Bot. Mag. Tokyo 101 : 459-471, 1988 The Botanical Magazine, Tokyo �9 by The Botanical Society of Japan 1988

Floral Ontogeny in Mazus pumilus (Scrophulariaceae)

RAMAVEER RAWAT*, DEVENDRA K. AWASTHI*

AND VISHNU KUMAR**

*Department of Botany, M.M.P-G. College, Modinagar-201204, India ; **Department of Botany, Meerut University, Meerut-250005, India

In Mazus pumilus, all the floral appendages are initiated in acropetal sequence in the second cell layer (except stamens) of the floral primordium by periclinal divisions. The actinomorphic calyx tube is formed due to zonal growth. The zygomorphy in corolla is evident from the inception of petal primordia which arise sequentially as independent units in order of one anterior, a pair of anterio-lateral followed by a pair of posterio-lateral. Later these primordia exhibit differential growth because of which zygomorphy becomes more pronounced. The upper corolla tube is formed by inter- primordial growth and lower corolla tube by zonal growth. Stamens are initiated in the third layer of the floral apex. Unlike sepals and petals, in the development of stamens (4) underlying cells of corpus also contribute. Posterior stamen is absent. The sta- mens become epipetalous because of interprimordial and zonal growth in the common region below the bases of petals as well as stamens. The two carpel primordia arise as crescent shaped structures which become continuous due to interprimordial growth. The ovary is formed by a ring of zonal meristem. The style develops later between stigma and ovary because of intercalary growth. The residual apex grows vertically along with the ovary and forms the septum of the ovary. All the floral appendages exhibit similar pattern of histogenesis and early growth suggesting thereby the ap- pendicular nature of these appendages.

Key words : Differentiation Floral histogenesis - - Mazus pumilus -- Scro- phulariaeeae.

A perusal of literature reveals that considerable attention has been paid to studies

of flower development in angiosperms but the family-Scrophulariaceae has received

little attention. Hartl (1956) studied development of gynoecium in members of

Scrophulariaceae. Singh (1979) and Singh and Jain (1979) studied early floral or-

ganogenesis in Digitalis purpurea L. and Antirrhinum majus L. respectively. Awasthi

et al. (1984) gave a detailed account of floral histogenesis in d . majus with emphasis

on corolla tube formation. Nishino (1983a) discussed formation of corolla tube in

Verbascum thapsus and Veronicastrum. The present communication deals with floral

ontogeny in Mazus pumilus (Burro. f.) Steen. (= M.japonicus (Thunb.) Kuntze) with

a view to study modes of initiation and early growth of different floral appendages,

formation of floral tubes with emphasis on development of zygomorphic sympetalous

corolla tube, epipetalous condition and syncarpous gynoecium using a combination of

light and scanning electron microscopy.

460 R. RAWAT et al.

Mater ia l s and Methods

The material of Mazus pumilus was collected at weekly intervals from the

naturally growing plants at Modinagar. Inflorescences in different developmental

stages were collected and fixed in formalin : acetic acid : 70% ethyl alcohol (5 : 5 : 90

v /v /v ) . For histogenesis, the flower buds were dehydrated in alcohol-xylene series

and embedded in paraffin wax (M.P. 58-60 C). Median longitudinal and transverse

sections of flower buds were cut at 6-8 ]lm and stained with Ferric chloride-Iron alum

and Heidenhain's Iron Haemotoxylin. Fast green was used as counter stain.

For organographic SEM studies, flower buds were stained with 1% Acid Fuchsin

in 70% ethyl alcohol for 72 hr and subsequently differentiated in 70-90% ethyl

alcohol. The flower buds were dissected Ufider Stereo-binocular and dehydrated in

Alcohol-Acetone series. These were then critical-point-dried with CO2 and coated

with Silver. The stubs were scanned and photographed on P SEM 501B model at 7.2

kv.

O b s e r v a t i o n s

Organography The flowers borne in terminal or axillary racemes are bracteate, hermaphrodite,

complete and zygomorphic with a sympetalous bilipped corolla. The bracts and

bracteoles are minute. Calyx tube is prominent, companulate and is made up of five

sepals which are green and hairy. I t persists even in the fruit as a complete covering.

