In this article we will discuss about the aperture found in the pollen and spore wall of plants.
It is a weak, preformed locus on the exine of spore and pollen through which the intra-exinous substance exits and pollen tube emerges during pollen germination.
Apertures may be absent from a pollen grain or spore when they are termed as inaperturate/atreme/non-aperturate (ex. Potamogeton). Some non-aperturate pollen grains belonging to the family Euphorbiaceae are omniaperturate. The sexine of omniaperturate pollen is either absent or very thin. The intine is thicker than exine. It is regarded that the whole surface of pollen wall is apertural in nature.
In aperturate pollen at the region of aperture the exine is either missing or greatly reduced. When reduced the exine represents a thin layer over aperture. The intine layer is thicker under apertures. The intine, in the aperture, may have a covering on the nexine side termed aperture membrane. Sometimes granular elements may occur on the membrane forming granulated aperture membrane.
An aperture may be formed on sexine only or on both sexine and nexine. In the former the aperture is described as simple. In the latter the aperture is compound or composite. The aperture formed on sexine is called ectoaperture (= ectopore). In contrast the aperture formed on nexine is termed as endoaperture (= endopore). Ecto- and endoaperture together constitute a compound or composite aperture (Fig. 4.19A & B).
i. Pore/porus (n., pl. -s/pori, adj. porate —Fig. 4.19C):
Pore is rounded or elliptic (ratio between length and breadth is less than two), more or less distinct aperture of a pollen grain. As a rule they are equally distributed on the equatorial plane or over the general surface of a pollen grain. The number of apertures is indicated by the prefixes mono-, di-, tri-, tetra-, penta-, hexa- and poly-.
A pore is formed on sexine only. At poral region continuity of sexine is interrupted, the nexine being continuous. This is simple aperture and ectoaperture as it is present only in one layer, i.e. sexine. Through the pore pollen tube emerges during pollen germination and intra-exinous material exits.
ii. Colpus (n., pl. colpi, adj. colpate-Figs. 4.19D &4.21M):
Colpus is an elongated aperture (the ratio between length and breadth is greater than two) of a pollen grain. Colpi are equally spaced on the equatorial plane or over the general surface of a pollen grain. In dicotyledon a colpus is a longitudinally elongated meridonial furrow that crosses the equatorial plane at right angle.
Exceptions are noted in some tetra-, hexa- and polycolpate grains where colpi do not always cross the equator. In palynology the term ‘meridonial’ is used to describe those longitudinal features of pollen surface that occur perpendicular to equatorial axis. The number of colpi is indicated by the prefixes mono-, di-, tri-, tetra-, penta-, hexa- and poly-. A colpus is an ectoaperture and formed on sexine only.
It is a simple aperture as it is present only in one layer, i.e. sexine. The sexine is interrupted at the region of colpus and the nexine is continuous. Colpus is the germinal apparatus of extant pollen and may function during the emergence of pollen tube. In extant pollen the colpus is the normal exit of intra-exinous substance only. It is interpreted that during evolution a colpus, by shortening and contraction developed to a pore.
iii. Sulcus (n., pl. sulci, adj. sulcate —Fig. 4.19E & F):
Sulcus is an elongated latitudinal ectoaperture present on the surface of most monocotyledon and primitive dicotyledonous pollen crossing the polar axis at right angle. It has the shape of a colpus, i.e. the ratio between length and breadth is greater than two. It differs from colpus on the basis of orientation. It occurs as a latitudinal aperture in contrast to colpus, which is longitudinal aperture.
In palynology the term ‘latitudinal’ is used to describe the features that run parallel to equatorial axis of a pollen grain. In most monocotyledons the term sulcus is restricted to distal aperture of a pollen grain only. Sulcus with its centre may occur at the distal pole or on proximal pole of a pollen grain and accordingly termed as anasulcate and catasulcate.
A pollen grain with one sulcus is called monosulcate. A pollen grain with two sulci is termed as disulcate.
Metroxylon, Chamaerops (Palmae) etc. In some pollen the sulcus may be in the form of three or four-slit like opening. Such pollen grains are respectively termed as trichotomosulcate (Fig. 4.20 C) and tetratomosulcate. It is simple aperture as it is present only in one layer, i.e. sexine.
iv. Sulculus (n., pl. sulculi —Fig. 4.19G & H):
The term sulculus describes a latitudinal aperture of a pollen grain. The aperture is an elongated furrow and situated more or less parallel to equator near a pole, generally distal. It is not centered at the distal pole like sulcus. Sulculi may unite encircling a grain thus forming a ring.
v. Lete (Erdtman, 1969):
Lete is always used as a suffix to denote the absence or presence of laesura(e). Laesura is the scar mark of a spore. The mark represents the original contact of spores at their tetrad stage.
