Abnormal secondary growth in dicot stem

bignonia ,boerhaavia, achyranthes, amaranthes, leptadinea, mirabilis

1. Anomalous structure in dicotyledonous stem DR. VISHNUPRIYA SHARMA DEPARTMENT OF BOTANY M.Sc. Sem –II , Paper BOT202 Anatomy and Reproductive Biology of Angiosperms

2. Anomalous secondary features in dicot stems The dicotyledons stems in most of cases have the normal structure in which a ring of collateral vascular bundles are found. At the time of secondary growth, inner and interfascicular cambium cut off xylem towards the inner side and phloem towards the outer side .However in a number of plants abnormal conditions are met with Abnormality is due to the two causes :
(1) In some the primary structure itself is abnormal.
(2) The primary structure is normal but secondary growth is abnormal

The anomalous structures in dicot stems may be due to : 1. The presence of medullary or cortical bundles. In Boerhaavia there are two rings of medullary bundles-inner of two, rather, big bundles and in outer of generally six or more vascular bundles. In Achyranthes there are only two medullary bundles, which in internodal part, point their proto xylem towards each other. In Mirabilis, Bouganvilea and other members of the family Nyctaginaceae medullary bundles are present. In Chenopodium the medullar bundles remain embedded in parenchymatous pith. Here they are said to be leaftraces 2. Due to the presence of cortical bundles. Some times a ring of vascular bundles is found in cortical region. These bundles may be referred as "cortical bundles" or they may be interpreted as leaf traces. The presence of such type of bundles has been studied in many families e.g. Bignonaceae,Casuarinaceae , (Casuarina) Cucurbitaceae, Proteaceae , Oleaceae and many others.

3. (2)Due to the absence of vessels. Though the presence of of vessels in angiosperm s is normal feature but some genera like Ballium , Drimays , Exospermum , Wintera , Zygogynum , Trochodendron Tetracentron do not posses vessels.
(3) Due to the abnormal behaviour of normal cambium: Sometimes when the normal cambium starts cutting cells at several places in irregular fashion and form at some places much larger proportions of xylem than of Poem, and at other places more phloem than xylem and a rigid and furrowed ylinder is produced. They may be simple type e.g. Bignoniaceous species or a complex one as in Bignonia.
(4) Due to presence of phloem bundles. Sometimes only phloem bundles are found developed in between outer collateral bundlese.g . Cuscuta .
(5) Due to presence of scattered vascular bundles in dicots e.g.Peperomla , Piper, Thalictrum, Nymphaea, Papever originale and. Elas-tostemma sessiliS .
(6) Due to presence to Interxylary or included phloem. In some plants like Combretum, Salvadora , Leptadenia etc. the interxylary phloem occurs

4. Abnormal Behaviour of Secondary cambium Normal cambium with abnormal activity eg Bignonia, Aristolochia Abnormal cambium with abnormal activity eg Amaranthus , Achyranthus , Boerhaavia Mrabilis , Chenopodium Abnormal secondary growth in Bignonia Stem Normal cambium with abnormal activity The transverse section of young Bignonia stem is like that of typical dicot. Stem, At the time of secondary growth the Inter fascicular cambium develop in the normal way forming a complete ring of cambium. Now the cambium ring forms xylem towards inner side and phloem towards outside in the normal way. The amount of secondary xylem formed is much more as compared to the amount of secondary phloem formed.
The cambium ring after performing, for a short time, its normal functions, begins at certain point to give off internally only a very small quantity of xylem and external a large amount of phloem. As a result of this ,deep wedges of irregularly widening phloem project into the outer xylem. In this way normal cambium shows abnormality as a result of which 4, 8 or more wedges are formed. The formation of cork cambium and cork takes places in the normal way. Abnormal cambium' with abnormal activity -In this case the normal cambium ceases its functioning after very short period and then an abnormal type of cambium development takes place from cortex or pericycle and behaves in an abnormal way, as for e.g., in Amaranthus, Achyranthus Boerhavia , Bougonvilla , Chenopodium

5. Abnormal secondary growth in Amaranthus stem The transverse section of young Amaranthus stem is some what circular in outline with a single layered epidermis, few layered endodermis and parenchyma. There is also present a single layered collenchyma and few layered pericycle and a central pith. In the central region their remain present two collateral ,open and conjoint vascular bundles known as medullary vascular bundles. In this stem at the time of secondary growth
(a) The normal cambium is inactive.
b) The cambium develops from the pericycle.
(c) The activity of this cambium is abnormal.

