In this essay we will discuss about the monocot and dicot stem.
Essay # 1. Monocot Stem:
Some monocotyledons belonging to the family Liliaceae have arborescent habit with woody stems like Dracaena, Yucca, Cordyline, Agave, Aloe etc. They exhibit an anomalous secondary growth in thickness producing a cambium which is abnormal both in position and function.
After the cease of primary growth a cambium ring of a few cells thick outside the vascular bundles in the cortex is formed. The cambial cells are fusiform or rectangular in shape. Instead of forming phloem and xylem in the outer and inner sides, as in normal conation, the cambial cells go on adding secondary tissues on the inner side first, and, later, little amount of secondary tissues are added on the outer side as well.
From the secondary parenchymatous tissues formed on the inner side differentiate into oval- shaped leptocentric vascular bundles. These parenchyma cells, in which the secondary vascular bundles remain embedded, are called the conjunctive tissue.
The cells of this tissue are radially arranged due to their origin by tangential divisions of the cambial cells. So they may be easily distinguished from the irregularly arranged parenchyma of the primary ground tissue. They may be thin-walled or thick-walled.
A few cambium derived cells divide longitudinally (anticlinal divisions followed by periclinal divisions) and even haphazardly to form xylem and phloem of the secondary bundles. These bundles differ from the primary bundles in presence of small amount of phloem and in absence of annular and spiral protoxylem elements. The secondary bundles are mostly amphivasal. Some of them may be collateral as well.
The small amount of phloem consists of short sieve tubes, companion cells and phloem parenchyma. The xylem is made of only tracheids, usually with scalariform thickening and small amount of xylem parenchyma with lignified walls.
The cambium adds small amount of parenchymatous cells on the outer side. The primary bundles are comparatively larger. They are also mostly amphivasal or rarely collateral ones. The secondary bundles are more numerous and the peripheral ones are smaller in size.
In Dracaena typical periderm formation by the activity of phellogen is not found but storied cork cells are formed by the continuous periclinal division of the outermost cortical cells followed by suberisation of their walls. This storied cork cells form a special type of protective layer outside the stem. In Aloe, coconut and royal palm periderm is formed in the same manner as in the dicotyledons.
Considerable thickening is often noticed at the bases of many palms. This is not due to the activity of a cambium cylinder, but it is due long-term primary growth. The apical meristem produces only a central column of parenchyma and vascular strands. Another primary meristem occurs just beneath the leaf primordia forming a mantle-like tissue zone, known as primary thickening meristem.
The cells of this meristem divide periclinally to form rows of cells which differentiate into ground parenchyma traversed by procambial strands. From the procambial strands vascular bundles develop. By the continued divisions of the ground parenchyma cells further increase in thickness takes place. Thus the primary thickening meristem produces the main bulk of the stem tissues, viz., ground parenchyma and most of the vascular bundles.
The structural anomaly may be directly influenced by external environmental conditions. The woody climbers or lianes and the storage organs show structural anomaly as well as abnormal secondary growth in thickness as they are different from the normal ones — from morphological and physiological points of view.
At the same time, there are many cases of anomalous condition which are not influenced by the environment and appear to be merely cases of variations of the design. Haberlandt has called these two cases as adaptive and non-adaptive anomalies of secondary growth in thickness, respectively.
Essay # 2. Dicot Stem:
Cambium — Normal in Position but Abnormal in Activity:
(a) In some lianes like Bignonia and other members of family Bignoniaceae, the cambium is normal in position and activity for the first time, but immediately it cuts off different proportions of xylem and phloem at four alternating points arranged in form of a cross. In one set of alternating points the cambium produces less amount of secondary xylem and much amount of secondary phloem, and vice versa in other set of alternating points.
As a result the woody cylinder appears to have four longitudinal grooves alternating with four ridges. The ridges are wider than the grooves. The cambium breaks up into a number of strips. The wider ones remain on the top of the ridges while the narrower ones remain at the bases of the grooves. As a result a peculiar structure with ridged and furrowed xylem cylinder is formed. In the secondary phloem formed in the groove parallel strips of bast fibres are formed.
This is a kind of adaptive anomaly. Bignonia is a woody climber (liane) and so its stem is twining and thus is inextensible as it hangs freely and has to bear its own weight. The stem is also flexible as it has to withstand high velocity of wind and again subject to radial compression due to unequal growth in thickness for twining purpose.
For those reasons the woody cylinder is not formed in the form of a solid mass, instead is split up into separate portions by the interpolation of softer tissues into the woody mass to achieve inextensibility and flexibility. In Bignonia softer phloem tissue is interpolated into the woody cylinder by the formation grooves.
