Proc. Indian Acad. Sci., Vol. 87 B, No. 2, February 1978, pp. 49-53, ~) Printed in .

Morphology of the and mechanism of seed dispersal of the freshwater weed tiara

B K NAYAR and K SWORUPANANDAN Botany Department, Calicut University, Calicut 673 635

MS received 27 December 1976

Abstract. is a freshwater weed introduced on the Kerala coast in 1960 and now spread over a vast area extending over 300 km. It has a very efficient seed dispersal mechanism which is described. In addition, the morphology of the carpel, fruit and seed are described. It is concluded that the effective reproduc- tory mechanisms make L. tiara a potentially dangerous weed.

Keywords. Seed dispersal mechanism; freshwater weed Limnocharis.

1. Introduction

Limnocharisflava (L.) Buch. (, Takhtajan 1966) is a herbaceous aquatic emergent weed of the rice fields and irrigation schemes in many tropical countries. It is listed among the more conspicuous freshwater weeds of Africa, Madagascar and other areas (Koch 1952; Wild 1961) and is regarded by Sculthorpe (1967) as a weed capable of spreading widely and colonising other warm areas of the world to the detriment of paddy cultivation, irrigation and drainage schemes, and in- land water transport. The seems to have been introduced into Kerala some time about 1960. It was first reported wild and spreading from the coastal town of Alleppey, and by 1967 had spread all over the coastal strip from Aroor to Ambalapuzha (Kammathy and Subramanyam 1967) ca 50 km apart. Six years later the weed was reported as widespread at Trichur (over 100 km from Alleppey) and spreading fast (Rao and Das 1973). During the course of a survey of the freshwater weeds of Kerala, especially Salvinia and Eichhornia, we came across L. tiara growing abun- dantly in the area extending from Ponnani to Chavara, a distance of nearly 300 km along the coastal belt. This plant has already become a nuisance in these areas and threatens to be as destructive as Eichhornia on the West Coast. A detailed account of the morphology of L. tiara has been reported recently (Wilder 1974) and it is pointed out how effectively the weed propagates vegetatively by means of bul- bils developed on the stalk. During the course of our survey, we observed certain interesting facts about the very efficient mechanism of seed dis- persal, which coupled with the vegetative propagation, makes L. flavaa very danger- ous weed. These observations, which are not recorded in the detailed studies on this taxon reported in recent years (Kaul 1967, 1976; Wilder 1974) are recorded here along with some aspects of the morphology of the carpel, fruit and seed.

49 50 B K Nayar and K Sworupanandan

2. Observations

The ofL. flava consists of 16-20 laterally compressed free carpels arranged in a tight and connected to a central non-vascularised parenchymatous column which does not extend to the apex, so that the anterior ends of the carpels are free. Kaul (1967, 1976) reports that the carpels of L. flava do not arise simultaneously but arise in a modified trimerous pattern, and that during ontogeny the receptacle elong- ates along with the carpels to form the central column. The carpel complex is ca 2 rnm across and 3 rnm high with a swollen base tapering upwards to a bluntly rounded stigmatic end (figure 1). Each carpel has an expanded basal half and is rather suddenly narrowed upwards to an obliquely placed radially elongated stigma. The stigmata of the carpel complex appear hemispherical, with a central hole and the stigma of the individual carpels radiating from it (figure 2). As reported by Kaul (1976) the stigma is a slit-like opening which gapes at anthesis, permitting pollen to be deposited inside the carpel chamber. The stigmatic surface is devoid of any ap- pendages. Each carpel is wedge-shaped in transection (figure 3) with the two lateral walls thin (composed of 4 or 5 layers of cells near the periphery and two cells near the central column) and papery; the peripheral wall is thick (14--18 layers of cells), chlorophyllous and provided with profuse latex canals. The carpel wall is paren- chymatous and the outer epidermis is composed of narrow cells. The laticiferous system consists of highly anastomosed narrow vessels. The vasculature of the carpel consists of one median large vascular bundle towards the middle of the thick outer wail, two smaller bundles on either side of the median bundle and a pair of closely placed slender highly branched short bundles close to the central column (ovular bundles). The ovular bundles form a network in the lateral walls of the carpel and their ultimate branches supply each individual . The branches of the ovular bundles ramify in the lateral walls of the carpel during fruit development and the network thus formed on either lateral wall (figure 6) extends over the outer wall of the follicle and merges with each other. Thus there is a mantle-like vascular reticulum over the median and lateral bundles of the fruit wall. The median bundle of the carpel does not branch but extends up to the stigmatic end; the two lateral bundles fork at the extreme apex and enter the stigmatic lobes on either side. All the vascular bundles of each carpel originate together as a single strand from the vasculature of the receptacle. The ovular bundles separate before the strand enters the carpel; the lateral bundles separate from the median bundle at the carpel base. are many and are scattered all over the inner surface of the lateral walls. They are anatropous and possess a prominent raphe (figure 4). Kaul (1976) reports that in the studied by him the open only very briefly. On the Kerala coast this is not the case; the flowers open completely and remain open for 5-6 hr (from ca i0 AM to 3 PM). Towards evening the flowers close and soon the which come to cover the carpels deteriorate and form a gum-like mass which encloses the carpels. The fruit is sub-globose, ca 15 mm across and 18 ram high and consists of wedge- shaped tightly adpressed follicles. During development the carpels increase markedly in size, and the cells of their wall enlarge and intercellular air cavities are developed, in the mature fruit wall the cells are highly vacuolated; the protoplast deteriorates later, and the cell cavities become filled with air. The outer epidremis of the lateral walls become thickened. As the follicles mature, the central parenchy- matous cohlrnn collapses, separating them and then they are shed on the water surface. Fruit and seed dispersal in Limnoeharis 51

