S T U D I E S I N T H E F A M I L Y S A X I F R A G A C E A E

H. Development of Ovule and Megagametophyte in Parnassia nubicola Wall.*

BY N. P. SAXENA (School of Plant Morphology, Meerut)

Received May 25, 1964

(Communicated by Prof. V. Purl, r.A.sc.)

INTRODUCTION

Parnassia, an interesting genus of the Saxifrageeeae, has reeieved considerable attention. But the embryological aspects have not been worked out since the time of Pace (1912). Mauritzon (1933) repeated the same species later and its embryogeny was worked out by Leb~gue (1953). The present communication is designed to give some information about the development of ovule and female gametophyte in Parnassia nubicola.

The flowers and flower buds of various ages, for the present investigation, were collected by Dr. Y. S. Murty from Mussoorie and by the author him- self from Kufri (Simla) and fixed in formalin-acetic-alcohol. Following the customary methods of dehydration and embedding, the material was sec- tioned at 8-12/~ and stained with safranin-fastgreen combination which gave satisfactory results.

OBSERVATIONS

Development of Ovule.--The ovule appears as a straight structure consisting of a homogeneous mass of parenchymatous tissue. In its hypo- dermal apical region, usually one (Fig. 1), but sometimes two (Figs. 2-3) archesporial cells make their appearance. At this stage, the primodium of the inner integument makes its appearance (Fig. 3) and the ovule starts bending to one side. As a result of further divisions, the integumentary cells begin to project as an annulus all round (Fig. 4). Soon after, the initials of the outer integument also become visible (Fig. 5).

The ovules continue to curve till they become perfectly anatropous (Figs. 6-7). At megaspore stage, the integuments do not cover the nucellus completely. Tannins also start filling in some of the cells of the outermost

* Research contribution No. 61, from the School of Plant Morphology, Meemt College; Meorut. 196

Studies in the Family Saxifragaceae--II 197

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layer of the ovule. To begin with they appear only in the chalazal region but later on they spread in other regions as well (Figs. 6-8).

198 N.P. SAXENA

The free end of the inner integument, forming the micropyle in the early stage, is comparatively thick (Fig. 7). But by the time embryo-sac is formed, the outer integument grows over the inner completely and the two lie very closely appressed. The micropyle is thus formed by both the integuments. The micropylar canal is not straight because the opening of the two integu- ments do not lie in one line. In almost all cases the ovules are stretched at the ehalazal side at maturity (Fig. 8). An abnormal case of haminanatropous ovule has also been recorded (Fig. 9) as in Saxifraga diversifolia (Saxem, 1964). But in this case the abnormal ovule seems to be unitegrnic.

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Megasporogenesis.--The arehesporial cell directly acts as megaspore mother cell, without cutting any parietal tissue and the ovule is tenninucellate. The reduction division in tile megaspore mother cell results in the formation of a dyad (Fig. 10) and subsequently a linear tetrad (Fig. 11) or a T-shaped (Figs. 15--16) tetrad. Tetrads of intermediate type (Fig. 17) are also seen

Studies in the Family SaxifragaeeaemlI 199

where the two micropylar megaspores are divided by an oblique wall with respect to the plane of the tetrad. There is considerable variation as to which megaspore i s going to function from the linear tetrad. Sometimes it is the chalazal (Fig. 12) or more frequently the second or third (Figs. 13-14) megaspore that functions. Among T-shaped tetrads too, any of the megas- pore may function.

Tetrads with two healthy megaspores are also seen. Figure 18 shows a uninucleate micropylar megaspore and a binucleate third megaspore. On the other hand, we also find a binucleate micropylar and a uninucleate chala- zal megaspore (Fig. 19) where the remaining megaspores lie side by side. Cases where two micropylar (Fig. 20) or two chalazal (Fig. 21) megaspores are enlarged, have also been recorded.

Thus, there seems to be a close similarity to what has been reported by Pace (1912) for Parnassia palustris.

Megagametogenesis.--The functional megaspore enlarges considerably and becomes highly vacuolated (Fig. 22). The nucleus divides into two (Fig. 23) and subsequently into four (Figs. 24-25). The mieropylar and the chalazal nuclei are separated by fairly large vacuole, the vacuolation being more pronounced at four nucleate stage. Several of the adjacent nucellar cells start degenerating even at the 2-nucleate stage. The four nuclei then divide to form eight nuclei (Figs. 26-27) and get organised into a Poly- gonum-type of embryo-sac (see Maheshwari, 1950) (Figs. 28-29). The egg in figure 28 possesses a vacuole towards the chalazal side which is unusual for it. The same figure also shows the two polars in a state of fusion. The nucellar epidermis disorganises and the embryosac lies directly enclosed within the integument.