The corolla tube (with five petals) is white to purple and bifid. Upper lip is made up

of two erect petals while lower lip, much larger and spreading, is made up of three

Figs. 1-12. 1 : Top view of inflorescence with inflorescence apex and flower primordium in the axil of bract. Also showing the initiation of five petal primordia, x 240. 2 : Flower bud in top view showing five sepal primordia on a circular rim. • 300. 3 : Flower bud in top view showing simultaneous inception of petal and stamen primordia. Note that anterio-lateral pair is the first to be formed and is comparatively large followed by posterio-lateral pair. X 300. 4: Flower bud showing remaining floral apex (pentangular in appearance) after the formation of stamens, x 270. 5 : Flower bud in side view showing growth on abaxial side of stamen primordium for the formation of corolla tube. x270. 6, 7: Flower buds in different developmental stages showing different stages of gynoecium. 6, the developing gynoeeial rim, x 270 ; 7, well differentiated stigmatic lobes, • 120. 8 : Longitudinal section of inflorescence apex showing inception of bract in second stratification, x 480. 9: Longitudinal section of inflorescence apex showing initiation of flower primordium in the axil of bract. Also visible acropetally differentiating procambium in the bract, x 720. 10 : Transverse section of inflorescence showing developing bract and flower primor- dium. x420. 11: Longitudinal section of globular flower primordium, x720. 12: Longitudinal section of flower primordium showing periclinal divisions for the initiation of sepal primordia, x840. A, stamen primordium ; B, bract primordium ; C, petal primordium ; Fp, flower primordium ; G, carpel primordium ; I, inflorescence apex; K, sepal primordium; Pc, procambium; rC, removed petal primordium ; Sai, subapieal initials.

Floral Ontogeny in Mazus pumilus 461

462 R. RAWAT et al.

petals. The lobes of the lower lip are free at the top. Two prominent ridges are present on the palate of the throat of corolla tube. The lips merge into a tube which is divisible into lower corolla tube and upper corolla tube. The lower corolla tube is longer than the upper corolla tube. Four didynamous stamens are inserted at the top of the lower corolla tube. The posterior stamen is absent. The anterior pair of stamens is longer than the lateral pair of stamens. Gynoecium is bicarpellary, bilocular and syncarpous. Ovary exhibits axile placentation. Style is simple and stigma is bilobed and lamellate.

Organogenesis The inflorescence apex is a dome shaped structure on which a number of bract

primordia develop sequentially in a spiral manner. Each bract primordium soon surpasses the height of the inflorescence apex and subtends single flower primordium (Fig. 1). The flower primordium before the inception of floral appendages appears as globular.

The five sepal primordia appear on the floral apex almost simultaneously (Fig. 2). The plastochron between the inception of five sepal primordia is minimum. As these grow vertically and laterally they become connected to each other because of inter- primordial growth and thus form a circular rim with five sepal lobes (Fig. 3). Soon after their inception, five petal primordia are initiated on alternate radii inner to the sepal primordia (Figs. 1 and 3). The longitudinal growth is slow as compared to sepals.

Immediately after the petal primordia, four antisepalous stamen primordia are initiated in a rapid succession (Fig. 3). The anterio-lateral pair of stamen primordia is the first to be formed followed by a pair of posterio-lateral pair of stamens. No rudiment of posterior stamen has been observed. Because of a very short interval between inception of stamen and petal primordia, and due to fast growth in the latter the stamen primordia appear bigger than the petals (Fig. 4). As the petal primordia grow laterally they become connected to each other in the interprimordial space (Fig. 5).

After formation of stamen primordia two ridges appear at the periphery in the anterio-posterior plane, which represent carpel primordia (Figs. 6 and 7). The ante- rior carpel primordium is larger than the posterior one (Fig. 7). These grow initially independently and soon become inter-connected because of lateral extension into inter-primordial space. The free portion of carpel primordium forms the stigmatic lobes. After the formation of carpel primordia, the petal primordia grow very fast and soon overlap the stamen primordia.

Histogenesis The dome shaped inflorescence apex exhibits tunica-corpus organization and

measures 43 to 46gm in height and 120 to 126gm in breadth. The tunica is uniseriate wherein the constituent cells divide only anticlinally. The corpus exhibits ill-marked cytohistological zonation pattern to the extent that almost all cells exhibit similar staining reactivity (Fig. 8).

