Aquatic Botany, 22 (1985) 277--289 277 Elsevier Science Publishers B.V., Amsterdam -- Printed in The Netherlands

OBSERVATIONS ON THE VEGETATIVE DEVELOPMENT, FLOWERING AND SEEDING OF CYMODOCEA NODOSA (UCRIA) ASCHERSON ON THE M E D I T E R R A N E A N COASTS OF FRANCE

G. CAYE and A. MEINESZ

Laboratoire de Biologic et d'Ecologie Marines, Groupe de Recherches Marines, Universitd de Nice, Parc Valrose, 06034 Nice Cedex (France)

(Accepted 22 April 1985)

ABSTRACT

Caye, G. and Meinesz, A., 1985. Observations on the vegetative development, flowering and seeding of Cymodocea nodosa (Ucria) Ascherson on the Mediterranean coasts of France. Aquat. Bot., 22 : 277--289.

Along the Mediterranean coasts of France, Cymodocea nodosa (Ucria) Ascherson oc- cupies wide expanses between depths of 0.5 m and 18 m. The stem of C. nodosa shows dimorphism. This is manifest: (1) as vigorously vegetative plagiotropic rhizomes which produce several secondary ramifications per year, and in which the growth rate can reach 1.80 m year -~ in seagrass beds of low density; and (2) by orthotropic axes which ramify little, grow slowly (1--2 cm year-l) , and produce flowers. The dimorphism may be switch- ed by environmental factors. Furthermore, C. nodosa shows a marked seasonality in growth, with rapid growth during April--May, and very much slower growth from Oc- tober-March. This seasonality is also shown by changes in the intensity of development of both ramifications and adventitious roots, as well as by gradual and regular variations in the length of internodes on the rhizome. These seasonal changes also apply to the number of leaves in leaf shoots, and to the length and width of these leaves. As a result, the leaf-area index varies from a maximum of 3.90 in spring to a minimum of 0.32 in early winter. Flowering is annual and may be abundant , particularly in dense beds (up to 226 flowers m-2). Flowers develop terminally on orthotropic axes, and growth of these axes continues by the development of the subjacent axillary bud. In the sediment of C. nodosa beds, seeds are found throughout the year, but germination of seeds has been observed only occasionally on the coasts of France.

INTRODUCTION

The seagrass Cymodocea nodosa (Ucria) Ascherson is widely distr ibuted in the Mediterranean and on the nor thern parts of the Atlantic coast o f A f r i c a ( D e n H a r t o g , 1 9 7 0 ) . A l o n g t h e coas t s o f F r a n c e , th i s spec ies o c c u p i e s w i d e e x p a n s e s b e t w e e n d e p t h s o f 0 .5 m a n d 18 m. I t g rows in d e n s e s t a n d s u s u a l l y n e a r m e a d o w s o f Posidonia oceanica (L. ) Del i le . H o w e v e r , i t s o m e - t i m e s f o r m s p o p u l a t i o n s a s s o c i a t e d w i t h t h e seagrass , Zos tera nol t i i H o m e - m a n n o r t h e alga Caulerpa prol i fera (Fo r s sk~ l ) L a m o u r o u x .

0304-3770/85/$03.30 © 1985 Elsevier Science Publishers B.V.

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T A B L E I

R e f e r e n c e s f o r C y m o d o c e a nodosa s e x u a l r e p r o d u c t i o n

A u t h o r s a n d r e f e r e n c e s P lace O b s e r v a t i o n D a t e

B a l a n s a in d e n H a r t o g ( 1 9 7 0 ) : p. 1 6 5

B a l a n s a in d e n H a r t o g ( 1 9 7 0 ) : p. 1 6 4

Pome2 in d e n H a r t o g ( 1 9 7 0 ) : p. 1 6 5

B o r n e t in d e n H a r t o g ( 1 9 7 0 ) : p. 1 6 3

B o r n e t ( 1 8 6 4 )

