In this article we will discuss about the anatomical structure of some dicotyledonous stems that show secondary growth.
1. Anatomy of Bignonia Stem (Fig. 7.4):
The following tissue layers are seen in the T.S. of the stem:
1. Epidermis:
It is uniseriate and cuticularised. It is wavy in outline.
2. Cortex:
It is parenchymatous and bounded internally by a starch sheath layer.
3. Perivascular Region:
There are scattered patches of sclerenchymatous cells, particularly under the ridges of the stem.
4. Vascular Bundles:
In young stem vascular bundles are arranged in a ring and cambium is normal in disposition. Soon secondary growth starts, the cambial ring is formed and then it cuts-off different proportions of xylem and phloem at different points.
At four points, the formation of secondary xylem is reduced and that of secondary phloem correspondingly increased. As a result of this, a peculiar structure with ridged and furrowed xylem cylinder is formed.
5. Pith:
It is parenchymatous.
Comments on Anatomical Features:
The specimen shows abnormal cambial activity during secondary growth in the intrastelar region.
The peculiarities are:
(a) Presence of grooved xylem and phloem cylinder due to unequal activity of cambial layer.
(b) Presence of scattered patches of sclerenchymatous tissues in the perivascular layer.
In addition, it has typical dicotyledonous stem structure as revealed by the following features:
(a) Presence of endodermal layer
(b) Presence of collateral and open type vascular bundles
(c) Presence of distinct pith zone.
2. Anatomy of Serjania Stem (Fig. 7.5):
A very peculiar structure is noticed in Serjania — a climber of the family Sapindaceae — where the cambium from the very beginning occurs in several separate strips, each strip having surrounding portions of primary tissues. It thus appears like a deeply-lobed compound structure made of several stems fused together.
The development of inter-fascicular cambium and its union with fascicular ones result in the formation of separate cambium rings. Each ring independently produces secondary xylem inside and a few layers of phloem outside. Thus, there is a large central stele surrounded by a few peripheral steles.
The detailed anatomical features of the stem is:
1. Epidermis:
It is uniseriate and distinctly cuticularised.
2. Cortex:
It is parenchymatous, several layers in thickness and extends up to the starch sheath layer.
3. Stele:
In fact there is a large central stele, surrounded by a few peripheral ones. All the steles are externally bounded by a lobed sclerenchymatous band. Each stele is composed of collaterally arranged end-arch vascular bundles. Distinct cambial layers are noticed in-between xylem and phloem.
4. Pith:
There is a distinct parenchymatous pith at the centre of each stele, among which the pith region of large central stele is conspicuous.
Comments on Anatomical Features:
The specimen shows abnormal cambial activity during secondary growth within the intrastelar region.
The other peculiarities are:
(i) The presence of conjoint, collateral, open type vascular bundles and pith in each stele reveals its dicotyledonous nature.
(ii) The presence of starch sheath layer (endodermis) and sclerenchymatous perivascular zone is also characteristic features of typical dicotyledonous stems.
(iii) Thus it appears that the polystelic condition of the stem axis definitely has some adaptive significance for its climbing habit.
3. Anatomy of Strychnos Stem (Fig. 7.6):
In Strychnos, anomalous structures result from the occurrence of unusual type of phloem which remains embedded in secondary xylem. This is termed interxylary or included phloem. This is due to occasional abnormal cambial activity, i.e., production of patches of phloem tissues while producing xylem at the inner side of cambial layer.
The detailed anatomical features are:
1. Epidermis:
It is composed of a single layer of parenchymatous cells, with distinct cuticularisation.
2. Cortex:
It is parenchymatous and several layers in thickness. Internally bounded by starch sheath layer.
3. Stele:
It is a typical di-cytostatic type. The vascular bundles are conjoint, collateral and open type. During intrastelar secondary growth there is the formation of a cambial ring which normally produces secondary xylem on the inner side and secondary phloem patches towards the outside.
After a period of secondary growth, at certain points cambium ceases to function normally and produces secondary phloem inside instead of xylem, as a result, several phloem patches are developed within the secondary xylem core, which is termed interxylary or included phloem.
Further, due to the activity of accessory cambium which develops in the pith region, intraxillary phloem or internal phloem is also developed. This accessory cambium may differentiate phloem only or phloem towards centre and xylem towards the periphery (i.e. inverted vascular bundle).
4. Pith:
It is distinct and parenchymatous, with intercellular spaces. (In mature stem, cork layer may be seen in the hypodermal zone in the T.S. of stem.)
Comments on Anatomical Features:
The specimen shows two major types of anomalous development:
(a) Presence of interxylary phloem due to the abnormal activity of cambium of the primary bundles.
(b) Presence of interxylary phloem due to the formation of accessory cambium (abnormal position) and their abnormal activity resulting in the formation of inverted bundles.