Floral Ontogeny in Mazus pumilus 463

On the inflorescence apex, bract primordia are initiated by periclinal divisions in the second stratification (Fig. 8). The underlying cells also divide and push out a small hump (Figs. 9 and 10). The cells of tunica iust above these periclinally dividing cells, divide only anticlinally adding cells only to the surface layer of the hump. As the primordium grows, a group of initials becomes discernible in the subapical position (the subapical initials) which contributes to the vertical growth of the bract. The derivatives of these subapical initials divide in all planes and contribute cells laterally to the abaxial and adaxial layers and basally to the middle layers. The procambium differentiates acropetally among the basal derivatives of subapical initials. The differentiating cells can be identified because of densely stained cytoplasm and prominent nuclei (Fig. 9). In later stages these cells get connected to the acropetally differentiating procambial strand of the inflorescence apex. Growth in the bract primordium is fast and soon it overtops even the inflorescence apex. LaUrel extension is by marginal and submarginal initials. Marginal initials divide strictly antielinally and contribute cells to the protoderm on both sides. Submarginal initials divide occasionally by periclinal and frequently by anticlinal divisions thus contributing cells to the adaxial and abaxial layers and to the middle layers. Lateral procambia differentiate among the basal derivatives of subapical initials.

When the bract primordium has attained a height of 30-35 ~m, a few cells of the third layer iust in the axil of floral bract divide periclinally pushing out small protuberance which increases in size due to divisions in the underlying cells of corpus (Fig. 9). Cells of tunica also divide but only anticlinally, thus contributing cells to the surface layer only. This becomes tunica of flower primordium. Soon a large globular structure is descernible in the axil of the bract (Fig. 11). This is the floral apex on which all the floral appendages develop acropetally. The floral apex has single stratification - - the tuniea, the cells of which divide only anticlinally, enclosing a homogeneous mass of corpus (Fig. 11).

On the flanks of the floral apex, five sepal primordia are initiated by periclinal divisions in the second layer of the apex (outermost layer of corpus) (Fig. 12). The underlying cells also divide and contribute cells to the developing sepal primordium. A group of subapical initials differentiate in the subapical position in the sepal when it has attained a height of 20 to 25/~m. Further vertical growth is by divisions in subapical initials, derivatives of which are added to the abaxial and adaxial layers and to the middle layers. The sepal primordia arch over the floral apex because of divisions and elongation in the cells of abaxial layer. Like bracts, the sepal primordia also grow laterally by marginal and submarginal initials (Fig. 13). These primordia initially are independent structures which extend horizontally by submarginal and marginal initials as well as tangentially by spreading of periclinal divisions in succes- sive lower layers and thus appear as coming close to each other. These remain closely appressed but do not show any sign of fusion (Fig. 14). Further vertical growth is because of zonal meristem which forms a complete ring below the sepal lobes (Fig. 15). The calyx tube is thus formed by zonal meristem. The calyx lobes which are formed due to the activity of subapical initials thus remain free. Differentiation of procam~

464 R. RAWAT et al.

Figs. 13-28. 13-15 : Transverse sections of flower buds showing different stages in calyx tube formation. • 360. 16 : Median longitudinal section of flower primordium show- ing sepal growth as well as simultaneous initiation of petal and stamen primordia. x 720. 17 : Longitudinal section of flower primordium showing stamen primordium which becomes massive due to divisions in all directions. • 18-21 : Transverse sections of anther showing different developmental stages. 18, • 360 ; 19, • 720 ; 20, 21, • 360. 22-25 : Transverse section of flower buds showing different stages in corolla tube formation. 22, 23, • 24, 25, x270. 26 28: Transverse section of carpel primordium showing different stages of developing carpellary tips and the gynoecial rim. 26, x480; 27, 28, x720. A, stamen primordium; C, petal primordium; G, carpel primordium ; K, sepal primordium ; Pc, procambium.

Floral Ontogeny in Mazus pumilus 465

bium is concomitant with the initiation of sepals (Fig. 12). It differentiates acropetal- ly in the basal derivatives of subapical initials.

After the formation of sepals, five petal primordia are initiated by periclinal divisions in the second layer (Fig. 16). Like sepals, initial growth is by divisions in the underlying cells of corpus and later vertical growth is by a group of subapical initials and lateral growth is by marginal and submarginal initials. The petal primordia also arch hyponastically over the floral apex like sepals. Procambium differentiation starts in conjunction with the periclinal divisions for the petal initia- tion in the corpus cells. Later, it differentiates acropetally among the basal deriva- tives of subapical initials. It is continuous with the procambium of sepals.