L e m a r u s in d e n H a r t o g ( 1 9 7 0 ) : p. 1 6 2

P o s p l c h a l in S i m o n e t t i ( 1 9 7 3 )

F e l d m a n n ( 1 9 3 7 ) : p. 2 4 4

R u g g i e r i ( 1 9 5 1 )

S i m o n e t t i ( 1 9 7 3 ) : p. 4 8 9

L i p k i n ( 1 9 7 7 )

Meinesz ( F i g u r e in J a u b e r t a n d Me inesz , 1 9 7 8 )

B a y ( p e r s o n a l c o m m u n i c a t i o n , 1 9 8 0 )

O r a n Male f l o w e r s , seeds (Alge r i a )

G u l f o f S m y r n i a ( T u r k e y )

F l o w e r s , f r u i t s

C h e r c h e l S e e d s (Alge r i a )

A n t i b e s Male f l o w e r s , ( F r a n c e ) f e m a l e f l o w e r s ,

seeds

A n t i b e s Male a n d f e m a l e ( F r a n c e ) f l o w e r s seeds

T o r r e v i e g a Male f l o w e r s (Spa in )

T r i e s t e F l o w e r s ( I t a l y )

B a n y u l s F l o w e r s ( F r a n c e )

S q u i l l a c e F r u i t s C a l a b r i a ( I t a l y )

9 J u n e 1 8 5 2

8 A u g u s t 1 8 5 2

1858

18 June 1861 August 1861

May- - - June

29 M a y 1 8 8 6

1 8 9 7

M a y - - - J u n e

W i n t e r - - S p r i n g

T r i e s t e F l o w e r s A p r i l - - M a y ( I t a l y ) 1 9 6 6 - - 1 9 6 9

Seeds J u n e - - A u g u s t 1 9 6 6 - - 1 9 6 9

I s rae l Seeds 16 A u g u s t 1 9 6 3

A n t l b e s Male a n d f e m a l e M a y - - J u n e M e n t o n f lowers , s eeds 1 9 7 5 - - 1 9 7 6 C a n n e s S e p t e m b e r - - D e - B e a u l i e u c e m b e r 1 9 7 6

Calv i F l o w e r s C o r s i c a F r a n c e

M a y 1 9 8 0

Pi rc e t al . 1 9 8 3 I s ch i a Seeds S e p t e m b e r 1 9 8 2 ( I t a l y )

The morphology o f C. nodosa has been described in detail by Bornet {1864) under the name Phucagrostis major Cavolini. This seagrass com- prises axes, or rhizomes, bearing adventi t ious roots and leaf bundles at their extremities. The leaves are al ternate and opposi te in arrangement and com- pletely envelop the rhizomes; at their base occur 10--14 brown squamulae intravaginales several mill imetres in length. When the oldest laminae become detached the sheath remains in place for several weeks. It then breaks down

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leaving the axis naked behind its apex. Thus, it is easy to observe the suc- cession o f nodes and internodes on the rhizomes.

During the last hundred years, reports of sexual reproduct ion in C. nodosa have been relatively few (Table I). Flowering in the other species, Cymodo- cea serrulata (R. Br.) Ascherson and Magnus, C. rotundata, Ehrenb. and Hempr. ex Aschers. and C. angustata Ostenfeld, has been also rarely observed in nature (Ostenfeld, 1916; Isaac, 1968; Den Hartog, 1970, Kirkman, 1975).

It has been accepted that reproduct ion is principally vegetative in the species o f Cymodocea (Kay, 1971).

MATERIALS AND METHODS

We have made observations and taken samples in several beds of C. nodosa. At Antibes, in the Golfe Juan, Cymodocea was associated with Caulerpa and Zostera, forming an extended, dense popula t ion over more than 100 hectares at depths of 1--3 m. At Port-Cros, the C. nodo~a bed, which is in places mixed with Zostera, covers several hundreds of square metres (0.5-- 1.5 m in depth) be tween a bank of Posidonia oceanica and the shore (Molinier and Picard, 1952). At Menton, at Beaulieu, and in the Baie des Anges at Nice, the C. nodosa beds are monospecific, of ten less dense, and they cover large areas at depths of 2--18 m.