All other major features indicate its typical dicotyledonous stem nature, i.e., starch sheath layer in the endodermal zone; conjoint, collateral open type of primary vascular bundles and also distinct parenchymatous pith.
4. Anatomy of Thunbergia Stem (Fig. 7.7):
A. T.S. of the stem of Thunbergia sp. shows the following pattern of tissue arrangement:
1. Epidermis:
It is uniseriate, parenchymatous and cuticularised.
2. Cortex:
It is distinguishable into a thick-walled parenchymatous hypodermis with patches of sclerenchymatous tissues, and thin-walled parenchymatous cortex. The innermost layer is transformed into a starch sheath layer.
3. Stele:
In the primary body, the stele is dyctiostele with conjoint, collateral, endarch vascular bundles. During secondary growth, accessory cambium develops below the starch sheath layer which produces secondary phloem outwardly, and secondary xylem inwardly, in the usual manner.
During the differentiation of secondary xylem, some scattered patches of secondary phloem are also developed within the secondary xylem layer due to abnormal activity of accessory cambium in localized zones. This is called interxylary or included phloem.
4. Pith:
It is distinct and parenchymatous in nature.
Comments on Anatomical Structures:
The specimen shows the features of a typical dicotyledonous stem with abnormal secondary growth due to some adaptive changes.
The reasons are:
(i) Presence of a typical uniseriate parenchymatous epidermis with thin cuticular layers.
(ii) Presence of a distinct three-layered cortical zone — hypodermis, cortex, and endodermis or starch sheath layer.
(iii) Presence of conjoint, collateral, endarch vascular bundles and distinct pith.
However, secondary growth takes place principally due to the formation of an accessory cambium layer just below the endodermis or starch sheath layer. Further, due to sporadic abnormal activity of cambium, interxylary phloem patches are also developed within secondary xylem.
This abnormal development of accessory cambium has some adaptive significance.
5. Anatomy of Boerhaavia Stem (Fig. 7.8):
In the families Amaranthaceae, Chenopodiaceae and Nyctaginaceae etc., anomalous structures are formed in rather different ways. The vascular bundles normally appear either in a ring or remain irregularly scattered in the ground tissues (medullary bundles).
They are, as usual, collateral open type. Cambial activity continues in the individual bundles, of course for a very short period, and soon ceases. However, secondary cambium arises on the outer side of the bundles. This cambium cuts of secondary bundles on the inner side, which remain embedded in a non-vascular tissue, are known as conjunctive tissue.
A transverse section through the stem of Boerhaavia Sp. (family Nyctaginaceae) shows the following characteristics:
1. Epidermis:
It is single-layered with distinct cuticle and scattered stomata.
2. Cortex:
It is distinguished into three layers:
(a) Hypodermis, composed of collenchymatous cells,
(b) General cortex, composed of parenchymatous cells with intercellular spaces, and
(c) Starch sheath layer or endodermis.
3. Stele:
Primary vascular bundles are scattered in the parenchymatous tissues, the bundles are collateral, conjoint and open type. Secondary growth in these bundles is very limited.
However, secondary cambial layer is formed just outside the primary vascular bundles. This cambial layer produces patches of secondary phloem outside and secondary xylem inside, along-with the formation of massive thick-walled conjunctive tissue inside the cambial layer.
4. Pith:
This region is not clearly demarcated due to the scattered distribution of primary vascular bundles in the parenchymatous ground tissue.
Comments on Anatomical Features:
The specimen shows the features of a typical dicotyledonous stem with accessory cambial activity during secondary growth.
The features are:
(i) Presence of collenchymatous hypodermis, parenchymatous cortex and starch sheath layer.
(ii) Presence of conjoint, collateral and open type primary vascular bundles.
In addition, secondary cambium develops just below the endodermal layer, and this cambium produces secondary vascular tissues along-with massive thick-walled conjunctive tissue.
The stem of Mirabilis jalapa (family Nyctaginaceae) more or less resembles that of Boerhaavia, but the medullary bundles are more numerous. Secondary growth is initiated by the formation of secondary cambium originating in the same manner an in Boerhaavia.
6. Anatomy of the Amaranthus Stem (Fig. 7.9):
The stem of Amaranthus (family Amaranthaceae) also shows anomalous secondary growth, almost identical to that of Boerhaavia sp.
In T.S. it shows the following tissue layers:
1. Epidermis:
It is uniseriate, parenchymatous and cuticularised.
2. Cortex:
It is differentiated into collenchymatous hypodermis, parenchymatous general cortex and starch sheath layer or endodermis.
3. Vascular Bundles:
These are scattered in the ground tissue. Each bundle is collateral, conjoint and open type. Secondary growth limited to each bundle, but soon it ceases. Secondary cambium develops just below the endodermal layer. It produces secondary xylem inside along with thin-walled conjunctive tissue and secondary phloem in patches towards the endodermal side.
