chapter v palynological studies - …shodhganga.inflibnet.ac.in/bitstream/10603/28986/10/10_chapter...

117

CHAPTER V

PALYNOLOGICAL STUDIES

INTRODUCTION

Palynology is the science of pollen grains and spores. Pollen grains are the

expressions of the structural and functional evolution in plants. They are used as

experimental material in biological investigations in order to rectify the problems

related to classifications. Pollen characters include pollen units, polarity, symmetry,

shape, size, apertures and exine (sporoderm) were used for plant classification. Pollen

morphology has played an important role in splitting of genera. Palynological evidence

may be used to place taxa of uncertain affinities to suggest rearrangement, withdrawal

and separations as well as collaborating other lines of evidence (Davis and Heywood,

1963).

The pollen grains of higher plants constitute the most vital unit of the flower with

regard to their form and function. These one-celled microscopic haploid units represent the

essential genetic bridge between generations. Description of the fundamental features of

pollen morphology by early Botanists has demonstrated the potential value of palynology

in phylogeny and plant taxonomy. The contribution of Woodehouse, (1935) through the

publication of the book “pollen grains” marked the beginning of purposeful studies on

pollen morphology in angiosperms. Erdtman’s, (1952) book “Pollen morphology and plant

taxonomy in Angiosperms” laid a solid foundation to this branch of study and

demonstrated the importance of pollen morphology in plant taxonomy. During the past

118

three decades the pace and progress of studies in Pollen Biology in general and pollen

morphology in particular, have been so rapid and varied that pollen based studies are now

relevant in various areas of Botany as well as in several other disciplines like Agriculture,

Forestry, Medicine, Geology and Archeology.

Palynology has been valuable aid for taxonomical rearrangements. It has helped

in identification and delimitation of taxa, elucidation of phylogenetic relationships,

placing taxa of uncertain affinities. Erdtman, (1952) has provided an excellent survey

of the use of pollen morphology in taxonomic studies. The pollen characters are more

conservative and may successfully be used for identification and classification of any

group of plants. Modern taxonomy has been shown to benefit much from external and

internal characteristics of pollen grains. In majority cases, the results were obtained

from pollen grains in conformity with the established classification. It may also be

remembered that they must be treated with certain amount of caution and must be

interpreted with reference to other lines of evidence.

Pollen grains data has increased the potentiality of research of biological

investigation. The first successful attempt using pollen characters in the classification

of plants was made by Lindley, (1830). Since then Woodehouse, (1935); Erdtman,

(1952); Bhoj Raj, (1961); Saad, (1960 and 1961) Vishnu Mittre, (1964); Nair, (1970

and 1979) and Takhtajan, (1980) have taken pollen characters as a basis for

classification of botanical relationship and palynology. Pollen character has been

119

considered by Gamble, (1923) in grouping the genera of Convolvulaceae of South

India.

Palynological evidence may be used to place taxa of uncertain affinities, to

suggest rearrangement and separations. It will also be of much value in the evolutionary

interpretation and in finding out the affinities of the taxa. Very often taxonomists over

look the pollen morphological characters in the treatment of taxa, early authors like

Lindley, Von Mohl and Fristscheze have realized the importance of such characters (vide

Davis and Heywood, 1963). It is quite unfortunate that seldom do taxonomists taken into

account of the pollen grains characters when describing or revising a group of plants.

There is very large scope to utilize pollen characters and it is hoped that a larger number

of critical data on pollen morphology would do a very great systematic and phylogenetic

value. It is opportune at this juncture to refer the classical works of Woodehouse, (1935)

and Erdtman, (1943 and 1952) dealing with the pollen grains in plant taxonomy. The

pollen morphological studies have already contributed much to the elucidation of

phylogenetic relationship in a few plant families.

The diagnostic characters of pollen are contained in the exine. The pollen grains

can be placed under artificial groups based on the morphological characters of the

exine. The morphological characters shall be categorized under apertures, size and

shape and exine sculpture. Among these the aperture character is considered to be of

primary importance, the exine surface pattern as secondary and others as tertiary (Nair,

1965). The aperture characters are based on their numbers and position.