So laesura is always proximal. A laesura comprises a commissure the line of dehiscence of spore along which the spore germinates. A spore is termed as monolete (Fig. 4.19K) when there is one un-branched straight tetrad scar on the proximal side (ex. Psilotum, Dryopteris).
A spore is called trilete (Fig. 4.20A & B) when there is a three-pronged laesura resembling the English letter capital ‘Y’ on the proximal side (ex. Lygodium, Pteridium etc.). Palynologists regard that trilete spores have a single tri-radiate or three-branched laesura. A spore is termed as alete when there is no laesura.
An alete spore has completely smooth surface that is devoid of dehiscence marks and tetrad scar. The spore shows no recognizable characters after separation from tetrad. Dilete spores are rare. Pollen grains of Simethis (Family: Liliaceae) exhibit a three-slit aperture resembling a trilete scar, which is very rare in angiosperm.
vi. Pantoporate (adj., syn. periporate — Fig. 4.21 A):
The term pantoporate describes a pollen grain with pores spread over the surface. Panto- (syn. pan-, peri-) is a prefix that illustrates global distribution. Pores are more or less circular where the ratio between length and breadth is less than two. The number of pores is always more than four and may be as high as 80.
The pores remain evenly distributed on the surface of a pantoporate pollen grain (ex. Chenopodiaceae). Though the terms polyporate and periporate are synonymous, the former is favoured. The term pantoporate is more widely used than other synonyms.
v. Ulcus (pl. ulci, adj. ulcerate —Fig. 4.191 & J):
Ulcus is an ectoaperture and a thin place on exine. The aperture is more or less like a circular pore, the ratio between length and breadth is less than two. The outline of a pore is irregular and often torn open widely (Traverse, 1988 and Chanda, 1966).
Some members of Restionaceae. Ulcus occurs either on distal or proximal pole. Accordingly a pollen grain is described as ana-ulcerate and cata-ulcerate.
vi. Pantocolpate (adj., syn. pericolpate —Fig. 4.21B):
The term pantocolpate describes a pollen grain with colpi spread over the surface. Panto- (syn. peri-) is a prefix that illustrates global distribution. Colpus is an elongated furrow where the ratio between length and breadth is greater than two. The number of colpi is more than three and they remain evenly distributed over the surface in a pantocolpate pollen grain.
Portulaca, Drymaria etc.
vii. Trichotomosulcate (Greek tricha = in three parts, tome = cut —Fig. 4.20C):
The term trichotomosulcate describes a monsulcate pollen grain in which the sulcus is three-branched. It appears more or less triangular and simulates a trilete laesura. It is an ectoapertue and latitudinal aperture. In patynology the term ‘latitudinal’ is used to describe the features that run parallel to equatorial axis of a pollen grain.
Elaeis guineensis (Palmae).
viii. Syncolpate (adj. —Fig. 4.21C & D):
The term syncolpate describes a pollen grain where the colpi run together at the poles. It appears that the ends of colpi join/anastomose normally near distal and proximal poles. Syncolpate pollen has two or more simple ectoapertures that are elongated furrows (the ratio between length and breadth is greater than two). The prefix ‘syn-‘ is used to indicate the fusion/anastomosis of features.
Illicium, Pedicularis etc.
ix. Parasyncolpate (adj. —Fig. 4.21E & F):
The term parasyncolpate describes a pollen grain where the apices of colpus divide into two branches and the branches anastomose/join with that of other colpus near the proximal and distal pole.
The branches join in such a way that they delimit an isolated area called apocolpial field. The apocolpial field normally has a definite shape that is usually triangular.
Eugenia (Myrtaceae), Nymphoides (Menyanthaceae) etc.
x. Brevicolpate (adj.):
The term brevicolpate describes a pollen grain where the colpus is short in length. The prefix ‘brevi-‘ is used to indicate a short feature. The distance between proximal pole and adjacent apex of colpus together with the distance between distal pole and adjacent apex of colpus is greater than the length of the colpus in a brevicolpate pollen grain.
Leptoma is a thin region on exine present on the distal pole of a pollen grain through which pollen tube emerges.
xii. Pseudoaperture (n., adj. pseudoaperturate):
Pseudoaperture is an area on the exine, which resembles an aperture. In this area the exine is thin and intine is also thin. Normally the intine is thicker where exine is thin. It is presumed that pseudoaperture is not the site of emergence of pollen tube. Pseudoaperture may be in the form of an elongated furrow and a pore.
