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Analele ştiinţifice ale Universităţii “Al. I. Cuza” IaşiTomul LII, s. II a. Biologie vegetală, 2006

MORPHOLOGICAL AND HISTO-ANATOMICAL ASPECTS AT SOMEDICOTILEDONATE SEEDLINGS RELATED TO THE

VASCULAR TRANSITION

RODICA RUGINĂ, C.TOMA, LĂCRĂMIOARA IVĂNESCU

Abstract: The authors investigate the morphology and structure of seedlings from 4 differentspecies of herbal dicotyledonate: Mirabilis jalapa L., Pisum sativum L., Ricinus communis L.and Tropaeolum majus L., some with epigeal and some with hipogeal germination. Studying thestructure of the vascular system during 21 days from the germination, we distinguished thetransformation from the primary structure of the root to the primary structure of the stem,underlining the level where the histological changes take place along the axis of the seedlings, theorder in which the xylem vessels appear, extinguish and other appear in different positions. Thegathered data support the theory of torsion as well as the theory of desmogenesis. The typeof germination does not influence the level where the vascular transition takes place.Key words: blastogeny, vascular transition.

Introduction

The metamorphosis of the alternative structure of the vascular apparatus of theroot into the overlapped structure of the stem has been a very interesting topic since thebeginning of the 20th century, especially for the French school of Botanic.

During the last decades, very many theories and hypothesis were stated, amongwhich the desmogenesis developed by G. Chauveaud, revolutionary as thinking mannerthrough its data about the hystogenesis.

Although numerous pieces of information accumulated conclude in this direction,it seams that the theory of rotation and torsion is assimilated by the majority of thebotanists in their studies regarding the hystogenesis of the vascular system .In this paper wewish to show based on the analyzed material that the theory of rotation actually providesdata in favor of the Chauveaud theory.

The lack of information regarding the hystogenesis of the vascular apparatus inthe Romanian literature of specialty encouraged us in this work, even if it will onlybecome a starting point for future research in the field of vegetal histology.

Botanical Garden of Iassy University “Al.I.Cuza” Iassy, Faculty of Biology

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Historical references

As we have already mentioned, at the beginning of the 20th century and duringits course the French botanists have approached many aspects regarding the structureof the vascular apparatus at seedlings and its transformations in ontogenesis.

Thus, Lenoir M. [3], as follower of torsion and deduplication, supports thistheory with the modifications of the vascular system at seedlings from Veronica andCucumis species, also given the histological data and the physiological argumenţs.

Chauveaud G. [1] rejects vehemently the previous theories regarding the vasculartransition, elaborating a new one related to the evolution of the vascular system and todesmogensis, distinguishing the , alternative position of the vascular bundles (characteristicto the root), the intermediary one (characteristic to hypocotyl) and overlapped(characteristic to the stem). The author also claims that the first elements conductor ofphloem and then of xylem appear from the direct differentiations of the fundamentalparenchyma and the next ones from the differentiation of a meristem (procambium). Thesame G. Chauveaud first defines the notion of contingent, taken over later by A. Tronchet[5, 6] and used to describe the structure modifications of the vascular system atBrassicaceae.

H. Hayward [1967] in his monographic work named "The structure ofeconomic plants", studying the morphology and anatomy of some culture plants, alsorefers to a few particularities of the germination of seeds, to the structure of seedlingsand to the hystogenesis of their vascular system.The school of vegetal morphology and anatomy from Iaşi, considering some aspects ofblastogenie at different species of Fabaceae [4] underlined the fact that the mostimportant characters with taxonomic value are offered by the shape and size of thecotyledons, of the folios and tendrils.

Material and methods

The morphology of germination and seedlings (starting with its cotiledonarphase and lasting until the appearance of the first 2-3 leaves) was studied on materialobtained from sowing in Petri boxes or flower pots.

We analyzed seedlings from 3 to 21 days old belonging to the followingspecies: Mirabilis jalapa L., Pisum sativum L., Ricinus communis L. and Tropaeolummajus L.

To observe the changes of the vascular tissues, the hysto-anatomical analyzewas done on transversal sections at three levels trough the root, the hypocotyls andepicotyls. The sections were done using the hand microtome but also using the one withparaffin and then they were colored with Heidenhaim hematoxyline and/or with greeniodine and alaunat carmine.

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The purpose being to research the evolution of the vascular system, our attentionwas focused on the central cylinder and less on the other anatomical zones of theinvestigated organs in which the modifications were not very significant during the first 21days from germination.