120

Pollen investigations have served to group plant families as eurypalynous and

stenopalynous. In the eurypalynous families, there are different morphological types

which form a basis to delimit taxa, while in the stenopalynous ones a single

morphological type typifies the whole family. In the former case, apertures are

considered to be of primary importance, ornamentation secondary and size, shape as

tertiary, where as in the latter, size and shape alone are significant. The use of pollen

morphology in providing key to taxa differentiation has already been reported in some

angiosperm families (Mathew and Philip, 1983; Koshy, 1986;Valsala Devi, 1987)

Contribution to the pollen morphology of the family Convolvulaceae have been

made by Gamble (1923), Sayeed- Uddin et al., (1942); Erdtman, (1952); Natarajan,

(1957) and Nair and Rehman, (1963). In the family Convolvulaceae, on the basis of

pollen morphology the plants may be separated into two major groups (Nair and

Rehman, 1963). The basic types of pollen grain in this family are the pantoporate and

spinose type from which the other morphotypes have evolved as outlined by Erdtman in

his Scandinavian pollen flora (Nair and Rehman, 1963). The genera Jacqemontia and

Evolvulus show pantocolpate pollen grains. The presence of pantocolpate pollen grains

and faveolate exine ornamentation show relationship between these two genera. The

genus Hewettia shows relationship with members of the genus Merremia by the

presence of 3- zonocolpate pollen grains and with reticulate exine ornamentation.

The taxonomic and evolutionary importance of pollen grains may be at specific,

generic or even higher levels are significant. It is quite convincing that in many cases the

121

type of pollen grains of taxon is quite characteristically constant in which case the taxon

is termed as stenopalynous. On the other hand, quite contrary to what is said above, the

types of pollen may vary considerably in size, nature of ornamentation or stratification,

diameter of the aperture such taxa are called as eurypalynous. The pollen grains of

related genera are usually more or less of the same type. Where ever the pollen grains of

related genera differ in their morphology there has been a tendency to separate the taxa

and split the genera. On contrary, pollen grains of same type are sometimes found in

plants not closely related. In certain genera, more than one type of pollen grains may be

found. Under such conditions, the different types are usually confined to particular

section of the genera. In other words, the pollen morphology has played a significant role

in splitting the genera.

Pollen gains are the expressions of structural and functional evolution in plants.

Palynological studies are of great significance in taxonomy, phylogeny and

palaeobotany. Analysis of fossil pollen is the most important approach to the

reconstruction of past flora, vegetation and environment (Faegri and Iversen, 1989).

Pollen morphology and pollen analysis have taxonomic value (Erdtman, 1969; Nair,

1970; Moore and Webb, 1978; Nilsson and Muller, 1978; Shivanna, 2003). The plants

can be placed under artificial groups based on the morphological character of the exine.

Based on the nature of pollen grains, Hallier, (1893) has grouped the genera

with Ipomoea type of pollen grains under the Echinoconiae and all others in Psiloconiae,

Ipomoea is under Echinoconiae. As far as the observations on the South Indian

122

Convolvulaceae are concerned, the pollen grains range in size. The pollen grains are non-

spiniferous and non-crassinexinous, contrary to what is described in the other groups.

The pollen grains may be termed as Convolvulus type. The pollen grains are colpate,

rugulate or forrate, being three. It is particularly noteworthy, are peculiar in having a

structure of Ipomoea type of pollen grains on the one hand and Convolvulus types on the

other. There are many pores (poly porate) to the size of equally spaced round germinal

aperture and the sexine is some what ornamental with very minute pits.

Major palynological characters in the order of their importance in taxonomic and

phylogenetic considerations are the germinal aperture, its form, number, and distribution,

exine ornamentation, pollen shape and size (Nair, 1970). The recent introduction at

Scanning Electron Microscopy (SEM) has proved to be a useful tool for palynological

studies with increased accuracy and precision. This has opened up possibilities for better

understanding of the exine ornamentation pattern and enabled application of exine features

in studies involved systematic relationships of microtaxa particularly sub species, varieties,

cultivar, cytotypes, bio forms etc. (Ravikumar and Nair, 1985). Several books are

available for a comprehensive coverage of pollen morphology and pollen analysis, which

are of taxonomic value (Erdtman, 1966 and 1969; Nair, 1970; Moore and Webb, 1978;

Nilsson and Muller, 1978; Thanikaimoni, 1978; Faegri and Iversen, 1989; Shivanna, 2003).

