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In this article we will discuss about the secondary growth in certain dicotyledonous stem:- 1. Aristolochia 2. Cucurbita.
Dicotyledonous Stem # 1. Aristolochia:
Aristolochia is a liane (woody climber) and its secondary growth in stem differs from the normal woody plants. The most striking differences are the formation of only parenchymatous medullary rays by the interfascicular cambium, and vascular tissues in the fascicular regions.
The primary structure of the Aristolochia stem shows following characteristics (Fig. 5.141):
(i) Epidermis:
It is uniseriate with cuticularised outer walls.
(ii) Cortex:
It is differentiated into outer collenchymatous hypodermis, middle parenchymatous portion and the innermost starch sheath. Both the collenchyma and parenchyma cells contain chloroplasts. Internal to the starch sheath a continuous band of sclerenchyma occurs, which are perivascular fibres, or may be said to form pericycle together with the adjoining parenchyma cells.
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(iii) Stele:
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 are arranged in a ring. The bundles are conjoint, collateral and open.
With onset of secondary growth in thickness a strip of parenchymatous cells of the broad medullary rays in the line of fascicular cambium become meristematic to form the interfascicular cambium.
Both of them join up to form the cambium ring. Only the fascicular cambium produces secondary xylem and secondary phloem on the inner and outer sides, respectively, in individual vascular bundles and 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. Annual rings are formed as a result of seasonal activities of the cambium. The primary phloem on the outer side gets crushed due to the pressure.
Bands of sieve elements alternate with the bands of parenchyma in the secondary phloem. 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 central pith gets more and more reduced in size. The continuous cylinder of sclerenchyma is disrupted and ruptured here and there, usually in the region of medullary rays. The adjoining parenchyma cells fill up the gaps thus formed.
The ruptured bands of sclerenchyma gradually undergo sclerosis and are ultimately transformed into sclereids. The hypodermal collenchymatous band also breaks due to pressure. The parenchyma cells of the cortex fill up the gaps thus produced.
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The epidermis also ruptures and periderm develops in the subepidermal layers. Phellogen is formed in patches to produce a thick layer of cork cells on the outer side and considerable phelloderm on the inner side. Lenticels are formed here and there on the stem surface (Fig. 5.142).
Dicotyledonous Stem # 2. Cucurbita:
The process of secondary growth in thickness is more or less similar to that of Aristolochia. But as this stem is not so woody, the cambial activities remain confined to the individual vascular bundles. Moreover, the perivascular fibrous band is not ruptured here.
Due to the tangential divisions of the cells of the outer cambium secondary xylem elements are produced inside and secondary phloem elements on the outside. The vessels are usually pitted and enormously large in size. So the characteristic radial arrangement and positions of other tissues are considerably disturbed.
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The secondary phloem elements are hardly distinguishable from their primary counterparts. The phloem consists of broad sieve tubes, narrow companion cells and parenchyma cells of intermediate size. Fibres and ray cells are absent. The sieve plates are simple and have large pores and connecting strands. The inner cambium strip remains inactive.