SCUBA gear was used to carry ou t observations and sampling in situ. The quantitative samples taken consisted of 3 sods, each of 0.05 m 2 and 15 cm thickness. On the beach, the sediment was eliminated by sieving, and the rhizomes and seeds o f Cymodocea were isolated and sorted.

In November 1982, at Port-Cros, 100 horizontal rhizomes were marked with plastic rings placed behind the apex. The marked plants were later col- lected for the s tudy of growth. Samples gathered at different seasons of the year, from beds in Port-Cros and Antibes, were brought to the laboratory for investigation of their morphology, b iometry and populat ion density.

RESULTS

Rhizome development and dimorphism

In situ, Cymodocea nodosa shows only its leaf bundles, the other parts of the plant are hidden by the sediment. If the sediment is removed, the leaf bundles may be seen situated at the extremities o f both vertical and hori- zontal axes. The horizontal axes, which show plagiotropic growth, branch by sending ou t secondary lateral axes of or thotropic growth. The secondary axes originate f rom an axillary bud which shows immediate development . At first, the leaf bundle of the secondary axis appears together with that of the main axis which has given rise to it. However, it is possible to determine the existence of precocious branching by the presence in the terminal leaf shoot of a short scale leaf. The latter, which always backs towards the horizontal

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axis, is the prophyll . It represents the first organ formed by the young meri- stem of the secondary axis. The first chlorophyll-bearing leaves which appear after the prophyl l are narrow and short. In shoots less than a year old, the leaves are smaller and fewer than in older shoots (Table II).

Those vertical axes aged one year or more, show port ions with short in- ternodes (1 mm) and por t ions with ones a little longer (2--3 mm) to much longer (up to 35 mm). On the port ion of the axis with long internodes, ad-

TABLE II

Biomet r ic analysis o f p o l y m o r p h i s m in C y m o d o c e a nodosa a

Types of Internode Number of Adult leaf Adult leaf Root Root axes length leaves/bundle length (ram) width (ram) length diameter

(ram) (mm) (ram)

Horizonta l 6 . 1 "*

Vert ical 1 ** (aged one year o r less)

Vert ical 1 . 5 "* (aged more than one year)

4.2 171"* 3.2 179"* 1 .65"*

1.6"* 132"* 2.5* 0* 0"*

3.7 316"* 3.6 69** 0.6 **

aThe samples were collected in June 1983 at Port-Cros. Ten axes of each type were analysed, the values given in the table are the calculated averages with significant differ- ences at the levels: * P = 5%, ** P = 2%.

Fig. 1. Port ' ion o f a p lagio t ropic rh izome showing the succession of series of short inter- nodes ( A u t u m n and Winter) and long in te rnodes (Spring and Summer ) hearing or tho- t ropic shoots.

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ventit ious roots develop at each node and sometimes or tho t rop ic branching occurs at one of these nodes. The fur ther the vertical axes are away f rom the plagiotropic meristem which gave rise to them, the longer they are. The longest vertical axes, reaching 20--30 cm, are no longer at tached to the old horizontal axes, which have broken down.

The horizontal rhizomes also show successive series of very short inter- nodes (1--2 mm) and much longer internodes f rom 3 mm to several cm in length, the greatest length observed was 7 cm (Figs. 1 and 2). The roots and branches are situated at the level of several successive nodes in the part of the rhizome with long internodes. On average, the internodes of the rhi- zomes are longer than those of the vertical axes and their adventit ious roots longer and more robust (Table II).

l c m

Fig. 2. Portion of a plagiotropic rhizome with series of short internodes (Autumn and Winter) without orthotropic shoots.

This dimorphism of the 2 types o f axis is also reflected in the mean length of the adult leaves; these leaves are longest at the extremit ies of vertical shoots (Table II).

