In this article we will discuss about the common anomalous structures that occur in stems.
A great diversity in the arrangement of vascular tissues is exhibited by the angiosperms in their stems. The arrangements, which exist in majority of the plants, are considered as normal. The variations are regarded as anomalous though the so-called anomalous structure is normal for the particular species.
The anomalous structure, which occurs in the primary body of a plant, is defined as primary anomalous structure. These anomalies occur in both monocotyledonous and dicotyledonous stem, and may persist throughout the life of the plant.
The followings are the common anomalous structures that occur in stems:
1. Occurrence of Scattered Vascular Bundles in Dicotyledonous Stem: (Fig. 19.1C):
Normally the vascular bundles, in the dicotyledonous stem, are arranged more or less in a ring. The followings are a few variations where the vascular bundles are more or less irregularly scattered in the parenchymatous ground tissue of the stem: Ex. Thalictrum (Ranunculaceae), Podophyllum peltatum (Podophyllaceae), Papaver orientale and P. somniferum (Papaveraceae), Bougainvillea (Nyctaginaceae), Piper betle and Peperomia langsdorfii (Piperaceae), Nymphaea (Nymphaeaceae) etc.
2. Occurrence of Vascular Bundles Arranged in a Ring in Monocotyledonous Stem (Fig. 19.1D):
In monocot stems the stele is atactostele, i.e. the vascular bundles are scattered in the ground tissue. But the followings are the exceptions where the vascular bundles are arranged more or less in one or two rings. Ex. Coix, Triticum, Oryza (Poaceae), Tamus communis (Dioscoreaceae) etc.
3. Occurrence of Cortical Bundles (Fig. 19.1A):
In dicotyledonous stem the vascular bundles occur in the stele. But there are some dicotyledonous stems where accessory vascular bundles are found in the cortex of stem in addition to stelar bundles.
The vascular bundle, which runs through the cortex independent of the ring of bundles present in the stele, is referred to as cortical bundles. In most species cortical bundles are leaf trace bundles. Metcalfe and Chalk (1950) listed thirty-seven families where cortical bundle occurs.
Mention may be made of Bombax malabaricum (Bombacaceae), Calycanthus occidentalis, Chimonanthns (Calycanthaceae) where four pericyclic bundles (leaf traces) occur with internal phloem and external xylem; Campanula pyramidalis (Campanulaceae); Crepis, Senecio, Vernonia (Compositae); Cotyledon (Crassulaceae); Momordica (Cucurbitaceae); Armeria, Plumbago europaea (Plumbaginaceae); Peltiphyllum (Saxifragaceae); petiole of Eryngium campestre, Siler trilobum (Umbelliferae); Nyctanthes arbortristis (Oleaceae) where four inversely oriented cortical vascular bundles occur with external xylem and internal phloem and these bundles are not directly connected with the main axial ring and in some genera of Rutaceae and Polygonaceae etc.
4. Occurrence of Medullary Bundle:
The supplementary bundle, which occurs in the pith of dicotyledonous stem in addition to the normal ring of bundles, is referred to as medullary bundles (Fig. 19.2).
Metcalfe and Chalk (1950) reported thirty-seven families where medullary bundles occur and a few examples are mentioned below:
Acanthus (Acanthaceae) where collateral and inversely oriented medullary bundles occur; Achyranthes aspera (Amaranthaceae) with two medullary bundles at internode and Cyathula prostrata with four medullary bundles; Obione portulacoides, Atriplex hortensis (Chenopodiaceae); Crepis, Dahlia, Lactuca, Sonchus (Compositae); Greenovia, Echeveria (Crassulaceae); Raphanus sativus (Cruciferae); Anthocleista (Potaliaceae); Bougainvillea, Mirabilis longiflora, Neea, Pisonia (Nyctaginaceae); Apium, Eryngium, Oenanthe (Umbelliferae) etc.
5. Occurrence of Internal Bundles:
The supplementary bundles, which occur below the normal ring of bundles and above the medullary bundles, are referred to as internal bundle. These bundles are not so deeply situated in the pith to be classified as medullary bundles. Ex. Rumex crispus, R. orientalis etc.
6. Occurrence of Internal Phloem or Intraxylary (Perimedidlary, Medullary) Phloem:
The phloem that occurs at the margin of pith in the form of a continuous cylinder or strands is referred to as intraxylary phloem or internal phloem. It is also known as medullary phloem due to its occurrence at the margin of pith.
Metcalfe and chalk (1950) reported the presence of internal phloem in twenty-seven families and a few examples are mentioned below: Wrightia, Vinca, Thevetia, Landolphia (Apocynaceae); Calotropis (Asclepiadaceae); Convolvulus, Ipomoea, Evolvulus (Convolvulaceae); Acanthus, Barleria (Acanthaceae); Solanum, Nicotiana (Solanaceae) etc.
7. Absence of Vessels in Angiosperm:
The xylem of angiosperms is characterized by the presence of vessels. Exceptions are noted in families Tetracentraceae (Tetracentron), Trochodendraceae (Trochodendron) and Winteraceae (Drimys, Pseudoivintera, Bubbia, Zygogynum) where vessels are absent.
8. Occurrence of Polystele in Dicotyledons:
Usually the stele of dicotyledonous stem is monostele, i.e. the vascular bundles are arranged more or less in a ring and lie within an endodermis. But variations occur when each vascular strand becomes completely enclosed by an endodermis.
As a result the stele becomes polystele. Ex. Nymphaea (Nymphaeaceae); Parnassia palustris (Parnassiaceae) whose nodes are polystelic; Dianthera americana where seven steles occur and each stele is represented by a vascular strand, which is completely enclosed by an endodermis.
One stele is situated at the centre and the remaining six steles are arranged towards the periphery of stem. Polystelic condition is also reported from Justicia. The stolons of Gunneraceae exhibit polystele. The fruit stalks of Malvaceae, Meliaceae, Moraceae, Sterculiaceae and Theaceae are also polystelic.
9. Occurrence of Separate Xylem and Phloem Bundles:
Usually the two vascular tissues xylem and phloem occur together to form bundles. In abnormal cases bundles consisting of either xylem or phloem strands only occur in addition to the normal bundles. In Cuscuta fully developed phloem occurs interspersed between other normal collateral bundles.
Incomplete bundles consisting of phloem only are observed in Rumex crispus, Ricinus communis, Xanthium strumarium, Antigonon leptopus, Achxjranthes aspera, Mirabilisjalapa etc., Maheswari (1930).
In Boerhaavia diffusa incomplete bundles are formed as a result of centripetal differentiation of phloem from procambium cells (Maheswari). In these procambium cells xylem is not formed on the inner side, instead the innermost undifferentiated cells divide tangentially to form cambium.
In Utricularia and Polypompholyx the two vascular tissues are not associated in bundles; instead they lie in alternate strands attached to a ring of sclerenchyma, consisting of fibres only. The two vascular tissues of Circaeaster agrestis stem like that of the main root lie separately at alternate radii. In this species there is a diarch xylem plate and phloem is situated on both sides.