The size and shape of pollen, pollen characters such as number and position of

furrows, number and position of apertures and the details of sculpturing of the exine are

of taxonomic value. The form, number distribution and position of apertures are

123

important palynological criteria in assessing relationships and phylogeny of plants.

According to the position, the aperture may be proximal, distal, zonal. In terms of

evolution the proximal position is most primitive and zonal position as most advanced.

Exine sculpturing is of great value in taxonomy. Pollen characters have helped

in differentiation of various taxa. The tribe Bombaceae of the family Malvaceae has

been recognized as a separate family Bombacaceae on the basis of palynology. The

genera Salix and Populus of Salicaceae can be distinguished on the basis of pollen

characters. Species of Anemone can be distinguished on the basis of aperture of pollen.

It is 3-zonocolpate in Anemone obtusiloba, pentaporate in Anemone alchemillaefolia

and pentacolpate in A.rivularis. Variation in the aperture and exine ornamentation may

help in the identification of various species of Bauhinia. Pollen size is helpful in

distinguishing two species of Malva, M. rotundifolia and M. sylvestris. Palynological

studies have also helped in the phylogenetic relationships. Palynological studies

suggest, two distinct phylogenetic stocks in the dicotyledons, monocolpate and

tricolpate represented by Magnoliaceae and Ranunculaceae respectively.

Pollen morphology and plant taxonomy

Nair and Sharma, (1962) suggested that pollen keys can be used to categorize

plants of a family at various taxonomic levels. Pollen characters have been considered

by Gamble, (1923) in grouping the genera of Convolvulaceae of South India into two

groups, spinulose and non-spinulose. The separation of several species of Ipomoea in

to species of Merremia and Operculina is supported by pollen morphology. Difference

124

in excrescence type between various genera and species are also a character for plant

identification. Hooker, (1885) included the genus Calonyction as a sub genus of

Ipomoea. But the excrescence type of spinose in Ipomoea were as verucose in

Calonyction. This supports the separation of two genera by Gamble (1923).

Evolutionary trends in pollen morphology in the Convolvulaceae

Regarding the phylogentic trend operating in pollen morphology, the opinion

differs to some extent with the individual authors. According to Erdtman’s Scheme of

evolution (1966), pollen grains having pore like or ill-defined proximal apertures have

given rise to those with zonal distal or global apertures. Vishnu Mittre (1964)

considered the trilete forms as ancestral type from which in-aperaturate type followed

by aperturate one evolved. On the other hand, Walker, (1976) has considered in-

aperturate pollen grain as primitive form, from which aperture ones evolved. Nair,

(1965 and 1970) suggested that in dicots evolution has taken place along two lines one

along the Magnolianeous and other along the Ranunculaceous represented

fundamentally by monocolpate and tricolpate forms respectively. Forms such as

colporate, porate, pororate and spiraperturate have evolved as a result of reduction and

specialization.

Exine ornamentation (sculpturing) comprises exposed surface details that are

present on the outer surface of the exine. Variations in sculpturing patterns constitute a

source of evidence for taxonomic considerations. Palynologists regard these characters

as of secondary taxonomical value. Twelve major sculpturing patterns have been

125

recognized in angiosperm families by Walker and Doyle, (1975). Nair, (1970) has

classified the exine surface pattern under two major categories (i) depression type

(faveolate, striate, reticulate scorbiculate, fossulate , rugulate, vermiculate and lophate)

and (ii) excrescence types (spinulate, baculate, clavate, gemmate, verrucate and

granulate).

