DEVELOPMENT OF ANOMALOUS SECONDARYVASCULAR RINGS IN THE ROOT OF SPERGULA, L.

By PRAXASH CHANDRA' JoSHT,

Department of Botany, Panjab University, Lahore.

Received March 4, 1935.(Communicated by Dr. H. Chaudhuri, Panjab University.)

1. Introduction.

PRESENCE of secondary vascular rings outside the central stele of the axishas already been recorded in about two species of Sjiergula and several othermembers of the family Caryophyllaceae (Solereder, 1908). As far as thewriter is aware, no detailed account of the anomalous growth has beenpublished by any author so far. Solereder believes that the developmentof the secondary zones is probably in all cases extra-fascicular, i.e., frommeristems formed in the primary cortex. Last year the writer came across

a similar secondary growth in the rootsof SSerg'uia rubra, D. and S. pentandra,L., growing in Lahore. The purposeof the present paper is to put this onrecord and further to describe thedetails of the process in S. rubra, D.The stem of these species shows aperfectly normal structure.

2. Investigation.

The primary structure of the rootof Spergula rubra is perfectly normaland similar to that of an ordinarydicotyledonous root (Text-Fig. 1).Within the piliferous layer are seentwo to three layers of cortex boundedinternally by a distinct endodermis.The cortical cells are quite large, more

TEXT-FIG. 1. Spergula rubra, D. Part of or less rounded in shape, and they area t.s. of a young root, showing normaldiarch primary structure. x 350. separated from one another by small

inter-cellular spaces. The cells ofthe endodermis are somewhat rectangular in t.s. and they have gotCasparian thickenings on their radial walls. These appear at first asminute dots, solitary or two on a wall, but later they become broader,

729

j 3p Prakash Chandra Joshi

after fusing when there are more than one. In the centre of the rootis the stele, consisting of a diarch xylem plate of the usual exarch typeand two groups of phloem on either side of it. The pericycle is mostly asingle laver of small cells, though at some places it is two-layered even atthis stage. This is apparently due to division of some of the original cells.There are a few parenchyma cells separating the xylem plate and the phloemgroups.

The secondary roots are similar in structure to the primary root; bothbeing diarch and secondary growth takes place in both of them in the sane way.

The primary growth in the stele of the root of SSergula rubra is concludedvery early. The rest of the growth is all of a secondary nature, and takesplace mostly through cambial activity. At places, however, some parenchyma

cells also undergo irregular divisionsand slightly increase the girth of theorgan. The cambial activity embracestwo distinct phases ; firstly, thedifferentiation and growth of theprimary cambium (first formed normalcambium), and secondly, the develop-ment of the secondary cambiums (laterformed) . The primary cambium formsa core of vascular tissue in the centreof the root, while the secondarycambiums form two to three super-numerary vascular rings on the outside,one after another, in a concentric

Trx.T-Fia. 2. Spergula rubra, P. Part of at.s. of the root, showing early secondarygrowth by means of a normal cambialring in the stele and the formation of aphellogen in cortical cells; the endo-dermis has disorganised to form anirregular space outside the central cylin-der; the outer parenchymatous cells ofthe central stele are comparatively largeand tangentially elongated. x 350.

lying just over the protoxylemcambium is thus differentiated.

manner.The beginning of normal cambium

is made, as usual, by the assumptionof meristematic activity by the paren-chyma cells lying in between the xylemand the phloem. These cells elongateand undergo tangential divisions.Later on this meristematic activityspreads on to the parenchyma cells

aoints. A complete ring of primary

The primary cambium functions in the normal manner (Text-I ig. 2).It cuts off secondary xylem to the inside and secondary phloem to the out-side. The secondary xylem merges into the primary metaxylem, and the

Secondary Vascular Rings in the Root of Spergula, L. 731

secondary phloem becomes continuous with and indistinguishable from theprimary phloem. On the outside of the protoxylem poles the cambiumfunctions as elsewhere, so that a complete cylinder of wood and bast isformed in the centre of the root. At places, however, where a rootlet traceis given out, the primary cambium in this region forms parenchyma alone andgives rise to wedge-shaped rays invading the central vascular core (Fig. 4) .