Modification of growth type in rhizomes

The apical meristems of bo th types of axis are not irreversibly differen- tiated, and their mode of growth can change. In very dense populations, where there is little space available, a horizontal rhizome has the capacity to grow vertically. On the o ther hand, vertical axes can assume plagiotropic growth. We have observed the following cases of change in mode of growth: (1) the apex o f a hor izontal rhizome has been cut or des t royed; plagiotropic growth is then assumed by the meristems of the 2 nearest (i.e., youngest) lateral axes; (2) within beds of average density, the apical meristem of aver -

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tical axis 10--30 cm long is able to spontaneously assume plagiotropic growth; (3) at the edges of beds or within certain beds of low density, on horizontal rhizomes with particularly fast growth, young lateral axes in the third or four th posit ion behind the apex of the principal rh izome may be- come plagiotropic af ter having first grown vertically. This precocious change in the mode of growth of a young lateral meristem nex t to the terminal meris tem occurs only when the plant is colonizing a free substrate.

Seasonal variations in growth and development

The gradual variation in length of internodes and the rhy thmic develop- ment o f the ramifications and adventi t ious roots, especially on the horizon- tal axes, demonst ra te regular progressive changes in growth.

Sampling at d i f ferent times o f year as well as placing a ring in situ on the rhizomes behind the apex (Fig. 1), has allowed us to follow the annual cycle of these variations. In the autumn, each horizontal rhizome presents 1--3 short internodes behind the terminal leaf bundle. On both types of axis, the internodes formed th roughou t the winter are short. At the beginning of spring internodes of progressively increasing length appear. The internodes formed in May are the longest. By the end o f June, the longest internodes on the horizontal rhizomes are formed in the second or third posit ion behind the apex, and on the vertical axis the new internodes are now short (1 mm). During the summer, the length of the internodes on the horizontal rhizomes decreases, while on the vertical axes the internodes remain short.

We have also no ted a seasonal variation in branching and the deve lopment of adventit ious roots. Thus, in springtime adventit ious roots develop on the horizontal axes immediately behind the apices as well as ramifications at the level of each node. The adventi t ious roots and the ramifications which form become less and less vigorous during the course of the summer. The last ramifications, appearing in late summer, fall of f in autumn.

The analysis of seasonal deve lopment and the calculations of the mean annual growth o f bo th hor izontal and vertical rhizomes f rom the bed at Port- Cros are given in Table III. The mean annual growth of horizontal rhizomes is 72.8 mm, with the format ion o f 11 internodes (i.e., 11 leaves formed per annum per meristem). For the vertical axes, the mean annual growth is 13.2 mm, with the format ion o f 7.7 in t emodes (or 7.7 leaves) per annum for each meristem growing or thot ropica l ly .

Leaf development also appears to be subject to seasonal growth rhy thm (Table IV). The foliage of the C. nodosa bed has a particularly dense appear- ance in May and June. In fact at this t ime o f year, the leaves are more nu- merous on the leaf shoots and the max imum lengths and widths are at tained by the adult leaves. In au tumn and winter these leaves are narrower, and their laminae are nearly always cut or torn. Consequently, the leaf area index (length × width) reaches its max imum value in May--June (Table IV).

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T A B L E III

Sea so n a l g r o w t h in n u n and d e v e l o p m e n t o f Cymodocea nodosa (Por t -Cros , d e p t h 0.5

m) a

N u m b e r of G r o w t h o f t h e N u m b e r o f N u m b e r o f i n t e r n o d e s axis ( m m ) a d v e n t i t i o u s b r a n c h i n g s

roo t s f o r m e d

H V H V H V H V

S e p t e m b e r - - F e b r u a r y 3.9 2.7 7.4 3.7 0 0 0 0 M arch - -Ap r i l 2.1 2.5 10.2 2 0 0 0 0 M a y - - J u n e 2.6 0 43 5 3.6 2.3 3.6 0 .4 J u l y - - A u g u s t 2.4 2.5 12.2 2.5 2.3 0 1.3 0 Dur ing a yea r 11 7.7 72.8 13.2 5.9 2.3 4.9 0.4

aAn a ly s i s was m a d e o n 30 vert ical axes and 30 ho r i z on t a l axes : H = h o r i z o n t a l ; V = vertical.