The morphological characters shall be categorized under aperture, size, shape,

exine ornamentation, nature and length of spine. The aperture characters are based on

their number and position. In the present investigation an attempt was made to analyze

the position of the genus Ipomoea after considering the palynological features. The

study of pollen morphology is highly significant and that could be more extensively

employed if the relevant data are readily available. There is a lot of scope towards

protecting the present system of classification of the Convolvulaceae members,

provided that a larger number of critical data are made available from palynology.

Reference may be made to the classical work of Hallier, (1893) for the

significance of pollen morphology in the classification of the different genera and

species. It is thought that the pollen characters of the Convolvulaceae have several

characteristic features in common with those of certain centrospermae, especially with

reference to the exine stratification and to the nature of aperture (Erdtman, 1952). The

resemblance to the grains of the polimoniaeae is quite unfortunately less marked. The

present study deals with the palynological studies of 16 South Indian species of

Ipomoea. Palynological characters such as pollen type, aperture morphotype, exine

126

ornamentation, spine type and length, grain size and shape, pollen fertility like feature

were taken into account. The plant specimens for the present study which were

collected from different parts of South India and cultivated in the college garden .

127

MATERIALS AND METHODS

For the present investigation 16 species of Ipomoea were selected. Palynological

characters such as pollen type, aperture morphotype, exine ornamentation, spine type

and length, grain size and shape, pollen fertility like features were observed ( Table:15).

Pollen analysis

Mature pollen grains from mature anthers were dusted on a clean slide and

stained with one percent acetocarmine. The acetolysed pollen grains were mounted in

glycerine jelly and the slides were sealed with paraffin wax. Acetocarmine was found to

be the most suitable stain for pollen grain studies in Convolvulaceae. The size of the

pollen grains was measured by using ocular micrometer. Twenty readings were taken in

each case. The pollen grains were micro photographed to study the shape of pollen

grains and the wall ornamentation. Photomicrographs were taken for all plant materials.

Jetner – Biolux research microscope was used for micro photographing.

Pollen size is calculated by taking measurements of polar axis and the maximum

breadth in the equitorial view of the grain and applying the formula P/E x100. The

shape of the pollen grain depends upon three factors (Knox, 1984; Sukhla, etal.,1998) ( i

) position of the microscope within the tetrad (ii) distribution and form of apertures (iii)

the mode of cellular expansion after the release of microspores from progenitor wall.

128

Pollen sterility was examined by smearing mature anther in 1:1 mixture of

glycerine and acetocarmine. The slides were kept 30 minutes for better staining and

then examined under microscope. Fully stained pollen grains were counted as fertile

and partially stained or unstained was counted as sterile. Pollen sterility was calculated

by using the following formula

% of pollen sterility = Number of sterile pollen grains x100

Total number of pollen grains

Table - 15 Qualitative and quantitative palynological features selected

Sl No Character Category

1 Pollen aperture morphotype qualitative

2 Pollen exine ornamentation qualitative

3 Spine type qualitative

4 Spine length( m) quantitative

5 Pollen grain size range ( m) quantitative

6 Pollen grain shape qualitative

7 Pollen fertility (%) quantitative

129

OBSERVATION

In the present investigation pollen characteristics of 16 species of the genus

Ipomoea were analyzed. Palynological characters such as pollen type, aperture

morphotype, exine ornamentation, spine type and length, grain size and shape, pollen

fertility like feature were observed. Plant materials used in the present study were

collected from different parts of South India. The observations revealed from the

present study are given below (Table -16 and 17 ; Plate – 25 and 26).

I. alba

Monad pollen grains, pantoporate, spinose, spine type pointed, spine length 11.8 m,

grain size 108-128 m, spheroidal and pollen fertility 92 %.

I. aquatica


grain size 120-130 m, spheroidal and pollen fertility is 78 %.

I. batatas

Monad pollen grains, pantoporate, spinose, spine type blunt, spine length 9.1 m, grain

size 115.6-132 m, spheroidal and pollen fertility is 71 %.

I. cairica


grain size 74.8-91.8 m, spheroidal and pollen fertility is 90 %.

I. carnea sub sp. fistulosa

130


grain size 92.8-101 m, spheroidal and pollen fertility is 95 %.

I. coptica


grain size 119-149.4 m, spheroidal and pollen fertility is 89 %.

