniseriate with cuticularised outer walls. Cortex is differentiated into collenchymatous hypodermis, parenchymatous portion and the starch sheath.
Chloroplasts are present in both collenchyma and parenchyma cells. A continuous band of sclerenchyma with strongly thickened walls occur. They may be called perivascular fibres, or may be said to form pericycle together with the adjoining parenchyma cells.
Broad medullary rays composed of parenchyma cells occur between the vascular bundles. The central portion of the stem is occupied by a large parenchymatous pith. The vascular bundles remain arranged in a ring. The bundles are distinctly collateral and open ones, with xylemand phloem on the inner and outer sides and having a strip of cambium in between the two.
With commencement of secondary growth in thickness a few parenchymatous cells of the broad medullary rays become meristematic in a line with the fascicular cambium of the vascular bundles. The newly-formed meristem, a secondary meristem, is known as interfascicular cambium.
It joins up with the fascicular cambium and thus a continuous cambium ring (Fig. 641 A) is formed. The fascicular cambium, in fact, the cambial zone goes on dividing tangentially and produces secondary xylem and secondary phloem on the inner and outer sides respectively.
Thus the primary xylem and primary phloem are pushed apart from each other. The secondary xylem has the usual elements arranged in vertical and horizontal systems. Metaxylem elements are fairly large in size. Annual rings with early wood and late wood are formed as a result of seasonal activities of the cambium.
The secondary phloem pushes the primary one on the outer side, and the latter usually gets crushed due to the pressure. Bands of sieve tubes and associated cells alternate with bands of parenchyma in the secondary phloem; fibres are absent. On the whole the vascular bundles increase enormously in size due to continued activity of the fascicular cambium.
The interfascicular cambium simply produces parenchyma cells on the outer and inner sides. Thus the medullary rays become increasingly more broad and long. The cells remain arranged in more or less radial rows.
The formation of the secondary tissues brings about profound changes in other portions of the stem. The central pith gets more and more reduced in extent. Distinct disruption in the continuous cylinder of sclerenchyma is caused by the increase, so that the band is ruptured here and there, commonly in front of the medullary rays.
The adjoining parenchyma cells fill up the gaps thus formed. These cells gradually undergo sclerosis and are ultimately transformed into sclereids. They, in fact, repair, so to say, the gaps caused by the onrush of the internal tissues.
The band of hypodermal collenchyma also suffers from pressure and breaks down frequently. The parenchyma cells of the cortex make their way into the breaks and thus occur as strips amongst hypodermal collenchyma.
The epidermis gets stretched and ruptures. Periderm develops in the subepidermal layers. Phellogen is formed in patches. They divide and produce a thick layer of cork cells on the outer side and considerable phelloderm on the inner. Lenticels are formed.