As secondary growth advances in the central stele, it exerts pressure onthe peripheral tissues. The epidermis and, a little later, the endodermisalong with some of the adjoining cortical cells rupture at several places.Some of the cells are finally broken down. This results in the appearanceof an irregular empty space to the outside of the vascular cylinder, traversedhere and there by remnants of the walls of the disorganised cells (Text-Figs.2 and 3) . Simultaneously with these changes the cells of the pericycle and

TaxT-Fia. 3. Spergula rubra, D. Part of a t.s. of a root older than one re-presented in Fig. 2, showing the origin of first secondary cambium in theform of small strips inside a band of parenchymatous cells formed as aresult of the activity of the pericycle cells and the adjacent phloemparenchyma; the cambial strips cut off xylem and phloem elements in thenormal manner. x 350.

some of the parenchyma cells of the adjoining phloem begin to grow andincrease in dimensions, especially tangentially. They undergo many divi-sions periclinally, and thus form several layers of parenchyma to the outside

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of the central stele. It is from this tissue that the different secondarycambiums take their origin.

The first secondary cambium is formed while the first formed primarycambium is still active, but after its formation the activity of the latterimmediately slows down. It is never formed as a complete ring. In thefirst instance it is differentiated as a ring of about a dozen cambial stripsof small and variable length. These arise from some of the cells in the middleregion of the parenchymatous zone formed from the division of the pericycleand phloem parenchyma cells. The boundary of the parent cells is easilyrecognised by their relatively thick walls. The cambial strips show no per-fectly regular sequence in their development, but it is invariably observedthat the first ones of these to differentiate lie opposite to the primary phloempatches.

The strips of the first secondary cambium act in the same manner as theprimary cambium. Cutting off xylem to the inside and phloem to the out-side, they give rise to a ring of collateral vascular strands separated from thecentral stele, formed from the primary cambium, by nearly four layers ofparenchyma. As growth proceeds, some of the adjacent cambial stripsbecome continuous with one another, but even then a complete ring ofsecondary cambium is seldom established. Generally, the resulting vascularzone is composed of three or four large arcs of vascular tissue, separatedfrom each other by parenchymatous rays three to four cells wide.

The parenchyma cells on the outside of the tissues formed by the firstsecondary cambium go on growing and dividing, and provide a similar seatof origin for the next secondary cambium (Fig. 5). This arises while theprimary and the secondary cambiums are still active to some extent, andappears in the form of small strips just as the preceding cambium. It alsofunctions in the same manner and gives rise to another ring of vascularbundles, separated from the previous one again by three to four layers ofparenchyma. These bundles are separated from one another by wider radialbands of parenchyma than in the preceding vascular ring, and it is seldomthat any two neighbouring bundles become continuous.

A third secondary cambium may arise in some roots in the same manneras the previous two and give rise to another ring of vascular bundles, butthis is very rare and the condition has been observed only in some exception-ally large roots.

As a result of this type of secondary growth, a t.s. through the base ofa mature Spergula root shows ordinarily two concentric vascular zonessurrounding a solid cylinder of vascular tissue in the centre. The innerzone is composed of three or four large arcs of vascular tissue, while the outer

Secondary IUascular Rine-s in the Root of Spergula, L. 1-33

one is made up of nearly a dozen small bundles (Fig. 4). The compositionof the vascular tissue is more or less alike in all the parts except that someof the peripheral bundles may be still immature and their tissues may stillbe not completely differentiated. The xylem is made of nearly radial rowsof simple pitted vessels separated by rows of prosenchymatous cells. Thewalls of the latter during early stages consist of pure cellulose, but later onsome become lignif ed while others remain unchanged. There are no dis-tinct medullary rays present in the secondary wood or phloem, but whentraces of the secondary roots, which arise opposite the protoxylem poles ofthe primary root, depart, large gaps of parenchyma appear in the centralcylinder simulating medullary rays. The phloem is as usual composed ofsieve-tubes, companion cells and phloem parenchyma. The cells of theradial and tangential bands of parenchyma separating the different bundlesof one vascular zone and different vascular zones respectively are somewhatcircular in t.s. and elongated in l.s.

The phellogen arises from the outer cells of the cortex. This happenswhen the epidermis has been ruptured by the growth of the stele within.It functions in the normal manner. The cork cells cut off from it to theoutside have the usual structure, but the secondary cortex consists of twotypes of tissues. The first formed portion consists of ordinary parenchyma-tous cells, while the portion developing afterwards consists mostly of scleren-chymatous cells forming a very distinct zone separated from the cork bythe cambium in the adult root (Fig. ö).