I. digitata



I. hederacea



I. hederifolia

Monad pollen grains, pantoporate, spinose, spine type blunt, spine length 11.1 m,


I. horsfalliae



I. indica


grain size 81.6-129.2 m, spheroidal and pollen fertility is 75 %.

131

I. obscura


grain size 78.2-88.4 m, spheroidal and pollen fertility 97 %.

I. pes-caprae sub sp. brasiliensis


grain size 117-186.7 m, spheroidal and pollen fertility is 91 %.

I. purpurea



I. quamoclit


size 92.5-157.5 m, spheroidal and pollen fertility is 95 %.

I. staphylina



All the 16 members of the genus Ipomoea showed pantoporate aperture

morphotype and have spheroidal shape. All the species showed spinose exine

ornamentation. Among the 16 species studied majority of the species have pointed

spines. Meanwhile I. quamoclit, I. batatas I. horsfalliae, I. hederifolia and I. hederacea

132

have blunt spine type. The maximum spine length is reported in I. carnea sub sp.

fistulosa and minimum in I. hederacea. Comparing the pollen size, large pollen is

reported in I. pes-caprae sub sp. brasiliensis and small pollen size is reported in I.

cairica. The fertility status is concerned 100% fertile pollen grains are reported from I.

hederacea and less in I. batatas and I. hederifolia.

133

Table -16 Particulars of pollen morphology

Sl.No

Name of taxa

Aperture

morphotype

Exine

orname-

ntation

Spine

type

Spine

length

( m)

1 I. alba pantoporate spinose pointed 11.8

2 I. aquatica pantoporate spinose pointed 9.1

3 I. batatas pantoporate spinose blunt 9.1

4 I. cairica pantoporate spinose pointed 11.3

5 I. carnea sub sp. fistulosa pantoporate spinose pointed 13.1

6 I. coptica pantoporate spinose pointed 11.8

7 I. digitata pantoporate spinose pointed 11.5

8 I. hederacea pantoporate spinose blunt 6.4

9 I. hederifolia pantoporate spinose blunt 11.1

10 I. horsfalliae pantoporate spinose blunt 9.8

11 I. indica pantoporate spinose pointed 10.8

12 I. obscura pantoporate spinose pointed 9.9

13 I. pes-caprae sub sp. brasiliensis pantoporate spinose pointed 12.1

14 I. purpurea pantoporate spinose pointed 10.1

15 I. quamoclit pantoporate spinose blunt 12.6

16 I. staphylina pantoporate spinose pointed

10.9

134

Table -17 Particulars of pollen morphology

Sl.No

Name of taxa

Grain size

range

( m)

Shape

Pollen

fertility

(%)

1 I. alba 108-128 spheroidal 92

2 I. aquatica 120-130 spheroidal 78

3 I. batatas 115.6-132 spheroidal 71

4 I. cairica 74.8-91.8 spheroidal 90

5 I. carnea Jacq.sub sp. fistulosa 92.8-101 spheroidal 95

6 I. coptica 119-149.4 spheroidal 89

7 I. digitata 88.4-102 spheroidal 98

8 I. hederacea 86.5-101 spheroidal 100

9 I. hederifolia 98.9-150 spheroidal 71

10 I. horsfalliae 117.6-157 spheroidal 80

11 I. indica 81.6-129.2 spheroidal 75

12 I. obscura 78.2-88.4 spheroidal 97

13 I. pes-caprae sub sp. brasiliensis 117-186.7 spheroidal 91

14 I. purpurea 90.9-150 spheroidal 97

15 I. quamoclit 92.5-157.5 spheroidal 95

16 I. staphylina 89.7-121 spheroidal 85

135

DISCUSSION

Based on the result obtained, an attempt was made to discuss and review the

taxonomy as well as the major evolutionary trends in pollen morphology within the

genera. In the present investigation pollen characteristics of 16 species of the genus

Ipomoea were analyzed. Palynological characters such as pollen type, aperture

morphotype, exine ornamentation, spine type and length, grain size and shape, pollen

fertility like features were observed. Plant materials used in the present study were

collected from different parts of South India. Morphological analysis of pollen grains is

made based mainly on the aperture characters. The other palynological characters such

as exine ornamentation, exine strata, pollen size and shape are also taken into

consideration as supplementary factors. The aperture characters are considered to be of

primary importance, exine surface pattern secondary and the others as tertiary.

