EXINE FORMATION IN CHAMAECYPARIS-LAWSONIANA (CUPRESSACEAE) AND A DISCUSSION ON PTERIDOPHYTE EXOSPORE AND GYMNOSPERM EXINE ONTOGENY

Exine ontogeny in Chamaecyparis lawsoniana is similar to that in other gymnosperms investigated as far as modes of wall layer formation are concerned. However, it differs markedly in timing of layer development , inasmuch as endexine elaboration is initiated at the cell surface pr ior to appareance of tectum and infratectum components at the level of the microspore surface coat. It is observed that the tectum and infra tectum are formed through deposition of sporopollenin on receptor site s provided by the microspore surface coat, whereas the foot layer appe ars to result from accumulation of similar sporopollenin against the e ndexine surface. It is proposed that the microspore surface coat is pa rt of the microspore glycocalyx and mediates tectum and infratectum fo rmation, while foot layer deposition must be mediated by a distinct, i nnermost part of this glycocalyx. Comparison of gymnosperm exine and p teridophyte exospore ontogeny shows that, (1) the whole solid, structu reless outer part of the exospores is formed through the same process as the foot layer of gymnosperm exines; (2) the ontogenetic and struct ural differences between the two types of walls are mainly due to the presence of the microspore surface coat in gymnosperms, which produces formation of exine outer strata having very varied organizations, usu ally including large gaps. The entire pteridophyte exospore appears th us to be ontogenetically homologous to the endexine and foot layer of the gymnosperm exine. It is assumed that the ''para-exospore'' of Isoe taceae and Selaginellaceae microspores and the comparable extra-exospo ral wall of certain fossil spores are formed through intervention of a microspore surface coat-like matrix, and are thus homologous to tecta l and infratectal structures of gymnosperm exines.