3. Discussion.

Anomalous thickening of the type such as has been described abovein the root of Sjergula, besides many other Caryophyllaceae, is known inseveral other families of the Centrospermales. In recent years it has beeninvestigated in detail especially in the families Amarantaceae (Schmid, 1928and Joshi, 1931), Chenopodiaceee (Arstchwager, 1926) and Nyctaginaceae(Maheshwari, 1930). Through the researches of these workers it has beenshown that all the supernumerary cambiums are genetically related andstand for the most part in direct lineage with the first secondary cambium.It is seen in these plants that every successive cambium as it begins to func-tion cuts off some parenchymatous cells to the outside and from these thenext cambium is formed. In Spergula, there is no such regular relation,but the two or three rings of the secondary cambiums are related in thisfact that all take their origin from the same parenchymatous band formedby the division of the pericycle cells and adjacent phloem parenchymatouscells.

734 Prakash Chandra Joshi

An interesting feature of the anomalous secondary growth in Sberg'idais that it is restricted only to the root. The stem always shows normalsecondary thickening in the vascular cylinder. A similar feature has beenseen in some other Caryophyllace e and the rest of the Centrospermales,such as, Polycar^on, Cerdia, Stipulicida, Ortegia, Lepigonum, Beta, Blitum,Hablitzia, etc. (Solereder, 1908). The root is usually regarded as a conser-vative organ and the hypothesis may be put forward that in the ancestorsof these plants anomalous secondary growth was more highly developedthan at present and extended to both the root and stem.

The physiological significance of this type of growth is unknown.There is no reason that it should be considered to be related to storage, forspecies of Spergula are mostly annual plants and no storage carbohydratesare seen in the root. Haberlandt (1914) classifies this type of growth amongnon-adaptive anomalies. It is not impossible that more work on such plantsespecially of the experimental type may throw light on the question, butno such study has been undertaken so far.

4. Summary.

The paper records the occurrence of an anomalous type of secondarygrowth in the vascular cylinder of the root of two species of Spergula growingin Lahore, namely S. rubra and S. pentandra, and describes the details ofsuch a development for S. rubra. It is found that as the primary cambiumis forming xylem and phloem in the normal manner, the cells of the pericycleand adjacent phloem parenchyma become meristematic and form a zoneof parenchyma to the outside of the vascular cylinder. From this zone ofparenchyma, the successive secondary cambiums take their origin one afteranother and form separate rings of vascular bundles or large arcs separatedfrom one another by three to four layers of interzonal parenchyma. Usuallyin a root two such secondary cambial rings appear and form two rings ofvascular tissue outside the central vascular core, formed as a result of theprimary growth and the activity of the primary cambium. In rare casesthree such rings are seen.

The writer is much indebted to the late Dr. S. R. Kashyap forseveral valuable suggestions and criticisms.

LITERATURE CITED.

1. Artschwager, E. F. .. Journ. Agric. Res., 1926, 33, 143.2. Haberlandt, G. .. Physiological Plant Anatomy (Eng. transl.), London, 1914.3. Joshi, A. C. .. .. Journ. bid. Bot. Soc., 1931, 10, 213.4. Maheshwari, P. .. .. Journ. Ind. Bot. Soc., 1930, 9, 42.5. Schmid, W. .. .. 1Vaturforschenden Gessellschaft, Zurich, 1928, 73, 542.6. Solereder, H. .. .. Systematic Anatomy of the Dicotyledons (Eng. transl.),

Oxford, 1908.

Prakash Chandra loshi. Proc. kid. Acad. Sci., B, vol. I, Pl. LXXII.

Fio. 4.

FIG. 5.

Secondary Vascular Rings in the Root of Spergula, L. 735

EXPLANATION OF THE PLATE.

FiG. 4.-Microphotograph of a t.s. from the base of a well-developed root of Spergula rubr.z,D., showing two concentric vascular zones surrounding the central solid cylinder;the inner zone consists of 4 large arcs of vascular tissue while the outer one ismade up of more than a dozen vascular bundles. X 21.

FIG. 5.—Microphotograph of a part of the same section as the above but more magnified. X 52.