NPC is an artificial system of classification of pollen and spore based on the three features of aperture only, i.e. number, position and character. Erdtman and Straka (1961) proposed NPC classification and palynologists all over the world accepted it.
According to NPC system each pollen grain has an arithmetic cardinal number consisting of three digits. The first digit reveals the absence or presence of aperture, and when present it mentions the total number of aperture(s) present in a pollen grain.
The second digit illustrates the position of aperture(s), i.e. distal, proximal, and latitudinal, meridonial, equatorial etc. The microspores reveal the position of aperture(s) with full clarity when they are in tetrad. The third digit explains the character of an aperture, i.e. circular/oval or elongated, simple or compound etc. ‘N’ from number, ‘P’ form position and ‘C’ from character of aperture compose the NPC-classification.
i. Classification of Aperture Based on Number(s):
In NPC system ‘N’ denotes the number (N; L. numerus) of aperture(s) present in a pollen grain. Aperturate pollen, i.e. pollen having apertures are divided into seven groups. The groups are mentioned as N1 to N7. Each group has characteristic number of aperture, i.e. N1 has one aperture and N2 has two apertures and so on. The N7 group has seven or more apertures.
N1 to N7 groups are also referred to respectively as monotreme, ditreme, tritreme, tetratreme, pentatreme, hexatreme, and polytreme (Greek trema means hole, opening, aperture; pl. tremata).
There are pollen grains where apertures are absent. Such pollen grains are termed as inaperturate or atreme and they are placed in N0 group. Another special group N8—termed anomotreme is created where the pollen grains and spores have one or several irregular or irregularly spaced apertures.
ii. Classification of Aperture Based on Position (Fig. 4.26):
In NPC system ‘P’ denotes the position (P; L. positio) of aperture in a pollen grain and spore. The position may be proximal, distal and equatorial. There are seven groups of aperture based on position namely –P0 to P6. Pollen grains having P0 group have uncertain or unknown position of aperture. P1 groups of pollen and spores are catatreme (Gr. Kata = down; -treme is suffix used as a synonym of aperture).
Catatreme pollen grains have one aperture that occurs on the proximal part of a grain. The proximal (L. proximus, nearest) part is the face of a pollen grain or spore that faces inward/nearest or toward the centre of tetrad (Fig. 4.31). P2 groups of pollen and spores are anacatatreme (Greek ana = up). Anacatatreme pollen and spores have two apertures.
One aperture with its centre occurs at the proximal pole. The other aperture with its centre occurs on the distal pole. The distal (L. distalis, remote, outer) part is the face of a pollen grain and spore that faces outward, i.e. away from the centre of tetrad and opposite the proximal part (Fig. 4.31). P3 groups of pollen and spores are anatreme, i.e. the aperture is distal in position.
P4 groups of pollen and spore are zonotreme. A zonotreme (zono-a prefix used to indicate the equatorial/subequatorial region) pollen grain is characterized in having apertures on equator or sub-equator. The equator is the part of a pollen grain or spore that runs midway between the proximal and distal poles and perpendicular to polar axis. P5 groups of pollen and spore are dizonotreme.
Dizonotreme pollen grains have apertures arranged in two or more zones. The apertures occur parallel to equator. P6 groups of pollen and spore are pantotreme (Greek pan, gen. Pantos, all, wholly). Pantotreme pollen grains have apertures scattered over the whole surface uniformly. As a rule, pantotreme pollen grains are spheroidal.
iii. Classification of Apertures Based on Character (Fig. 4.27):
In NPC-system ‘C’ denotes the character (C; L. character) of an aperture in a pollen grain and spore. The character groups of pollen and spore are seven and they are mentioned as C0 to C6. C0 groups have apertures whose character cannot be established with certainty. C1 groups of pollen and spore have leptoma (Greek leptoma means thin place).