The stamens are initiated immediately after the petal primordia. For the initia- tion of stamens, periclinal divisions occur in the third layer (topographically) and deeper layers of corpus, inner to petals but opposite to the sepals (Fig~. 16 and 17). The stamen primordia grow further by the activity of sub-apical initials. Apical initials contribute cells to the surface layer by dividing anticlinally, thus keeping pace with the increasing mass of underlying cells. Stamen primordia from the very beginning appear massive and club-shaped and in contrast to sepals and petals tend to grow straight. Lateral growth is by marginal and submarginal initials but it ceases very early. A single procambium strand differentiates acropetally when the stamen primordium has attained a height of 60-66 gm (Figs. 18-21) Only four stames are initiated. Posterior stamen is absent.

In early developmental stages, stamen appears oval in transverse section (Fig. 18). It is made up of homogeneous mass of cells with a procambial strand in the centre surrounded by a single layer i.e., epidermis. A group of archesporial cells in the hypodermal layer becomes discernible at each corner (Figs. 19 and 20). Thus four groups of archesporial cells as large sized, densely stained with large nuclei can be identified as occupying the four corners of the stamen. It becomes trapezoidal in transverse section (Fig. 20). The archesporial cells elongate radially and divide periclinally (Fig. 19). Filaments develop later by growth at their base. In later stages, the outer primary parietal cells divide periclinally and form wall layers of each microsporangium. The inner sporogenous cells produce microsporocytes (Fig. 21).

When the hump of the staminal primordium becomes prominent, mitosis occurs on the abaxial side just at the level of petal primordia (Fig. 17). After the formation of stamens and carpels, the abaxial hump grows vertically as well as laterally along with the petal primordia as a single structure filling the gap between the corresponding petal primordia (Fig. 22). First of all the interprimordial space between the anterior and anterio-lateral pair of petal primordia is filled up forming the lower lip of corolla. Simultaneously interprimordial space between two posterio-lateral petal primordia which form the upper lip of corolla, is filled up. Later on, the interprimordial gap between the petal primordia of two lips is also filled up, so that, a complete tubular structure is formed. This structure grows further by the activity of zonal meristem at the common base of petals and stamens (Fig. 23). It is difficult to draw sharp boundaries of zonal meristem but the densely stained cells with more meristematic

466 R. RAWAT et al.

a c t i v i t y are wel l mani fes t . Th is zonal me r i s t ema t i c a c t i v i t y c o n t r i b u t e s to the lower

p a r t of coro l la tube. The t angen t i a l sec t ions do no t reveal any sor t of fusion be tween

marg ina l mer i s t em of a d j a c e n t corol la lobes, r a the r t hey become connec ted due to

g rowth in t he i n t e r p r i m o r d i a l space (Figs . 23-25). The zonal mer i s t em also ex t ends

Figs. 29-37. 29 : Longitudinal section of flower bud after the formation of stamen primor- dia. Note divisions for the formation of hump on the abaxial side of stamen primor- dium at the level of petal insertion. • 30-34 : Longitudinal sections of flower buds showing different developmental stages in the formation of gynoecium. The septum grows out with in the gynoecial rim and appears as remaining apex. 30, • 480 ; 31-33, • 360 ; 34, • 480. 35 : Longitudinal section of flower bud showing fully form- ed septum and a number of ovules, x 120. 36: Transverse section of flower bud passing through the upper part of ovary. • 270. 37 : Transverse section of flower bud passing through the lower part of ovary showing bilocular condition and highly swollen placentae. X 300. A, stamen primordium; abx, abaxial growth; C, petal primordium ; G, carpel primordium ; K, sepal primordium ; Oa, ovary ; P1, placentae ; Se, septum; Sy, style.

Floral Ontogeny in Mazus pumilus 467

below the connected bases of stamens and thus takes all the stamen primordia upward and the stamens become epipetalous.

After forming stamen primordia, the floral apex bears two carpels which are initiated as two hemispherical structures because of periclinal divisions in the second layer in anterio-posterior plane (Figs. 26 and 29). Anterior carpel primordium is initiated slightly earlier than the posterior carpel primordium. Like sepals and petals, the original lobes grow vertically by subapical initials and laterally by marginal and submarginal initials (Fig. 27). Later, growth occurs in the interprimordial space also so that the whole structure forms a circular rim (Fig. 28). The carpel primordia in the form of a rim initially grow as erect structures but in later stages, close in because of more adaxial meristematic activity. The free tips continue as such and form stigmatic lobes (Figs. 31-33). This complete rim grows further because of zonal meristematic activity which forms the style. Like sepals and petals, procambium differentiates acropetally among the basal derivatives of subapical initials as usual.