Accordingly they are termed as pseudocolpus and pseudopore. In pseudocolpus (pi. pseudocolpi, adj. pseudocolpate) the pseudoaperture is colpus like, i.e. the ratio between length and breadth is more than two.
Myosotis (Boraginaceae). In pseudopore (pl. pseudopores, adj. pseudoporate) the ratio between length and breadth of aperture is less than two. Traverse (1988) defines the thin area present in leptoma as pseudopore.
Pollen of Cupressaceae and Taxaceae, and the fossil family Cheirolepidiaceae.
xiii. Spiraperturate (adj.-Fig. 4.21G & H):
Spiraperturate pollen grains have one to several colpi. The colpi are fused giving the appearance of a set of spirals surrounding the whole pollen grain. The apertures may be irregular in outline, irregularly spaced and spiral around the pollen grain.
Eriocaulon aquaJcum (Eriocaulaceae) and Crocus etc.
i. Pororate (adj., n. pororus Fig. 4.21I):
The term pororate is used to describe a pollen grain that has compound aperture consisting of ectoaperture and endoaperture where both ecto- and endoaperture are pores and they are not congruent. Ectoaperture is the pore (the ratio between length and breadth is less than two), which is formed on sexine.
The endoaperture is also a pore that is formed on nexine. The endoaperture is also called os (pl. ora, adj. orate). Pore with os forms the pororate pollen. This term is commonly used in adjective form.
ii. Colporate (adj., sing, colporus, pl. colpori – Fig. 4.21 J, K & L):
The term colporate is used to describe a pollen grain that has compound aperture consisting ectoaperture and endoaperture where ectoaperture is colpus and the endoaperture is os. Colpus is formed on sexine and os is formed on nexine. Colpus with os forms the colporate pollen grain.
Pollen grains of Compositae and Solanaceae etc.
The term is commonly used in adjective form. In a colporate pollen grain the colpus is meiidonial aperture, i.e. it occurs parallel to polar axis. The endoaperture os may be longitudinally elongated (Fig. 4.21L) termed lolongate (ex. Cassia sofera, Mangifera indica).
The endoaperture may be also Iatitudinally elongated (Fig. 4.21 K), i.e. elongation occurs perpendicular to polar axis-termed lalongate (ex. Datura metel, Lippia Alba), Banik et al. (1986). Apart from meridonial position colpori may spread over the surface of a pollen grain. Such grains are described as pantocolporate pollen (Fig. 4.22H).
iii. Multiorate (adj.-Fig. 4.22A):
The term multiorate describes a colporate pollen grain where the number of ora is more than two in a colpus.
iv. Colporoidate (adj.):
The term colporoidate describes a colporate pollen grain where the os is indistinct in a distinct colpus, i.e. a colporate pollen grain with a weakly developed pore. In a colporoidate pollen grain the pore is either absent or not clearly recognizable.
Scoparia dulcis, Undemia crustacea etc.
v. Colpoidorate (adj.):
The term colpoidorate describes a colporate pollen grain where the colpus is indistinct and the os is distinct.
vi. Heterocolpate (adj. – Fig. 4.22B & C):
The term heterocolpate describes a pollen grain where both simple aperture colpus and compound aperture colporus are present.
Lythrum, Peplis etc.
vii. Porocolpate (adj.-Fig. 4.22D & E):
The term porocolpate describes a pollen grain where the simple apertures both colpus and pore occur, and their arrangement is alternate to each other round the equator.
viii. Colpororate (adj.-Fig. 4.22G):
The term colpororate describes a compound aperture of a pollen grain consisting an ecto-, endo- and mesoaperture. The ectoaperture is an elongated furrow and simulates a colpus. The endoaperture is lalongate. The mesoaperture is lolongate and short. Thanikaimoni (1980) defines mesoaperture as the middle part of a composite aperture. In this region there exist also an ecto- and endoaperture.
ix. Synorate/synclinorate (Fig. 4.22F):
The terms synorate/synclinorate describe an os of a colporate pollen grain. The os is latitudinally elongated and joins with other of similar os.
In many pollen grains the margin of aperture is distinct from the remainder of exine. The margin may be thin or thick, or may have different ornamentation. The followings are a few common examples.
x. Annulus (n., pl. annuli, adj. annulate):
Annulus is an area on sexine that occurs encircling a pore (Fig. 4.23A). The area can be sharply differentiated from the rest of sexine. The area may be elevated (Fig. 4.23B) or depressed (Fig. 4.23C), or may have different sculpturing than the remaining part of sexine. The area appears as a halo surrounding a pore.
