in which tissues of one genetic type completely surround tissues of another genetic type. The ornamental shrub Laburnocytisus adami is a periclinal chimaera that originated from the grafting of Cytisus purpureus onto a Laburnum anagyroides stock. The epidermis is composed of Cytisus cells and the internal tissues of Laburnum cells. Externally it is not obvious that this graft hybrid is a chimaera but occasionally shoots arise consisting solely of tissues from one or other parent. The seeds, which derive from the internal tissues, are always Laburnum. Investigations into the stability of this chimaera led to the *tunica-corpus theory
of apical organization. The variegated leaves of certain plants are examples of periclinal chimaeras that have originated from a plastid mutation preventing chlorophyll synthesis. The mutation may occur in either the second layer of the tunica (L2) or the corpus (L3). Differences in colour appear between the leaf margins and the centre of the leaf because the margins are derived from the L2 while the centre is composed of both L2 and L3. Thus if the mutation occurs in the L2 then the margins are white and the centres green (the underlying green L3 tissues are not masked by the white L2 tissues). This type of variegation is seen in Pelargonium.