TABLE IV

Quantitative parameters relating to leaf growth in a bed of Cymodocea nodosa growing at 0.5 m at Port-Cros, compared at 2 times of year

Date of sampling Density of Number of Number of Leaf Leaf area Leaf area index shoots m -2 leaves/ leaves m -2 width (ram 2) m 2 m -2

bundle (ram) (length x width x I)

24 and 25 November 1982 1647 2.1 3459 1.85 87.50 0.32 22 and 23 June 1983 2060 3.9 8034 3.20 48.6 3.90

Flowering

Observations o f flowering in situ The first t ime we found flowers of C. nodosa was in 1975 at Antibes in

the Golfe Juan (Jaubert and Meinesz, 1977; Meinesz, 1978), but it is only since 1982 tha t we have observed the C. nodosa beds at each season. From this date, we have found flowers, fruits and seeds each year. In Antibes, on 3 June 1982, we found a bed o f C. nodosa in f lower between depths of 1 and 2 m. The anther o f each male flower and the stigma o f each female f lower was still inside an axillary leaf sheath. After screening 3 samples, we found on average 48% fertile vertical axes, 29% sterile vertical axes and 23% vegetative hor izontal rhizomes; the flowering densi ty was 226 flowers m -2.

Diving on 17 June at the same place permi t ted the observation of the end o f flowering; nearly all the stigmata had withered and the stamens had been shed, leaving in place the bases o f their peduncles. On Juan les Pins beach, in the Golfe Juan, we found several hundred stamens of C. nodosa

284

stranded. Thus, anthesis and poll ination must have happened between 3 and 17 June, the sessile stamens becoming detached at matur i ty .

In June 1983 and in June 1984 the f lowering o f this phanerogam in the Golfe Juan was again noted. On 22 June 1983, at Port-Cros, C. nodosa was at the end of flowering with a densi ty o f 220 flowers m -2. This bed also f lowered during June 1984.

In all the beds, each flower, male or female, occurred at the ex t remi ty of a vertical axis at least one year old.

In all samples taken in the Golfe Juan, s taminate flowers and pistillate flowers occurred. However, we also observed beds divided into areas com- posed of individuals o f the same sex, i.e., male zones and female zones. These zones could each cover a few square metres, as at Port-Cros, where we found 90 flowers m -2 in June 1982.

Fig. 3. (Right) Female flower with its 4 filamentous stigmata issued from 2 styles. (Left) Male flower with its solitary stamen.

Floral morphology According to Bornet (1864), flowers are solitary, lack a perianth, and are

unisexual (Fig. 3). The male flowers are reduced to one stamen. The anther is composed o f 2 pollen sacs joined by a connective. At matur i ty the pink- coloured stamen stands up f rom a stalk and each pollen sac opens by 2 slits.

A sample o f 30 stamens, gathered f rom the Golfe Juan on 17 June 1983, were subject to biometric analysis. The largest stamen measured 21 × 4 mm

285

and the smallest 10 × 3 mm; over the whole sample the mean length was 16 mm (variance 7.22) and the mean width was 3.7 mm (variance 0.26). The stalk of the stamens measured 3--4.7 cm. At the base of the stalk a ring of brown squamulae occurs, 1--1.5 mm in length, which appear identical to those situated in the axils of the leaves.

The female flowers are inconspicuous and can be easily overlooked in situ. Their existence is only apparent because of a group of 4 filiform stigmata, each of which projects from the vagina of a chlorophyll-bearing leaf.

The 4 stigmata arise from 2 short styles in the apical region of 2 free ovaries. Each ovary (mean height 2 mm, mean width 1.2 mm) bears a small lateral appendage in its apical region. The 2 ovaries are situated side by side on a short peduncle at the base of which occur the squamulae {Fig. 4C). The flower of C. nodosa may be considered to be composed of 2 free ovaries.

