A new ontogenetic classification of stomatal types is proposed which replaces the three ontogenetic types of Pant (1965) with seven new ones. The new classification clearly differentiates between the developmental involvement of the subsidiary cells and the purely structural relationship of the neighbouring cells. All known, and hypothetical, ontogenetic pathways of stomatal development can he incorporated into the new classification. An, hitherto unknown, ontogenetic type which incorporates neighbouring, mesogene subsidiary, and perigene subsidiary cell elements into the stomatal complex is described from the fern, Polypodium vulgare L.

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Differential cell wall thickening in developing guard cells of Polypodium vulgare L. has been studied with particular reference to guard cell protoplast deformation and the eventual formation of the stomatal pore. Concomitant studies on the development of guard cell chloroplasts and their starch inclusions during ontogeny of the stomatal complex have provided data which have been incorporated into a model to account for the formation of the pore. Guard cell starch inclusions reach a maximum density per unit volume at the same time as the guard cell walls achieve maximum differential thickening. These events coincide with the development of the pore. It is suggested that, whilst pore formation is initiated enzymatically, the mechanical forces required to bring about the separation of the two guard cells are of an osmotic nature derived from starch hydrolysis. The development of the mesophyll in relation to the epidermis is examined in respect of the formation of substomatal chambers.

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