Poaceae. Some members of Caryo- phyllaceae have annulus with thinned sexine.
xi. Margo (n., pl. margines, adj. marginate – Fig. 4.23D):
Margo is an area on sexine that occurs encircling a colpus. The area can be sharply differentiated from the rest of sexine. The area may be elevated or depressed, or may have different sculpturing than the remaining part of sexine. The area appears as a halo surrounding a colpus.
xii. Costa (n., pl. costae, adj. costate, costal [Traverse, 1988] – Fig. 4.23E, F & G):
Costa is a rib-like thickening that occurs on endonexine. It is associated with colpi. Costae are usually meridonial and border colpi. The thickening may also occur following the outline of an ectoaperture.
Melia. Costae are best observed in equatorial view of a pollen grain. Punt et al (2007) reported the presence of costa in pore also.
xiii. Operculum (n., pl. opercula, adj. operculate – Fig. 14.23H, I & J):
Operculum is a thickening of pore membrane that can be observed distinctly and measured. It is an isolated part of sexine and separated from the rest by a narrow zone where sexine is absent. So the structure of operculum is similar to that of the remaining exine. Operculum covers aperture partially or wholly.
It remains surrounded by aperture. So opercula are more or less circular in pore and elongate in colpi. The zone of operculum is less strong than the rest of exine. A pollen grain ruptures along operculum when pressure is exerted from inside. In fossil pollen pore-opercula are frequently lost.
xiv. Arcus (adj. arcuate-Fig. 4.23K & L):
The term arcus describes a kind of thickening of sexine. The thickening is in the form of a band that occurs between the apertures of a pollen grain. The bands are curved or bend like a bow.
Alnus (Betulaceae), Punt et al.
xv. Oncus (pl. onci – Fig. 4.24A):
The term oncus describes a structure that occurs beneath the aperture of certain unacetolysed pollen grains. The structure is lens- shaped and not resistant to acetolysis.
Corylus (Corylaceae), Punt et al.
xvi. Aspis (adj. aspidate/aspidote-an orthographical variant of aspidate-Fig. 4.24B & C):
The term aspis describes a kind of thickening around a pore. The thickening is in the form of a dome-like protrussion where pore occurs. It differs from annulus in not having any thinning or depression of sexine.
Betula (Betulaceae), Dorstenia (Moraceae), Punt et al
xvii. Atrium (Fig. 4.24D):
The term atrium describes the shape of a space that occurs within the aperture of a complex pore. The endopore is much wider than the ectopore. As a result the pore canal is narrow towards sexine and wider towards nexine.
Myrica (Myricaceae), Punt et al.
xviii. Vestibulum (Fig. 4.24E):
The term vestibulum describes a small chamber formed by the sexine and nexine of a pollen grain. Sexine and nexine split apart near a porus thus forming a chamber. The chamber is the site of communication between outside and inside of a pollen grain.
The main function of an aperture is to allow passage for the emergence of pollen tube. Polyaperturate grains have better opportunity for the pollen tube to emerge close to stigma surface than monoaperturate one. The apertures also provide exits of proteins that recognize the compatible stigma.
The number of porus, colpus and colporus may be indicated by prefixes di-, tri-, tetra-, penta-, hexa- and so on. It is also of common use to write numbers instead of prefixes, e.g. 3-colpate instead of tricolpate and 4-porate instead of tetraporate etc. Many palynologists prefer to use prefixes. The term pantoaperturate is used to indicate the globally distributed apertures of a pollen grain.
The aperture membrane (e.g. colpus, porus and leptoma etc. of a pollen grain) exhibits harmomegathy (n.; pl. harmomegathi; adj. harmomegathic). Apart from aperture membrane, pollen wall exhibits harmomegathy. Extant pollen grain exists either in dry or hydrated condition. In harmomegathic mechanism pollen wall and aperture membrane either expand or contract to accommodate the change of osmotic pressure in the cytoplasm during dehydration or hydration.
Harmomegathic effect protects the male gametophyte of a pollen grain against desiccation during dispersal. Harmomegathic effect depends on pollen size, apertures, the thickness of pollen wall and pollen coatings etc. When coating is abundant it acts as a sheath against desiccation. Pollen grains collected from herbarium materials also exhibit harmomegathy.
Regarding apertural evolution trichotomocolpate (= trilete) form is regarded as the basic type from which other apertural forms (e.g. colpate, porate, colporate, pororate, pantocolpate, pantoporate and spiraperturate etc.) have evolved. The lines of evolution are illustrated in Fig. 4.25. The relative primitive and advanced apertural forms are depicted in the Box 4.2.
Box 4.2 illustrating relative primitiveness among apertural forms.