(b) In Vitis (Vitaceae), Clematis (Rannun- culaceae), Aristolochia (Aristolochiaceae, Fig. 5.141), Tinospora (Menispermaceae) etc. a normal cambium ring is formed by the union of fascicular and inter-fascicular cambia. But, during function, the fascicular cambium produces secondary xylem and phloem as usual on the inner and outer sides, respectively, within individual vascular bundles, whereas the inter-fascicular cambia produce only ray-like parenchyma cells on both sides. As a result, broad and enormously elongated medullary rays and fluted vascular cylinder is formed.
Here again, interpolation of soft tissue into the woody cylinder takes place due to abnormal activity of the inter-fascicular cambia, which is helpful for climbing purpose. The woody cylinder is broken by wide rays and the sclerenchymatous cylinder encircling the vascular bundles is ruptured and the resulting gaps are filled by parenchyma cells. So, this is again an example of adaptive type of anomaly.
(c) In Bauhinia divaricata, B. sericella and Prestonia macrocarpa the stem is flat, ribbon-like at maturity having the flattened strap shaped stele. Secondary growth starts by the activity of a normal cambium ring producing secondary xylem and phloem towards inner and outer sides, respectively, as usual. At that early stage the vascular cylinder as well as the stem is more or less round with uniform amount of secondary xylem consisting of small tracheary elements surrounding the pith.
After such a period of secondary growth the cambium becomes more active at two opposite sides, and at alternate opposite sides it becomes less active. The nature of the secondary xylem produced by the alternating segments of cambium is different.
In the less active region the secondary xylem shows small tracheary element, whereas that of adjacent more active region exhibits vessels with larger diameter. With such continued growth the stele gradually becomes flattened strap or band shaped and the shoot becomes flat and ribbon-like.
This type of stem is usually observed in those species of Bauhinia which are vines. Here again, the solid woody cylinder is broken and become flat by the abnormal activity of the cambium to make the plant flexible and inextensible with increased conductivity by forming vessels with large lumens. This is again a kind of adaptive anomaly.
Cambium Abnormal in Position but Normal in Activity:
Serjania is a woody climber of the family Sapindaceae exhibiting several discrete vascular cylinders (Fig. 5.148) each of which has its own cambium and pith and is encircled by an endodermis. Thus a polystelic appearance is observed from the very beginning, as if, several stems are fused together.
During secondary growth each cylinder of primary vascular bundles becomes notched at certain points and thus folds and ridges are formed. The ridges are constricted-off from the cylinder and the separated bundles behave as independent cylinders. Each bundle ring undergoes secondary growth in thickness by means of a ring of cambium present in them.
These cambia, though unusual in position, normally give rise to internal xylem and external phloem. At maturity each vascular cylinder exhibits bast fibers and wood enclosing parenchymatous pith at the centre. Certain amount of parenchyma also occurs between the vascular cylinders.
With age and consequent development of periderm the compound structure becomes more marked. In fact, there is a large central stele, surrounded by a few peripheral ones. The secondary xylem is composed of vessels and tracheids, on the inner side, and a few layers of phloem on the outer. A sclerenchymatous cylinder occurs encircling the vascular cylinders. It is again notched and wavy in outline with less amount of tissue at the notches.
In Serjania and other members of the family again, instead of forming single solid woody cylinder the stele is broken into many — even in the procambial stage to make the stem compatible with twining habit. The vascular cylinders remain encircled by soft parenchymatous tissue, which helps to make the stem tough, inextensible but flexible. So, this peculiarity is obviously adaptive in nature.
Anomaly due to the formation of Accessory Cambium:
In the families Nyctaginaceae, Amaranthaceae, Chenopodiaccae, etc., different types of anomaly is observed. The collateral and open vascular bundles normally appear either in a ring or they remain irregularly scattered in the ground tissues called medullary bundles. Cambial activity in the primary bundles commences but soon ceases.
This is followed by the formation a secondary accessory cambium encircling the primary bundles. This cambium cuts off secondary bundles inside, which remain embedded in a nonvascular tissue, known as conjunctive tissue consisting of either thin-wailed parenchyma or thick-walled lignified elongated cells.
The parenchyma cells, through gradual lignification, become thick-walled. The cambium produces very little tissue on the outer side. The secondary bundles may be irregularly scattered or may remain arranged in concentric rings.
(a) A transverse section through the stem of Boerhaavia sp. of the family Nyctaginaceae shows the following characteristics (Fig. 5.149 & 150):
1. The section is more or less circular in outline with uniseriate epidermis having stomata.
2. A few layers of collenchyma in patches with intervening parenchyma cells form the hypodermis followed by parenchyma with distinct intercellular spaces.