So at the time of secondary growth any layer of the pericycle becomes meristematie and forms pericyclic cambium. The cambium starts to cut xylem on the inner side in patches alternating with pårenchyma patches and phloem towards outer side just above the xylem patches and phloem towards outer side just above the xylem patches and parenchyma at rest of places. The previously formed parenchyma get shifted upward phloem than form secondary cortex. Now at this stage the vascular bundles so formed from a structure like that of original collateral, open and conjoint vascular bundles Now the activity of this cambium cases and becomes functionless. After it another cambium development take place from secondary cortex. Its activity is just similar to the of pericyclic cambium as a result of which one more ring of vascular bundle formed outside the first ring of vascular bundles. After this, cambium so becomes functionless and third cambium develops from the secondary cortex which also shows the similar type of activity and as result of which third ring of vascular, bundles formation takes place and in this large number of rings of vascular bundles formed formation takes place. The cambium formation takes place in the normal way. In this way Amaranthus stem shows abnormal secondary growth, in which abnormal cambium shows abnormal activity.

6. Abnormal Secondary growth in Boerhaavia Stem Young stem of Boerhaavia (family Nyctaginaceae ) structure is just like a typical dicotyledonous stem with the only difference that it has three rings of vascular bundles. In the centre there are present two big sized vascular bundles forming first ring of vascular bundle and known as medullary vascular bundles. Surrounding the medullary bundles there remain present a ring o f6 to 20 normal sized vascular bundles forming the second ring of vascular. Outer the second ring of vascular bundles arranged in a ring and contain 15 to 20 vascular bundles. At the time of secondary growth the cambium of first and second ring of vascular bundles remain inactive and show a very little activity within the vascular bundles only. The cambium of the third ring of vascular bundle remain active and by the development of interfascicular cambium completes the cambium ring. Now this normal cambium ring shows abnormal activity.
In the regions of vascular bundles the cambium forms xylem towards innerside and rest of places it cuts sclerenchyma, while towards outerside it forms simple parenchyma only for a short period and then cambium begins to cut the phloem on the outer side just above the xylem which alternates with the sclerenchyma and in this way the previously formed parenchyma gets pushed up ,and the cambium ceases its activity. After some time the formation of cambium takes place from the parenchyma. Thus in Boerhaavia stem the secondary growth takes place by the abnormal cambium with abnormal activity.

7. Abnormal secondary growth in Achyranthus stem- In this case the anomalous growth in thickening takes place by the development of the successive rings of the collatral bundles from one ring of secondary meristematic tissue in the pericyclic region. The secondary arch of rings or secondary meristematic tissue in the pericycle give rise to collateral vascular bundles successively. The conjunctive tissue between bundles may be consisted of parenchyma in some species and in others of lignified or unlignified parenchyma.

8. Mirabilis Stem (family Nyctanginacéne ) The transverse section shows a outer most layer epidermis which is covered with a thin cuticle. Below epidermis there is a broad cortical region consisting of collenchyma. parenchyma and endodermis. Below endodermis few layer of parenchymatous cell represent pericycle, Below the pericycle the phloem is present. The xylem is represented by a continuous cylinder of xylem parenchyma and xylem vessels.
The central portión of the stem consist of thin walled rounded oval shaped or polygonal parenchymatous cells, called ground tissue. The abnormal secondary thickening occures in the form of a succession of rings of vascular bundles. The secondary bundles exhibit a concentric or irregular arrangement becoming embedded in parenchymatous tissue.

9. Topographical sketch of Mirabilis stem after secondary growth

10. TS of Mirabilis stem after secondary growth

11. Abnormal secondary growth in Leptadenia stem : The T.S. of stem of Leptadenia (family Asclepiadaceae ) shows an outer most layer epidermis, consist of single row of compact cells and is covered with a thick cuticle. Below epidermis, hypodermis is present which consist of one or two layer of thin walled parenchymatous cells. Below it few layers of chloroplasts and there cells known as chlorenchyma. Below it endodermis is present which consists of barrel shaped and compact cells. Below the endodermis, a broad pericycle is present. This zone consists of thin walled parenchymatous cells interrupted by unlignified patches of sclerenchyma. Below it the phloem is present. The secondary phloem is conspicuous consisting of sieve tubes, companion cells and phloem parenchyma. The primary phloem is also seen at certain places in crushed froms . Below secondary phloem cambial zone is present. The cambium consists of thin walled rectangular cells arranged in radial rows. Below cambium the xylem is present which is represented by both primary and secondary xylem vessels and vascular tracheids . The primary xylem is confined towards pith region. The secondary xylem which is continuous remain interrupted by the strands of interxylary or included phloem which develop centripetally. In the central region a less develop parenchymatous pith also remain present. The latex tubes are also found among the parenchymatous tissue. The stem of Leptadenia shows anomalous structure due to the presence of included phloem in secondary xylem and xerophytic characters due to the presence of ( i ) a thick cuticle over epidermis. (ii) photosynthesis tissue is present in stem, (iii) well developed sclerenchymatous pericycle is present, (iv) the latex tubes are present and (v) the mechanical tissue is well developed