Less frequently, the three antipodals do not enlarge in size or show any division (Figs. 30-31) but in some cases their nucleus has been seen with loose chromatin (Fig. 32). Generally, the antipodals enlarge in size con- siderably and one can mistake them for egg apparatus. They continue to increase either in length or in breadth (Fig. 33). They are mostly filled with dense cytoplasm. Sometimes, the number of antipodals increases owing to their big size and dense contents (Fig. 34). Johri (1960) has attributed nutritive function for such a behaviour of antipodals. Similar cases of in- creased number of antipodals have also been reported in Saxifraga diversi- folia (Sexena, 1963).

A case has also been found where in addition to the normal binucleate embryo-sac we find a secondary sporogenous cell (Fig. 35). The latter over-

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Studies in the Family Saxifragaceae--II 201

laps the former and shows a characteristie vaouolation. This may possibly lead to the formation of a second embryo-sac.

SUMMARY

1. The ovules of Parnassia nubicola are bitegmie and anatropous. They are tenuinucellate.

2. The megaspore mother cell generally forms a linear tetrad but T-shaped tetrads are also known to occur. Persistence of more than one healthy megaspore is also seen.

3. The embryo-sac is Polygonum-type. Enlargement of antipodals is a common feature. Their number may also increase sometimes.

The author wishes to express his sincere thanks to his teacher Dr. Y. S. Murty for his valuable guidance and encouragement. He is equally thankful to Prof. V. Puri for his unfailing interest in the work and to the Ministry of Scientific Research and Cultural Affairs for financial assistance.

Johri, B. M.

Maheshwari, P.

Lcbegue, A.

*Mauritzon, J.

Pace, L. Saxena, N. P.

* Not seen in original.

REFERENCES

.. "Nutrition of the embryo-sac," Prof. Sum. Shoo. Bot., (Darjeeling), 1960, 106-18.

.. "An Introduction of the embryology of Angiosperms," McGraw Hill, New York, 1950.

.. "Embryog6nie des Parnassiac6es: D~veloppement de rembryon chez le Parnassia palustris L. ," C.R. .4cad. Sei., Paris, 1953, 236, 1693-95.

. . "Studien fiber die Embryologie der Familien CrassRlaceae und Saxifragaceae," Diss. Lurid., 1933.

. . "Parnassia and some allied genera," Bot. Gaz., 1912, 54~ 306-29.

. . "Twin Embryosacs in Saxifraga diversifolia Wall.," Sci. and Cult., 1963, 29, 614-15.

. . "Studies in the family Saxifragaceae. 1. A contribution to the morphology and embryology of Saxifraga diversifolia Wall.," Proc. Ind..4cad. SoL, 1964, 60, 38-51.

EXPLANATION OF FIGURES

Flos. 1-9. The ovule of Parnassia nubieola. Figs. 1-8. Development of ovule. Fig. 3. Showing the origin of the inner integument. Fig. 8. Ovule showing the protruded chalazal part, Fig. 9. A hemianatlopous ovule.

(amh., archesporium; eh., chalaza; ii., inner integument; m., micropyle; m.m.e., megasport mother cell; o.L, outer integument; t., tannin).

202 N.P. SAXENA

FIos. 10-21. Megasporogcnesis in P. nubicola. Fig. I0. A dyad. Figs. 11-21. Tetrads. Figs. 11-14. Linear tctrads. Figs. 15-16. T-shaped tetrads. Fig. 17. Tetrad of intermediate type. Figs. 18-21. Tetrads with two healthy megaspores.

(d.m., degenerating rnegaspores; n.e., nucellar epidermis.)

Flos. 22-35. Female garnctophytc of P. nubicola. Figs. 22-29. Development and organi- zation of embryo-sac. Fig. 22. Funclional rnegaspore. Fig. 28. Fusion of polars, also note the position of vacuole in the egg. Fig. 29. Fully organiscd embryo-sac. Figs. 30-34. Anti- podals. Fig. 31. Normal antipodals of the ovule of Fig. 30. Enlarged. Fig. 33. Enlarged antipodals with dense nuclear contents. Fig. 34. Increased antipodals with daxk contents ~. Fig. 35. A secondary sporogenous cell overlapping the normal binucleate ernbryo-sac.

(ant., antipodals; d.n.e., degenerating nucellar cells; e., egg., end., endosperm; emb., embryo; n.e.s., normal embryo-sac; p., polars; s.n., secondary nucleus; s.s.c., secondary sporogcnous cell; syn., synergids).