3. The stele includes primary vascular bundles in three rings.
4. Two fairly large bundles occur at the central region surrounded by quite a few comparatively much smaller ones forming a loose ring.
5. The outermost ring consists of pretty large number of much smaller bundles occurring just beneath the pericycle.
6. The central bundles are collateral and open but cambial activity remains confined to individual bundles.
7. Secondary growth in thickness commences for the first time through the formation of a continuous cambium cylinder in the outer ring.
8. Secondary xylem and phloem formation remains restricted in the fascicular regions only and lignified conjunctive tissue in the interfascicular regions.
9. This cambial activity declines soon and a new accessory cambium arises in the parenchyma outside phloem.
10. In this manner other cambia may arise further outside producing a few growth rings.
11. The newly-formed bundles remain embedded in hard lignified conjunctive tissue.
12. Scanty secondary phloem is formed outside opposite to the xylem vessels.
(b) Transverse section of stem of Mirabilis jalapa of the same family shows the following characteristics (Fig. 5.151):
1. The section is rather quadrangular in outline.
2. It more or less resembles that of Boerhaavia, but the medullary bundles are more numerous.
3. Those occurring towards periphery are smaller in size and more numerous, whereas those at the centre are larger and more scattered.
4. Secondary growth is initiated by the formation of secondary accessory cambium originating in the same manner as in Boerhaavia.
5. This cambium cuts off secondary tissues, usually secondary xylem elements on the inner side which remain embedded in the conjunctive tissue.
6. Secondary phloem elements are occasionally formed.
7. The cambium produces very little secondary tissues on the outer side.
(c) The transverse section of the stem of Amaranthus of the family Amaranthaceae shows the following characteristics (Fig. 5.152):
1. The transverse section is more or less circular in outline with uniseriate epidermis.
2. The ground tissue is well-differentiated.
3. Collenchyma cells occur in the hypo- dermal region interrupted by chlorophyll-containing parenchyma cells here and there.
4. The vascular bundles are medullary ones, numerous and scattered in the pith.
5. The bundles are collateral and open.
6. Cambial activity is confined to the individual bundles, and it ceases soon.
7. Secondary growth occurs due to development of an accessory cambium outside the stele.
8. The cambium cuts off similar bundles with xylem on the inner side and phloem on the outer side.
9. The secondary bundles remain embedded in thin-walled conjunctive tissue, which is wavy in outline on the inner side.
(d) The stem of Chenopodium album of family Chenopodiaceae shows the following characteristics (Fig. 5.153):
1. There is a concentric ring of primary medullary bundles which are collateral and open.
2. They secondarily grow little in thickness.
3. Then a peculiar anomalous secondary growth starts when concentric zones of collateral vascular bundles arise from successive rings of secondary accessory cambia originating in the pericycle or phloem.
4. Conjunctive tissues with lignified walls are also formed by the secondary cambium, which embed the bundles.
5. A very interesting feature is the formation of isolated strands of phloem, called phloem islands which remain buried in the secondary xylem. During development, phloem patches, as usually, arise outside, which is followed by the formation of short arcs of secondary meristem on their outer side.
6. This meristem goes on producing normal tissues, completely enclosing the phloem patches.
In all the above cases the nature of anomalous secondary growth is non-adaptive type as it is not influenced by the environment. In these plants, conjunctive tissues are formed and these tissues give mechanical strength in the usual manner.
Formation of Intraxylary and Interxylary Phloem by the Abnormal Activity of Abnormal Accessory Cambium
(a) In Campsis radicans (=Tecoma radicans) and C. grandiflora (=Tecoma andrepens) anomaly is produced during secondary growth (Fig. 5.154):
1. In this genus, the normal cambium ring forms the typical xylem and phloem, inside and outside, respectively.
2. Later accessory cambia are formed in two arcs at the outer margin of pith and on the inner side of the normal vascular cylinder.
3. These cambia produces secondary xylem and phloem in inverse order i.e., xylem outside and phloem inside.
4. As a result, two arcs of vascular bundles are formed at the margin of pith, showing inverse orientation of xylem and phloem in contrast to normal bundles.
5. The two patches of internal phloem, thus formed, gradually crush the pith.
6. So, these phloem patches here are intraxylary and secondary in origin.
7. In the developing stem, some parenchyma cells are found between the wood formed by normal and accessory cambia.
(b) In some members of family Combretaceae — Combretum, Entada, and Strychnos of family Loganiaceae anomalous structures result from occurrence of unusual type of phloem which remains embedded in secondary xylem. This type is known as interxylary or included phloem.