Pollen size and shape are relatively less diagnostic as they are not stable characters.

A wide range of grain size and shape may occur in the same taxon. Moreover the method

of pollen preparation also affects considerably the pollen size and shape (Walker and

Doyle, 1975). According to Muller, (1979) increase in pollen size primarily caused by

increased adaptation to different systems of animal pollination. The taxonomic value of

pollen size has been reported in some varieties of cereals like Sorghum valgare and Zea

mays by Nair, (1962). Grain size as an index to ploidy level emergence of plant dioecism

had been shown in Trichosanthes palmata.

136

The pollen grains are spherical in shape. The pollen grains in all the species studied

are pantoporate, echinate or baculate in ornamentation. Larger pollen have been noted in

I. pes-caprae sub sp. brasiliensis and smaller in I. obscura. In most of the species studied,

sterility of pollen have been recorded and only in four species such as I. carnea, I.

hederacea, I. quamoclit and I. purpurea, the pollen fertility was 100%. The pollen studied

were comes under the characteristic group of Convolvulaceae called spiniferous.

Ipomoea type known as echinoconiae, as evidenced by the present investigation, which

confirms the work of Hallier, (1893) cited in Erdtman, (1952). The detailed study on the

pollen morphology of Ipomoea species have been carried out by previous workers Nair

(1965). In his studies on the Himalayan species of I. purpurea, Nair, (1965) suggested

that the pollen of I. purpurea, differ from the rest of the species of Ipomoea, in having

large sized pollen.

The significance of pollen analytical data in the evaluation of systematic kinship

existing among the taxa of Convolvulaceae has been studied by Sampathkumar and

Rangaswami Ayyangar, (1980). According to them, the species and varieties of

Convolvulaceae, like Ipomoea, Cressa, Evolvulus, Hewittia, Jacquemontia,

Merrimmea, Operculina, Porana and Rivea, suggested that the family Convolvulaceae

as euripalynous and the pollen of Ipomoea and Rivea are polyforate and echinate, while

the other taxa possess colpate or rugulate or psilate (smooth) pollen. It is opportune at

this moment to point out the evolution of palynological criteria will have to be made

critically and cautiously in advanced investigations (Erdtman, 1952). Erdtman believes

137

that palynology has contacts with cytology as well as genetics in questions covering

chromosome number, meiotic disturbances and others.

Variation in exine structure, sculpture, variation in the number, position and

complexities of apertures in the pollen grain wall gives the morphological diversity of

Convolvulaceae have been made by Gamble, (1921); Sayeed-Uddin et al., (1942);

Erdtman , (1952); Natarajan, (1957); Nair and Rehman, ( 1963); Rao and Ong, (1974) ;

and Vij and Sachdeva, (1974). Six pollen types were recognized by Erdtman, (1952),

Nair and Rehman, (1963) in this family.

As per the present investigation in the species of Ipomoea, larger pollen have

been noted in I. quamoclit than I. purpurea, which differs from the work of Nair

(1965). In their studies on the pollen grains of the species of Ipomoea and other genera

of Convolvulaceae, Sampathkumar and Rangaswami Ayyangar, (1980), reported

polyforate (pantoporate) and echinate pollen in the species of Ipomoea, it is slightly

differ from the present investigation. In the species of Ipomoea, that in our studies both

polyforate echinate and polyforate baculate pollen have been reported.