The chlorophyll-bearing leaves, modified only slightly or not at all, play the role of floral bracts (Figs. 4A and B). The flowers develop and grow in- side the sheaths of the "floral leaves". Only those sheaths containing female flowers show any modification; this is slight and consists of a pronounced superposition of the stipulae, leaving only a narrow opening, at the head of the sheath f rom which the stigmata project. This arrangement is reminiscent

3 lcm

a, 1 r a m

Fig. 4. Cymodocea nodosa: (A) differentiated sheath leaf of the pistillate floral leaf; (B) w i t h the normal sheath leaf; (C) the ovary w i t h the squamulae (sq) at its base and a lateral appendage (la) with the stigmata at its apex.

~86

)f a spa the and a l lows t h e close c o n f i n e m e n t o f s ty le and fer t i le s t igmata . O n the fer t i le apex , a vege ta t ive b u d occu r s a t t he side o f t he f lower , ir-

,espective o f w h e t h e r t h e l a t t e r is ma le o r f e m a l e (Fig. 5). T h e b u d is in t he ucil o f t h e l ea f s i t ua t ed i m m e d i a t e l y p r o x i m a l to t h e f lora l leaf. This b u d ' .omprises 2 y o u n g leaves, t he f irst o f wh ich appea r s in the f o r m o f a t rans- )arent b r ac t l ack ing a l amina , its b a c k is c lose agains t t h e b a c k o f t h e s ta lk ff t he f lower (p rophy l l ) . Thus , t h e o rgan i za t i on o f t h e a p e x shows a f l ower n t e rmina l p o s i t i o n and a d e v e l o p i n g vege ta t ive b u d o f ax i l la ry origin. Each Lpex t h a t bears a f em a l e f l ower o f t e n has 2 axi l la ry b u d s d e v e l o p i n g in t h e udls o f t he 2 leaves p r e c e d i n g t h e f lower .

?ig. 5. Fructiferous apex of Cymodocea nodosa; pistillate flower and staminate towers in terminal position with the bundles of leaves formed from the development of :he axillary buds: pf = pistillate flower; sf = staminate flower; fb = floral bract; pr = )rophyll; sl = subtending leaf; ba = bud axillary development.

T h e g r o w t h o f t h e ver t ica l axes is thus s y m p o d i a l at t he t i m e o f f lower ing ; Lhe axi l la ry b u d p r e c e d i n g t h e f lower t akes over g r o w t h o f t h e ver t ica l axis • f t e r t he t r a n s f o r m a t i o n o f t h e t e rm i na l m e r i s t e m in to a f lower .

Fruiting Pairs o f f rui ts f o r m in s u m m e r on the o r t h o t r o p i c r h i z o m e s o f f e m a l e

plants. Each f ru i t bears a f l eshy pe r i ca rp , a b o u t 0 . 2 - m m th ick , which de- 3omposes in a u t u m n . Thus , seeds are lef t a t t a c h e d in pairs t o t he r h i z o m e .

287

Progressively, the seeds fall off the plant and may persist in nearby sediment for at least one year.

We have found C. nodosa seeds in the bed sediment all the year round in several localities. In the bed in Antibes, seeds were present in all samples (mean density 220 seeds m-2). At Port-Cros we only found seeds in the female zones, at a mean density of 80 seeds m -2. The abundance of seed production appears to be related to the proximity of male plants to female ones in the seagrass bed.

DISCUSSION

The growth of the leaves appears to be influenced by a seasonal rhythm. The adult leaves are longer and wider in June than in autumn. The number of leaves in each leaf bundle is also greater in June. For both reasons, the leaf-area index is much higher in June. Similar variation in leaf length has previously been noted in other marine phanerogams, notably in Syringodium, Halodule and Thalassia (Phillips, 1960; Strawn, 1961; Patriquin, 1973; Zieman, 1975).

