The following points highlight the four common types of spore found in embryophytes. The types are: 1. Isospore (n., pl. -s, syn. Homospore) 2. Heterospore (n., pl. -s; hetero- a Greek Prefix Meaning ‘different’, ‘not normal’) 3. Meiospore (n., pl. -s) 4. Miospore (n., pl. -s; in Greek Meion means ‘less’, ‘smaller’).
Type # 1. Isospore (n., pl. -s, syn. Homospore):
Isospores (in Greek ‘isos’ means equal)/homospores (homo-, a Greek prefix, meaning ‘alike’, ‘similar’) are characterized in having one kind of spore only. They all are of same size.
They are generally small, morphologically similar and not distinguishable as to sex. Plants producing isospore/homospore are described as isosporous/homosporous (adj.). Homospores are produced in the sporangium borne on homosporous plants as a result of reduction division. The unicellular isospore/homospore develops into multicellular gametophytic plant.
In homothallic gametophytes there originate both male and female sex organs and accordingly male and female gametes develop, e.g. homosporous pteridophytes like Lycopodium, Equisetum, Ophioglossum, and Polypodium etc. In heterothallic gametophyte, e.g. Equisetum arvense, two types of gametophyte develop. In one type male sex organ develops while the other has female sex organ only.
Type # 2. Heterospore (n., pl. -s; hetero- a Greek Prefix Meaning ‘different’, ‘not normal’):
Heterospores are characterized in having two kinds of spores- microspore and megaspore. Microspore and megaspore are distinguishable on the basis of sex and size. The megaspores are always larger than the microspores of same species. It is interpreted that the larger size of megaspore is due to an adaptation for the storage of food, which nourishes the embryo arising later.
In heterosporous embryophytic plants megaspore germinates to produce multicellular female gametophyte, also called megagametophyte. The microspores are smaller in size than the megaspores of the same species. In heterosporous plants microspore germinates to produce multicellular male gametophyte, also called microgametophyte. Plants producing heterospores are described as heterosporous (adj.).
Microspore and megaspore are the product of reduction division that occurs in the respective sporangia, accordingly termed microsporangium and megasporangium produced on heterosporous plants. Example: Selaginella, Isoetes etc.
Coal microscopists describe all small spores as microspores regardless of biological function. Traverse (1988) suggests avoiding this practice since it may tend towards confusion when more precise information becomes available.
In palaeopalynology the term macrospore is used as a synonym of megaspore, though macrospore may not have similar biological function like megaspore. All spores including pollen grains that are greater than 200 micra in diameter are grouped under macrospore.
Type # 3. Meiospore (n., pl. -s):
Spores produced as a result of reduction division or meiosis is termed as meiospore. Microspore, megaspore, isospore, pollen etc. all are meiospores as they develop respectively from their mother cells as a result of meiosis.
Type # 4. Miospore (n., pl. -s; in Greek Meion means ‘less’, ‘smaller’):
A pollen grain or spore having diameter less than 200 micra, regardless of biological function is termed as miospore. When the term miospore is used, pollen grains and spores having diameter more than 200 micra are referred to as megaspore irrespective of their biological function.
In palaeopalynology sometimes it becomes difficult to distinguish spores from isospores/homospores of homosporous plants, microspore of heterosporous plants, small megaspores, pollen and pre-pollen when they all are observed individually. So palynologiste adopted the term ‘small spore’ to describe the spores of relatively small size, regardless of their biological function.
Later the term ‘miospore’ is proposed to denote isospores/homospores, true microspore of heterosporous plants, small megaspores, pollen grains and all fossil spores having diameter less than 0.20 mm. On the other hand spores having diameter greater than 0.20 mm are referred to as macrospores (Guennel).
The 0.20 mm measurement is used as the demarcation line because of technical reasons only and bears no biological meanings. The standard sieves that are used in the preparation techniques of spore and pollen slides have meshes of that size.
[Prepollen is defined as a pollen grain characterized by the presence of proximal aperture, presumed proximal germination and distal aperture. The pollen tube that emerges through proximal aperture is meant for releasing fertilizing nuclei, presumably in antherozoids-thus exhibiting zoidogamy. The haustorial pollen tube emerges through distal aperture. Prepollen is haploid and is observed in some extinct (e.g. Cycadofilicales) and extant (e.g. Cycas and Ginkgo) seed plants. Prepollen grains have laesurae. In this sense one may exclude Cycas and Ginkgo because they lack proximal laesurae].