The layer of silica between striae. Plural form is “virgae”. From Latin for “rod”.
The growth (morphogenesis) of pennate diatom valves with a single row of areolae in their striae resembles the making of a basket where the main supports were called virgae and the connections woven between them were called vimines. In diatoms, the virgae and the vimines join, creating the network of gaps which become the areolae (Cox and Ross, 1981).
See also costa.
Also described the silica between striae (Ross et al., 1979) but has been replaced by virga, which better describes the structure and morphogenesis of this valve feature.
Cox, E.J. (2011). Morphology, cell wall, cytology, ultrastructure, and morphogenetic studies. Overview and specific observations. In: The Diatom World (J. Seckbach and J.P. Kociolek, Eds,), Cellular origin, life in extreme habitats and astrobiology, Vol. 19, pp. 21–45, Springer, Dordrecht.
Cox, E.J. (2012). Ontogeny, homology, and terminology - wall morphogenesis as an aid to character recognition and character state definition for pennate diatom systematics. Journal of Phycology 48: 1–31. 10.1111/j.1529-8817.2011.01081.x
Cox, E.J. and Ross, R. (1981). The striae of pennate diatoms. In: R. Ross, ed., Proceedings of the Sixth Symposium on Fossil and Recent Diatoms. O. Koeltz, Koenigstein. pp. 267–278.
Round, F.E., Crawford, R.M. and Mann, D.G. (1990). The Diatoms. Biology and Morphology of the Genera. Cambridge University Press, Cambridge, 747 pp.