In Strychnos, Thunbergia etc. anomaly is produced (Fig. 5.155):
1. The normal cambium initially gives rise the usual inward xylem and outward phloem.
2. Small segments of accessory cambia originate outside the secondary phloem.
3. These accessory cambia give rise secondary xylem and phloem usually towards interior and exterior, respectively.
4. The accessory cambia, after a brief period of activity, gradually join with the normal cambium ring.
5. As a result, patches or bands of secondary phloem, formed by normal cambium, become included between xylem mass produced by normal and accessory cambial ring.
6. The secondary phloem is thus buried in the secondary xylem, and is termed included or interxylary phloem.
7. It should not be confused with internal or intraxylary phloem which is normal and primary in origin in some cases, but secondary in Tecoma.
So, in these plants anomaly is due to the formation of interxylary phloem by the functional and positional anomaly of the cambium. This is, again, a case of non-adaptive anomaly.
(c) In Nyctanthes of the family Oleaceae the following anomaly is observed (Fig. 5.156):
1. The cross-sectional outline of the stem is quadrangular with bulged corners.
2. Here, apart from normal vascular bundles occurring more or less in a ring, there are four cortical bundles at the bulged corners of the stem.
3. These bundles are inverted i.e., the phloem remains towards the centre and the xylem towards the periphery.
4. The cambium in these leaf trace bundles shows anomalous function.
5. It adds xylem outside and phloem inside. They are obviously leaf trace bundles.
6. The central vascular bundles are rather compact collateral and open ones with intervening patches of parenchyma in form of rays.
7. The secondary growth in this central vascular cylinder is normal.
In a number of dicotyledons, fleshy roots are developed to serve as organs of storage. Anatomical anomalies are often noticed in these organs which develop considerable amount of storage parenchyma. Again, due to these anomalous cambial activities, the conducting elements are formed with suitable arrangements for translocation.
Massive development of storage parenchyma is noticed in many cases in the region of cortex or secondary phloem. Therefore, many fleshy roots exhibit a notable deviation from the normal structure in the comparatively poor development of woody tissues.
(a) In the fusiform underground tap roots of radish (Raphanus sativus of family Cruciferae) the storage tissue mainly originates by proliferation of parenchyma of pith region. Massive storage parenchyma may also be produced from the secondary xylem parenchyma and xylem rays. The vascular elements — with some poorly developed fibres — remain arranged in concentric rings here.
(b) The fleshy nature of the conical tap root of carrot (Daucus carota of family Umbelliferae) is also due to massive development of storage parenchyma in the phloem and xylem.
(c) In napiform root of sugar beet (Beta vulgaris) of family Chenopodiaceae a striking anomalous form of secondary growth is observed (Fig. 5.157):
1. In this root several successive accessory cambial layers are formed.
2. The first cambium layer forms a ring of vascular bundles near the primary xylem.
3. Several secondary accessory cambia very soon arise in the pericycle with rapid succession.
4. All the cambial layers continue to function, but the rate of activity becomes gradually slower after the early period of growth.
5. Each cambium produces separate bundles with surrounding conjunctive tissues.
6. The newly-formed cambia divide and multiply very rapidly to build-up parenchymatous layers.
7. As a result, alternate layers of proliferated pericycle and vascular elements are formed. The bundles are largely parenchymatous with very scanty lignified elements.
8. Growth continues in the bundles by cambial activity and by proliferation of xylem and phloem parenchyma.
9. Thus concentric growth rings are produced with the formation of a complex system as incomplete cylinders irregularly associate with other layers.
10. The major bulk of the storage parenchyma (sink), however, belongs to phloem which communicates with the source cells. Sucrose is the storage form of carbohydrate in the parenchyma cells.
(d) The fleshy adventitious root of sweet potato (Ipomoea batatas) of family Convolvulaceae exhibits another complex type of anomalous growth (Fig. 5.158):
1. Cambium arises in the normal position in between the xylem and phloem.
2. Both primary and secondary xylem elements develop in the usual manner with the formation of a large proportion of parenchyma.
3. With further development many secondary accessory cambia arise in the parenchyma around individual vessel or vessel groups.
4. These cambia produce a few tracheary elements adjacent to vessels, a few sieve elements and laticifers away from the vessels, but a fairly large number of parenchyma in both the directions.
5. At a still later stage, tertiary cambia may be formed in the parenchyma. Periderm of pericyclic origin occurs as normal in the periphery.
In all these roots secondary growth occurs mainly for the storage purpose with formation of scanty conducting tissues but large amounts of storage parenchyma in concentric rings. So, the anomaly in these cases is adaptive in nature.