Seasonal variation in growth has also been described for Posidonia ocea- nica (Ott, 1980; Panayotidis and Giraud, 1981; Caye and Rossignol, 1983).

The seasonal alternation between short and long internodes allows the age of C. nodosa plants to be determined easily. The maximum age of a vertical rhizome was found to be 10 years, and that of a horizontal rhizome 6 years (Caye and Meinesz, 1985). Variations in the thickness of the leaf sheaths which persisted on the rhizomes also allowed the age of Posidonia oceanica to be determined (Crouzet, 1981; Crouzet et al., 1983; Pergent et al., 1983).

The dimorphism of the C. nodosa axes is related to a difference in its intensity of growth and development. In one year, horizontal rhizomes grow from a few centimetres in dense beds, to nearly 2 m in beds of low density, while the vertical axes grow approximately 13 mm year-~ in both types of bed. Vertical axes ramify on average less than once per year; in contrast, horizontal axes produce on average 5 secondary axes per year in dense beds, and much more than this in less dense beds. In the Baie des Anges, we counted 26 secondary axes formed in one year. Rapid growth of the hori- zontal rhizomes permits vegetative expansion of the species, whereas vertical axes grow slowly with only occasional ramifications, but assure the occur- rence of sexual reproduction.

The differentiat ion between the 2 types of axis is not always permanent; the apical meristem can modi fy its mode of development under certain con- ditions and pass from one type of growth to the other. It is possible to dis- tinguish 2 sets of circumstances which can change this mode of development. If the plagiotropic apex is mechanically damaged, it is replaced in funct ion by the nearest, and the youngest , secondary orthotropic axis, which then assumes plagiotropic growth. This process occurs frequently in most higher

288

plants, and is explained by the breaking of inhibi tory relations between meristems {Nozeran, 1978). However, in the o ther cases, the factors which induce switching of the meristem mode of growth are external ; plant densi- ty, available space, perhaps light. This reversibility of the mode of growth allows the plant to adapt to changes in its environment , with a tendency to develop a plagiotropic axis whenever there is a free space.

The first stages in the t ransformat ion of the terminal meristem on the flowering vertical axes occur in March--April, at a tempera ture of around 13°C and about 12--13 h day-length. The floral bud develops in spring, under the influence of increasing tempera ture and lengthening day. Flower- ing always occurs between late May and late June, at temperatures 17--20°C and for 15--16 h day-length. Given the present state of knowledge, it is dif- ficult to be precise about the role of each of these factors in determining flowering in C. nodosa under Mediterranean condit ions.

The tropical species are known to f lower rarely and sparsely (Kay, 1971). However, C. nodosa flowers every year, but the density of its flowers varies from one bed to another, and fruiting is abundan t only if male and female stocks are near each other in the bed. Associated with the separation of individuals into more or less extensive male and female zones, is a decrease in densi ty o f both flowering and fruiting.

Germinat ion of C. nodosa has been only previously recorded in France by Bornet (1864), although numerous seedlings have been found in southern Italy (Pirc et al., 1983). However, we have found only 2 germinated seeds during our sampling and diving. Thus, despite abundant flowering and fruit- ing, and at t imes germination, the comple t ion o f sexual reproduct ion , may occur only occasionally on the coast o f France.

ACKNOWLEDGEMENTS

This research was sponsored by the Parc National de Port-Cros and the GIS Posidonie. We thank Dr. I. Jenkinson for his help with the English translation.

REFERENCES

Bornet, E., 1864. Recherches sur le Phucagrostis major Carol. Ann. Sci. Nat., 5 Ser., Bot., 1: 5--51.

Caye, G. and Rossignol, M., 1983. Etude des variations saisonni~res de la croissance des feuilles et des racines de Posidonia oceanica. Mar. Biol., 75: 79--88.

Caye, G. and Meinesz, A., 1985. Evaluation de la long~vit4 des rhizomes de Cyrnodocea nodosa d'apr~s les variations cycliques de la longueur des entre-noeuds. Rapp. Comm. Int. Met M~dit., 29: 187--188.

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