Leptoma is a thin area, aperture like and functions like an aperture. Pollen grains having one leptoma are termed as monlept. The leptoma may occur on either proximal-or distal face of a pollen grain and spore and accordingly termed as catalept and analept. C2 groups are trichotomocolpate (Gr. Tricha, in three parts; tome, cut; kolpos, depression, furrow).
Trichotomocolpate is a three- branched aperture, the branches of which are more than two times longer than breadth. Trichotomocolpate pollen and spores having aperture on porximal face are termed as trilete.
The group C3 has colpate grains. The group C4 comprises porate pollen grains. The group C5 comprises colporate pollen. The group C6 comprises pororate pollen. C3, C4, C5 and C6 groups of aperture are previously discussed under aperture.
In NPC classification a grain is mentioned in three-digit number (Figs. 4.28, 4.29 & 4.30), e.g. 343 instead of N3P4C3. Pollen grains having NPC 343 are tritreme zonocolpate, which is also described as tricolpate pollen.
NPC 764 characterizes those pollen grains that are polytreme pantoporate, which are also described as pantoporate or polyporate. Pollen grains of Amaranthaceae, Chenopodiaceae etc. have NPC 764. Examples of tricolpate pollen grain, i.e. NPC 343 are Rumex, Vitex, Tectona, Argem one etc.
Merits of NPC classification:
1. It is a simple system of classification and illustrates the apertures of a pollen grain and spore.
2. NPC makes the description of apertures precise.
3. With the aid of NPC pollen grains and spores of pteridophyta, monocotyledon and dicotyledon, to some extent, can be differentiated. Most of the spores of pteridophyta are monolete or trilete. Monocots are characterized by inaperturate, monosulcate and monoporate pollen grains. Dicots, with a few exceptions, have pollen grains that are mostly with three meridonial furrows and polyaperturate. Thus NPC narrows the search list of identification of unknown sporomorphs.
4. NPC is supposed to be of primary classificatory character because apertures are most conservative. It is supplemented by surface ornamentation, size and shape etc. of a pollen grain. Sometimes it becomes possible to identify the family or genus or even species of a pollen grain with the aid of NPC in combination with other morphological characters.
5. Palynologists all over the world accepted NPC-classification as it is basically simple and consistent where pollen grains and spores could be arranged easily. This helps to identify unknown sporomorphs.
6. NPC, sporoderm stratifications, exine patterns, size and shape etc. of a pollen grain are genetically stable. This property is utilized for various purposes and the followings are a few illustrations. With the aid of NPC and other characters a key can be formulated that helps to identify unknown pollen and spores.
Identification of pollen and spores is the essential prerequisite in the applied aspects of palynology, i.e. aeropalynology, melissopalynology, forensic palynology and palaeopalynology etc.
The interfamily and intra-family affinities of taxa, to some extent, can be determined with the aid of NPC. As for example the family Gramineae seems to be closely related to Restionaceae, Centrolepidaceae and Flagellariaceae, because pollen grains of above taxa are monoporate.
7. NPC and the various types of exine patterns and ornamentation provide characters of taxonomic significance and thus become one of the sources of alpha taxonomy.
Example:
Bombacaceae is segregated from Malvaceae; Zingiberaceae, Cannaceae and Musaceae are amalgamated into Scitamineae.
Demerits of NPC classification:
1. It is an artificial system of classification.
2. Syncolpate and parasyncolpate pollen grains do not fit neatly in NPC system.
3. Pollen grains that are characteristically present as aggregates in tetrads, e.g. Ericaceae, Typhaceae and polyads, e.g. Orchidaceae, Mimosa etc. are not grouped in NPC system.
NPC-system of classification is always compared with Linnaeus’s system of classification, because the latter is also an artificial system of classification. The characters of stamen were the basis of classification.
‘Linnaeus accepted the weakness of his classification but claimed that it was propounded mainly as an aid to identification’. This is also applicable to Professor Erdtman. Palynologists from every discipline of palynology utilize Erdtman’s NPC classification and other characters related to pollen morphology as an aid to the identification of unknown sporomorphs.