Results

Mirabilis jalapa L. (Nyctaginaceae)

The morphology of the seedlings. The germination is epigeous and takes placeafter 14 days from the sowing, when at the surface of the soil the two asymmetricalcotyledons appear. After another 2-3 days, the root measures 6-7 cm and. the hypocotyl 2cm. At seedlings of 21 days the growth is obvious the growth in length of the root and ofcotyledons and less at the hypocotyl that forms tubers (just 2.5-3 cm). The cotyledons, bigand foliaceous have the limb of 1.5-2 times wider than its length, with a long andemarginated tip. The slightly heterocotily and the forming of tubers are characteristic alsoto other species from the same family (for example Abronia umbellata).

The structure of the seedling. The structure of the central cylinder of the rootfrom a 14-day old seedling (at the upper and middle level) is diarch. The ligneous fasciclesare next to the centre trough the metaxylem vessels relatively large and the phloem formstwo blades tangential at the pericycle.

In the upper third of the root new metaxylem vessels appear, lateral to the onesalready mentioned above, along the axis of the organ forming a compact xylem corp. After2 (3) days, in the same area but especially at the lower third of the hypocotyls, the centralcylinder becomes very thick; in its axis we can find a parenchyma mass with very big cells.Meanwhile, other new ligneous vessels have formed in the shape of tangential blades thatalternate with the two fascicules of primary xylem still persistent; almost simultaneouslynew groups of phloem elements are formed.

During this phase of siphonostele, the vascular system is represented by 4fascicles with overlapped xylem and phloem, typical structure for the stem. The hypocotylprepare a stem structure with 4 fascicles at the lower and 8 at the upper part. The conductortissue of the epicotyl (the epicotyledonary internode) has a ring shaped type, withprocambium in two layers, which has produced in the exterior groups of phloem elementsoverlapped on some vessels of xylem. In the medullar parenchyma we can distinguish morelibero – ligneous fascicles (12-13).

Pisum sativum L. (Fabaceae)

The morphology of the seedling. The germination is hypogeous and takes place12 (14) days after the sowing. It is visible when trough the micropyl the axis root-

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hypocotyls appear at 1-2 (3) mm in length at the beginning. In 5-6 days it reaches 2.5-3 cm;in this stage the first layer of epicotyl internode is also visible of 2-3 mm in length. It iscurved at the beginning but in 14 days it becomes straight and measures 2-2.5 cm; on thesurface of the root we observe 3 orthostichies of lateral roots as nipples shaped. The firsttwo leaves look like some bractei and it seems that they have formed previous to thegermination; only the third leaf is a typical nomophyll.

The structure of the seedling. Inside the central cylinder of a root belonging to a14-day old seedling (in the lower and middle third) and then to a 21-day old (in the lowerthird) we can observe 3 fascicles of xylem and 3 of phloem. Each fascicule of xylempresent 7-8 narrow vessels of protoxylem (of 10-20μm) and 4-6 wide vessels of metaxylem(of 20-40 μm). The phloem fascicules contain 4-5 (6) vessels next to the pericycle.

In the small length (3-4 mm) of the hypocotyl the axis becomes parenchymatic.The xylem vessels group in 3 tangential blades neighbors to 2 groups of phloem vesselsfascicles. Under the cotyledons, 2 of the phloem fascicles together with the new formedwill penetrate toward their inside.

In the epicotyl internode (in the lower and middle third) we find a typical structurefor the root, with the xylem occupies the center at this day with 4 xylemic poles; next tothem, at side, we distinguish 4 groups of phloem elements.

The next internodes of the epicotyls (2 and 3) have a intermediary structurebetween root and stem: 4 blades of tangential xylem and 4 of phloem opposed to the fîrst;next to them, in the centre, the initial protoxylem and metaxylem persist. In the upper thirdof the internode number 3 and all along internode number 4 the typical caulinar structureofthe conductor tissue consists of 6 libero – ligneous fascicles, among which 2 belong to[Hayward H., 1967] the 3 and 4 nomopylls; these are vîsible since the fîrst epicotilarinternode, as the 2 cortical fascicles probably belonging to l and 2 bracteant leaves.

Ricinus communis L. (Euphorbiaceae)

The morphology of the seedling. At 14 days from the sowing, the epigeangermination takes place. The growth of the root's length happens simultaneously withthe hypocotyl’s. At the beginning, the hypocotyl is curved and then becomes straightreaching the extraction of the cotyledons from the albumen. Until the first leaves appear(after 21 days) the cotyledons grow and become foliaceous so able of photosynthesiswith a 5-6 cm petiole and a slightly oval limb of 6-7 cm in length and 5-6 cm wide.

