A propagative plant body consisting of a gametophyte enclosed in a non-cellular coat. Spores are enclosed within a capsule (sporangium) and are produced in groups of four (tetrads) when the parent cell divides meiotically. In more primitive plants the spores are identical (isospore) and the condition is called ‘homospory’. In more advanced, vascular plants spores of two sizes are produced and the condition is called ‘heterospory’. Small, male microspores are contained within a microsporangium; larger, female megaspores within megasporangia. Where spores occur in tetrads the contact surfaces produce a ‘trilete’ mark on each of the four spores, marking the point for the germination of the prothallus. Less commonly, two contact surfaces are produced, resulting in a ‘monolete’ marking. Spores which were probably produced singly are ‘alete’, with no obvious marking.