The genera Lettsonia, Ipomoea, Calonyction and two members of the genus

Merremia show pantoporate pollen grains. But the pollen grains of Merremia are non-

spinose and pollen grains are spinose in the other genera. In the genera Jaquemontia

and Evolvulus the pollen grains of pantocolpate, whereas M. dissecta has 3-zonocolpate

pollen grains, M. tuberosa has 4-zonocolporate pollen grains and M. umbellata has 6-

zonocolpate pollen grains. The porate aperture pollen grains are considered to be an

138

advanced character (Woodehouse, 1935). On the basis of this M. tridentata and M.

tridentata sub sp. hastata are regarded more advanced when compared to the other

species. This indicates that both primitive and evolved type of pollen is found in the

same genus. Regarding the pollen grains in M. tridentata similar observation was

reported earlier by Austin and Staples, (1980) on the basis of pollen structure they

separated M. tridentata into a new genus Xenostigia. The major groups of pollen grains

in Convolvulaceae were found here as reported by Nair and Rehman, (1963).

Pollen grains provided with excrescences are always pantoporate, while the

reticulate types are either 3-zonocolpate, 4-6 zonocolpate or pantocolpate. The surface

ornamentation of the exine is a significant morphological character helping a great deal

in the categorization of various genera and species. In the case of stenopalynous

families and as a supplementary factor in eurypalynous families (Nair and Sharma,

1962). According to Nair, (1971) the ornamentations formed of excrescences are

primitive and those formed of depressions are advanced.

The thickness of the exine is found to using in all the genera studied. Data on

pollen size among the genera studied vary slightly. There is uniformity in the shape of

the monad. It is opportune at this moment to point out the evolution of palynological

criteria will have to be made critically and cautiously in advanced investigations

(Erdtman 1952). Erdtman believes that palynology has contacts with cytology as well as

genetics in questions covering chromosome number, meiotic disturbances and others.

139

Evolutionary tendencies in the Convolvulaceae are also apparent from the plant

studied. The basic type in the family perhaps is the pantoporate and spinose type from

which the other morphotypes have evolved by the process of zonation, reduction and

fusion as outlined by Erdtman in his Scandinavian pollen flora (Nair and Rehman,

1963). From the pantoporate spinose type, pantoporate verrucose type evolved. In

addition to germinal apertures which are of primary importance in the morphology of

pollen grains can be seen. From the basic pantoportate type 5 and 3 zonocolpate

evolved in one line and panto colpate types on another line. Morphological evolution of

the pollen grain is very evident within the genus Merremia.

The genera Jacquemontia and Evolvulus show pantocolpate pollen grains. The

presence of pantocolpate pollen grains and faveolate exine ornamentation show

relationship between these two genera. The genus Heweittia shows relationship

between these two genera. The genus Hewettia shows relationship with members of the

genus Merremia by the presence of 3- zonocolpate pollen grains and with reticulate

exine ornamentation.

Pollen characters of taxonomic and phylogenetic importance relate to aperture

type, pollen wall architecture (exine ornamentation, exine strata), polarity, symmetry,

shape and grain size (Walker and Doyle, 1975) of these the germinal aperture character

forms the stable fundamental basis and hence given. Primary importance in the

interpretation of inter relation while exine surface pattern is secondary and the other

characters such as pollen size and shape are tertiary in the degree of significance

140

(Erdtman,1952; Vishnu-Mittre, 1964; Nair, 1965; Walker and Doyle, 1975). The

aperture characters refer to their form, number and distribution and to a lesser extent to

the position. They show variation in different plants at various taxonomic levels so as to

be of use in the identification of general species and sometimes even varieties. The exine

surface pattern serves as supplementary character in reaching taxonomic and phylogenetic

conclusions (Erdtman 1952; Nair, 1970). Features of exine ornamentation are particularly

useful in stenopalynous taxa, while the tertiary characters such as exine strata, pollen size

and shape are insignificant characters in applied taxonomy.

In the present investigation the polyploidy species such as I. batatas shows

comparatively larger pollen grains. Vij and Sachdeva, (1974) reported that in

Convolvulus pluricaulis the tetraploids shows larger grains than diploids. Chaturvedi et

al., (1997) also reported that the size range of the pollen widens with the increase of the

ploidy level. It was reported that increased size of pollen grains due to polyploidy is

most noticeable in the intraspecific level.

chapter v palynological studies - …shodhganga.inflibnet.ac.in/bitstream/10603/28986/10/10_chapter...

Documents