Figures 1-11. Morphology of carpel, fruit and seed ofLimnocharisflava (c, latex vessel; d, dorsal vascular bundle of carpel ; f, frill-like ridges;/, lateral vascular bundle of carpel; o, ovular bundle; r, raphe; s, vascular supply of seed; v, vascular bundle). 1. Longisection of , showing parietal ovules. 2. Dorsal view of stigmatal complex. 3. Transection of carpels. 4. Ovule. 5. Transection of portion of carpel wall (dorsal surface downwards). 6. Vasculature of follicle, showing vascular supply to ovules (the hatched line on left indicates outer surface of follicle). 7, 8. Two views of the seed. 9. Transection of a portion of seed coat. 10. Diagrammatic representation showing how frill-like ridges are formed on the seed coat. 11. Two floating follicles, one floating with the flat side upwards (A) and the other floating with suture down wards (B).

The follicle walls being spongy and their chamber air-filled, they float on the water surface. Initially they float flat, with one of the lateral flat surfaces uppermost (figure 11A) and maintain this posture for three or four days, during which time they are carried relatively long distances by water. The parenchyma tissue of the central axis of the fruit, which plugs the follicle chamber at the thin end, deteriorates and water enters the chamber. The cells of the lateral walls of the chamber become water-logged and nearly transparent, clearly showing the dark seeds inside. As the cells become water-logged the lateral walls arch out so that the follicle opens out at the suture. The peripheral thick wall of the follicle alone being buoyant at this 52 B K Nayar and K Sworupanandan stage, the follicle turns over in water so that the opening faces downward and the peripheral wall upward (figure 11B). The seeds drop off one by one from the follicle through the ventral aperture. It usually takes two or three days for the follicle to discharge all the seeds. Meanwhile the follicle is carried about in water and the seeds are thus distributed over a wide area. When released from the follicle, the seeds sink immediately. The seeds of L.flava (figures 7, 8) are numerous in each follicle, laterally compressed, ca 1.0 nun long, 0.75 mm broad, 0.3-0.4 mm thick and ovate in outline. They are dark brown in colour (turning black later on) and have a slightly swollen parenchy- matous region next to the hilum at one end. There is a thin depressed area extending from the hilum to about 2/3 the length of the seed on either fiat side (so that the seed appears horseshoe shaped, with the arms of the horseshoe closely placed and connected together by a thin area). The testa is hard but thin, being composed of only two layers of radially compressed cells. The inner layer consists of compactly arranged cells, while the outer is made of much larger ceils (figure 9). The outer wall of each of the otlter cells is protruded either at the middle or at one end to form a flat ridge- like protrusion. The cells being regularly arranged, the protruded region of contigu- ous cells form flat narrow frills (figure 10) transversely encircling the seed in a regular manner. The frills are wavy, interrupted at the median groove on the flat sides (figure 7) and some of them occasionally interrupted irregularly on the narrow sides of the seed (figure 8). The embryo is conduplicately horseshoe-shaped as in all the (Lubbock 1892).

3. Conclusions

The efficiency with which L. [lava has become established and rapidity with which it has spread during the past 15 years along the coastal belt of Kerala emphasize the potential danger posed by this weed. The annual rate of its spread is progressively increasing; about 7 km in the first 5 year period, 20 km during the next 5 year period and currently over 30 km. The floating mechanism of the follicles, with the change in orientation of the floating follicles brought about several days after shedding, ensures wide dispersal of seeds. The gradual opening of the follicle itself ensures that seeds are widely scattered. The tendency of the petals to deteriorate fast to form a gummy layer over the carpel complex is possibly a protective measure to retain the pollen deposited on the smooth stigmatic surface. Also, the gummy layer seems to prevent excessive loss of water from the developing carpels. The efficient mechanism of protection of fertilised carpels and seed dispersal, coupled with its ability for extensive vegetative multiplication by means of detachable bulbils (Wilder 1974) make L.flava a potential threat to waterways, irrigation schemes and paddy cultivation along the West Coast. As it often happens, the warnings of the potential dangers of a weed are usually ignored till the situation gets beyond control and the weed cannot be easily destroyed or its spread checked. It is high time that effective steps are taken to check the spread of L. [lava in India. Fruit and seed dispersal in Limnocharis 53

References

Kammathy R V and Subramanyam K 1967 Limnocharis H. B. K.,ka new to India; J. Bombay Nat. Hist. Soc. 64 389-390 Kaul R B 1967 Ontogeny and anatomy of the of Limnocharis tiara (Butomaceae); Am. J. Bot. 54 1223-1230 Kaul R B 1976 Conduplicate and specialised carpels in ; Am. J. Bot. 63 175-182 Koch W 1952 Zur Flora der ober italienschen Reisfe|der; Ber. Schweiz Bot. Ges. 62 628-663 Lubbock J B 1892 Contribution to our Knowledge of Seedlings (London: Trubner & Co.) Rao A T and Das G C 1973 Limnocharis tiara (L.) Buch. : an on the move in Kerala State; J. Bombay Nat. Hist. Soc. 70 577 Sculthorpe C D 1967 The Biology of Aquatic Vascular Plants (London: Edward Arnold Ltd.) Takhtajan A 1968 Systema et Phylogenia Magnoliophytorum (Leningrad: Komarov Inst.). Wild H 1961 Harmful aquatic plants in Africa and Madagascar; Kirkia 2 1-66 Wilder G J 1974 Symmetry and development of umbellatus (Butomaceae) and Limnocharis flava (Limnocharitaceae); Am. J. Bot. 61 379-394