4. Pith:
It is more or less inconspicuous and parenchymatous in nature.
Comments on Anatomical Features:
The specimen reveals the features of a typical dicotyledonous stem with anomalous secondary growth.
The features are:
(i) Presence of hypodermis, cortex and starch sheath layer.
(ii) Vascular bundles (primary) are conjoint, collateral and open type.
(iii) Pith distinct and parenchymatous.
In addition, due to abnormal formation of secondary cambium just below the endodermal layer, secondary vascular tissues along with conjunctive tissues are formed. This is a case of abnormal secondary growth.
7. Anatomy of Tecoma Stem (Fig. 7.10):
In Tecoma sp., a secondary cambium arises on the inner side of the normal woody cylinder. This internal cambium cuts off both xylem and phloem, but in reverse order. So phloem is here intraxylary and secondary in origin.
A T.S. through stem of Tecoma reveals the following tissue layers:
1. Epidermis:
It is uniseriate, cuticularised and parenchymatous.
2. Cortex:
It is parenchymatous and 2-3 layers in thickness. The inner layer is represented by starch sheath layer.
3. Vascular Bundles:
Primary bundles are arranged in a ring. They are conjoint, collateral and open type. During secondary growth they form a cambial ring and produce secondary xylem inwardly and secondary phloem outwardly. After a period of secondary growth these bundles form a concentric ring around the parenchymatous pith.
There is a distinct pericycle region where scattered sclerenchymatous patches are seen. In the meanwhile, secondary cambium develops in the inner part of stele, i.e., within the pith region instead of pericycle zone. The internal cambium produces inverted bundles (i.e. internal phloem and external xylem). Thus, this internal phloem is secondary in origin.
4. Pith:
It is distinct and parenchymatous with intercellular spaces.
Comments on Anatomical Features:
The specimen shows the features of a typical dicotyledonous stem with anomalous secondary vascular bundles at the central region.
The reasons are:
(i) Presence of parenchymatous cortex with starch sheath layer.
(ii) Presence of conjoint, collateral, open and endarch vascular bundles with distinct pith.
In addition, secondary cambium development in abnormal position (i.e. in the inner core of stem) and abnormal activity (i.e. outward production of xylem and inward production of phloem) resulting in the formation of secondary inverted vascular bundles, are very interesting features.
8. Anatomy of Chenopodium Stem (Fig. 7.11):
In the stem of Chenopodium album of the family Chenopodiaceae there is a concentric ring of primary bundles (medullary) which are collateral and open ones. They grow in thickness to some extent and then a peculiar anomalous secondary increase ensures when concentric zones of collateral vascular bundles arise from successive rings or arcs of secondary cambium originating in the pericycle or phloem.
Conjunction tissues with lignified walls are also formed by the secondary cambium in which the bundles remain embedded. A very interesting feature is the occurrence of isolated strands of phloem — called phloem islands — buried in the secondary xylem.
It becomes possible because phloem patches arise centrifugally as usual, and then short arcs of secondary meristem develop on their outer side, which go on producing normal tissues, resulting in the complete enclosure of the phloem patches.
A. T.S. of the stem shows the following features:
1. Epidermis:
It is single-layered, cuticularised and parenchymatous.
2. Cortex:
It is distinguishable into collenchymatous hypodermis, parenchymatous cortex proper and innermost stretch sheath layer.
3. Vascular Bundles:
The primary bundles are collateral, conjoint and open type. These bundles are arranged in a ring. Secondary cambium is developed in the pericycle region, which produces phloem patches within the secondary xylem inwards along with thick-walled conjunctive tissues. These phloem patches are interxylary in nature, formed due to abnormal secondary cambial activity.
4. Pith:
It is distinct and parenchymatous.
Comments on Anatomical Features:
It reveals the features of a typical dicotyledonous stem with anomalous cambium activity.
The reasons are:
(i) Presence of hypodermis, cortex, starch sheath layer and pith.
(ii) Primary vascular bundles are conjoint, collateral and open type.
Secondary vascular tissues are formed from the secondary or accessory cambium which arises in the pericycle region. This cambium also produces interxylary phloem along with secondary xylem and thick- walled conjunctive tissues arranged in concentric rings.
9. Anatomy of Mirabilis Stem (Fig. 7.12):
The stem of Mirabilis jalapa of family Nyctaginaceae is rather quadrangular in outline. It more or less resembles that of Boerhaavia of same family described earlier, but the medullary bundles are more numerous. Those occurring towards the periphery are smaller in size and more crowded, whereas those at the central region are larger and more spaced out.
Secondary growth is initiated by the formation of secondary cambium originating in the same manner as in Boerhaavia. This cambium cuts-off secondary tissues, usually secondary xylem elements on the inner side which remain embedded in the conjunctive tissue. Secondary phloem elements are occasionally formed as the cambium produces very little secondary tissues on the outer side.