The remaining floral apex after formation of carpellary wall, is represented by a few densely stained cells (Fig. 30). This again becomes active and grows vertically by divisions in subapical initials and along the wall of the gynoecium as unit structure. The basal part of carpel i.e., overy is formed by zonal meristem. The septum increases in thickness because of mitotic divisions in the hypodermal layers (Fig. 34). On these thick placentae, ovules are initiated by periclinal divisions in the hypodermal layer (Figs. 35 and 37). In the lower part, ovary is bilocular with axile placentation and in the upper part it becomes unilocular (Fig. 36).

D i s c u s s i o n

The flower has been described as having a modified compressed shoot and its lateral members are homologised with the foliage leaves (Takhtajan, 1959, 1969; Zimmerman, 1959; Eames, 1961). It was thought that variation in structure and form of lateral appendages is because of differential distribution of growth activity of meristem and rates of maturation (Esau, 1965). A number of workers (McCoy, 1940 ; Sprotte, 1940 ; Kaussman, 1941 ; Engard, 1944 ; Boke, 1948, 1949 ; Kasapligil, 1951 ; Tepfer, 1953 ; Tucker, 1959 ; Tucker and Gifford, 1966 ; Kaplan, 1968a, b ; Awasthi et al., 1984) homologised the floral appendages with the foliage leaves using floral histogenesis (modes of initiation and early development of floral organs) as evidence. In Mazus pumilus, too, all (with the exception of stamens which are initiated in third layer) the floral appendages viz., sepals, petals, and carpels are initiated acropetally by perielinal divisions in the second layer like foliage leaves, thus supporting the observa- tions of earlier workers.

Histogenesis of bracteole in many taxa is reported to be similar to that of foliage leaves (Gifford, 1951; Tucker, 1959). In Downingia, Kaplan (1967, 1968a) reported that the development of bract, sepal and petal is similar to that of foliage leaf. In M. pumilus bract, sepal and petal exhibit similar mode of initiation and early histogenesis i.e., vertical growth by subapical initials and lateral growth by submarginal initials.

468 R. RAWAT et al.

The pattern of division of marginal and submarginal initials correspond to Hara's (1957, i959) submarginal type in which separate histogens have been traced for protoderm and subjacent mesophyll tissue.

In Mazus pumilus, a very prominent calyx tube is present. It is about half of the total length of the calyx and persists even in the mature fruits completely covering it. The sepal primordia are invariably independent and dorsiventrally flattened. They grow vertically by subapical initials and laterally by submarginal initials. Because of this marginal activity the two independent surfaces of the sepal primordia come very close to each other but do not show any sign of fusion e.g., adpression, inter- penetration of protodermal cells etc. All the primordia are pushed up because of zonal meristem activity below their bases and in the interprimordial space. Such type of fusion falls in the second category of Boke.-(1948) who reviewed the literature on ontogeny of corolla tube in angiosperms.

A sympetalous bilipped corolla is divisible into upper corolla tube and lower corolla tube. In the lower corolla tube four epipetalous stamens are inserted. According to developmental morphologists, the phylogenetically united portion of the corolla tube is formed by zonal growth in the interprimordial region and below the petal primordia (McCoy, 1940; Miller and Wetmore, 1946; Cusick, 1966). Boke (1948) concluded that different plants fall into three main categories with regard to the manner of corolla tube formation. In Vinca, for example, upper corolla tube above the site of stamen insertion is formed by ontogenetic fusion and the lower corolla tube as a result of zonal growth (third category of Boke, 1948). Kaplan (1968a) also reported corolla tube formation by zonal growth before the union of adiacent petals, whereas McCoy (1940), Miller and Wetmore (1946) and Robyns (1955) reported sympetalous corolla tube formation exclusively by zonal growth in some other taxa. However Sattler (1977) and Daniel and Sattler (1978) reported corolla tube formation by extension and fusion of marginal meristems of petal primordia. Nishino (1976, 1978, 1982, 1983a, b) on the basis of his observations on several taxa suggested three modes of corolla tube formation : (i) Post-genital fusion (ii) connection of petal bases (iii) elongation of the ring like common base. Awasthi et al. (1984) studied corolla tube formation in a related species viz., Antirrhinum majus and have shown that it is formed due to zonal growth preceeded by joint growth of hump on the abaxial side of stamen primordia and interprimordial region between petal primordia.