The structure of the seedling. The central cylinder of the root is tetrarch. At a14-day, the xylem from the lower third of the organ is represented by 4 fascicles with 9-10tight vessels each, with polygonal contour in transversal section, alternating with the samenumber of phloem fascicles. After a week, at the same level at the root, between thementioned fascicles 8-9 long metaxylem vessels appear. At the same age (21 days) at theupper part of the root, the axis of the central cylinder becomes parenchymatic trough the

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resorbtion of the metaxylem. We can still see protoxylem vessels (alternative) and thephloem vessels opposed to them; the new vessels of xylem are wide, with thin and stillnonlignified walls.

Discussions and conclusions

A long time scientists considered that the transition from the root's structure tothe stem's one was done in the collet area. But, as we have already seen, the transition fromthe alternative structure of the root to the overlapped structure of the stem takes place atdifferent levels in the root, hypocotyl and epicotyl.

According to G. Chauveaud [1] the three organs represent different stadiums ofevolution of the vascular system. During the ontogenetic evolution different phenomenatook place: appearance of xylem and phloem vessels, their resorbtion, re-settlement of thenew vascular elements, passing from the altern stage to the intermediary (tangential) oneand overlapped.

The start of the overlapping phase is given by the appearance of a meristematictissue which will produce xylem and phloem in centrifugal direction and respectivelycentripetal. These structures are not different types but are different phases of the generalevolution type of the conductor system.

The alternative structure we observed at the 4 species studied is found only atthe lower third of the root; at the middle and superior level (basal) ,the tangential phase isobvious at Ricinus communis L. and even overlapped at Mirabilis jalapa L. andTropaeolum majus L.

At Pisum sativum L. the modifîcation of the conductor system take place in thel, 2, 3 and 4 internodes of the epicotyl. In the basal internode of the epicotyl (1) on a smallarea of its base, we find a xylemic central core characteristic to the root.

The vascular evolution in the hypocotyl, even if only conserving the tangentialphase, makes us believe that this formation belongs to the hypocotyl – root axis.

The type of germination epigeal and hipogeal does not influence the level ofvascular transformations along the axis; it can be produced in the epicotyl (Pisum sativumL.) or in the root (Tropaeolum majus L.).

BIBLIOGRAPHY

1. CHAVEAUD G., 1911 – L’appareil conducteur des plantes vasculaires et les phases principales de sonévolution. Ann. Des Sci. Nat., Bot., sér. 9, 13: 114 – 4382. HAYWARD H., 1967 – The structure of economic plants. Ed. Cramer J., New York3. LENOIR M., 1920 – Evolution du tissus vasculaire chez quelques plantules de Dicotylédones. Ann. Des Sci.Nat., Bot., sér. 10, 2: 1 – 1234. TOMA C., GEORGETA TEODORESCU, ANGELA TONIUC, 1975 – Donées morphologiques concernantles plantules de quelques Légumineuses. An. st. Univ. “Al. I. Cuza” Iasi, s. II a (biol.), 21 73 – 76

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5. TRONCHET A., 1930 – Recherches sur les types d’organisation les plus répandus de la plantule desDicotylédones. Leurs principales modifications, leur rapports. Extr. d’ Arch. De Bot., 4, 1: 1 – 2376. TRONCHET A., 1952 – La valeur de notion de convergent chez Phanérogames et la réalité des phénomènesd’acceleration basifuge. L’Année biol., 56e ann., sér. 3, 28, 7 – 8 : 181 - 18

Explanation of platesPlate I Mirabilis jalapa L. Transections at various level of root and hypocotylPlate II Ricinus communis L. Transections at various level of root and hypocotylPlate III Pisum sativum L. Transections at various level of root and epicotyl12, 14, 16, 21 d – day of the seedlings

AbreviationsA, B, C – root, at lower (A), middle (B) and upper (C) levelD, E – hypocotyls, at lower (D) and upper (E) levelF, G - epicotyl, 2 (F) and 3 (G) internode1 – cortical parenchyma; 2 – endodermis; 3 – pericycle; 4, 5 – alternative xylem: proto- (4) and metaxylem (5); 6– tangential xylemic vessels; 7 – superposed xylemic vessels; 8 – alternative phloem; 9 – tangential phloem; 10 –superposed phloem; 11 – cambium; 12 – pith ray; 13 – pith.

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