In Mazus pumilus, for the corolla tube formation, periclinal divisions occur in the interprimordial space between petal primordia on the abaxial side of stamen primor- dia. The underlying cells also divide and contribute to the abaxial growth which fills up the gap between the two adjacent petal primordia. As the adioining petal primordia are growing laterally also by marginal and submarginal initials, a complete ring or a cup like structure is formed. This cup like structure grows further by perielinal and anticlinal divisions at the base of petals as well as their interprimordial space. The growth on the abaxial side of stamen primordium is comparable to the connecting region of Nishino (1978, 1982, 1983a, b). This is then followed by zonal growth below the bases of petal and stamen primordia due to which stamens are also

Floral Ontogeny in Mazus pumilus 469

raised on the corolla tube. Thus, it is concluded that the upper part of corolla tube is formed by filling up of interprimordial space between the petal primordia and that the lower part of corolla tube is formed by zonal growth.

The zygomorphy in the corolla tube is evident from the inception of petal primordia. The anterior petal is first to be formed and remains larger in size. The anterio-lateral pair of the petals is the next to be formed and is comparatively smaller. This is followed by the filling up of interprimordial space and thus the lower lip is formed. Posterio-lateral pair is the last to develop. These are the smallest. Inter- primordial space between the two is so little that they undergo little tangential expansion. Lastly the interprimordial space between the two lips is filled up. Therefore, zygomorphy in the corolla tube is evident from the inception of petal primordia and later it continues and become more pronounced due to differential growth in the petal primordia.

The extension of zonal growth below the common bases of stamen and petal primordia results in the formation of epipetalous stamens in Mazus pumilus. Other workers have also made similar observations in Catharanthus (Boke, 1948), Pharbitis (Nishino, 1976), Solanaceous species (Nishino, 1978) and Echinops (Leins and Gemme- ke, 1979). The stamens are inserted at the top of lower corolla tube. Awasthi et al. (1984) reported just the reverse case in Antirrhinum majus L. (Scrophulariaceae) where the stamens are inserted at the base of the corolla tube. In it, the zonal activity below the common base of stamen-petal primordia ceases early in its ontogeny because of which they are left at the base. Daniel and Sattler (1978) have also reported a similar case in Solanum dulcamara where the stamens remain at the base of corolla tube. The insertion of the stamens at the top of the lower corolla tube is an indication that the later growth of the corolla tube is confined to the lower part of corolla tube only (Leins and Gemmeke, 1979).

Both the carpel primordia originate as independent structures and later due to interprimordial growth become connected and form a circular rim which grows further by zonal growth. So the syncarpous gynoecium in Mazus pumilus is formed by interprimordial growth and not by the process of post-genital fusion. Hagemann (1970) also reported that all sorts of cups and cylindrical structures are formed by the process of meristematic fusion i.e., a type of zonal growth and not by congenital fusion. Kaplan (1968b) reported post-genital fusion of carpel primordia in Downingia bacigalupii.

The free distal regions form the stigma where as the style is interpolated by intercalary growth between stigma and ovary while the ovary is formed by zonal growth.

Varghese (1969) on the basis of vascular anatomy reported a central placental cord which is a fusion product of placental strands in Mazus pumilus. The vascular strand furnishes traces to the ovules borne on the fused margins of the same carpel. On the basis of the criteria laid down by Puri (1952) placentation is axile. Present observa- tions based on ontogeny also support the observations of Varghese (1969) that the placentation on the basis of vascular anatomy is axile. The septum in typical axile

470 R. RAWAT et al.

p l a c e n t a t i o n arises f rom the s u m m i t of the floral apex and grows u p w a r d a long wi th

the wal l of the o v a r y and remains a t t a c h e d to i t t h r o u g h o u t i ts l eng th d i v i d i n g i t in to

two locules. Therefore , the s ep tum grows as a un i t s t ruc ture a long w i t h the inner wal l

of the ovary . The mass ive p l acen t ae deve lop on it. The ovules are i n i t i a t e d b y

pe r i c l ina l d iv i s ions in the s u b h y p o d e r m a l layer . The p l acen t ae are thus c a r p e l l a r y in

na tu r e and no t ax ia l s t ruc ture .

Au tho r s are g ra te fu l to Prof . Y.S. M u r t y and Dr. V.P. Dube for the i r keen in te res t

and faci l i t ies . W e are also t h a n k f u l to Director , A.I .I .M.S. , New Delhi for SEM

fac i l i t y and to Mr. S.C. S h a r m a for t echn ica l assistance. F i n a n c i a l ass is tance to R R

from U.G.C, New Delhi , is g r a t e fu l l y acknowledged .

R e f e r e n c e s

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Accepted June 2, 1988 Received February 22, 1988